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#751 2020-08-03 00:46:47

ganesh
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Re: Miscellany

629) Machu Picchu

Machu Picchu, also spelled Machupijchu, site of ancient Inca ruins located about 50 miles (80 km) northwest of Cuzco, Peru, in the Cordillera de Vilcabamba of the Andes Mountains. It is perched above the Urubamba River valley in a narrow saddle between two sharp peaks—Machu Picchu (“Old Peak”) and Huayna Picchu (“New Peak”)—at an elevation of 7,710 feet (2,350 metres). One of the few major pre-Columbian ruins found nearly intact, Machu Picchu was designated a UNESCO World Heritage site in 1983.

Although the site escaped detection by the Spaniards, it may have been visited by the German adventurer Augusto Berns in 1867. However, Machu Picchu’s existence was not widely known in the West until it was “discovered” in 1911 by the Yale University professor Hiram Bingham, who was led to the site by Melchor Arteaga, a local Quechua-speaking resident. Bingham had been seeking Vilcabamba (Vilcapampa), the “lost city of the Incas,” from which the last Inca rulers led a rebellion against Spanish rule until 1572. He cited evidence from his 1912 excavations at Machu Picchu, which were sponsored by Yale University and the National Geographic Society, in his labeling of the site as Vilcabamba; however, that interpretation is no longer widely accepted. (Nevertheless, many sources still follow Bingham’s precedent and erroneously label Machu Picchu as the “lost city of the Incas.”) Evidence later associated Vilcabamba with another ruin, Espíritu Pampa, which was also discovered by Bingham. In 1964 Espíritu Pampa was extensively excavated under the direction of the American explorer Gene Savoy. The site was much deteriorated and overgrown with forest, but Savoy uncovered remains there of some 300 Inca houses and 50 or more other buildings, as well as extensive terraces, proving that Espíritu Pampa was a much larger settlement.

Machu Picchu was further excavated in 1915 by Bingham, in 1934 by the Peruvian archaeologist Luis E. Valcarcel, and in 1940–41 by Paul Fejos. Additional discoveries throughout the Cordillera de Vilcabamba have shown that Machu Picchu was one of a series of pucaras (fortified sites), tambos (travelers’ barracks, or inns), and signal towers along the extensive Inca foot highway.

The dwellings at Machu Picchu were probably built and occupied from the mid-15th to the early or mid-16th century. Machu Picchu’s construction style and other evidence suggest that it was a palace complex of the ruler Pachacuti Inca Yupanqui (reigned c. 1438–71). Several dozen skeletons were excavated there in 1912, and, because most of those were initially identified as female, Bingham suggested that Machu Picchu was a sanctuary for the Virgins of the Sun (the Chosen Women), an elite Inca group. Technology at the turn of the 21st-century, however, identified a significant proportion of males and a great diversity in physical types. Both skeletal and material remains now suggest to scholars that Machu Picchu served as a royal retreat. The reason for the site’s abandonment is also unknown, but lack of water may have been a factor.

The high level of preservation and the general layout of the ruin are remarkable. Its southern, eastern, and western portions are surrounded by dozens of stepped agricultural terraces formerly watered by an aqueduct system. Some of those terraces were still being used by local Indians when Bingham arrived in 1911. Walkways and thousands of steps, consisting of stone blocks as well as footholds carved into underlying rock, connect the plazas, the residential areas, the terraces, the cemetery, and the major buildings. The Main Plaza, partly divided by wide terraces, is at the north-central end of the site. At the southeastern end is the only formal entrance, which leads to the Inca Trail.

Few of Machu Picchu’s white granite structures have stonework as highly refined as that found in Cuzco, but several are worthy of note. In the southern part of the ruin is the Sacred Rock, also known as the Temple of the Sun (it was called the Mausoleum by Bingham). It centres on an inclined rock mass with a small grotto; walls of cut stone fill in some of its irregular features. Rising above the rock is the horseshoe-shaped enclosure known as the Military Tower. In the western part of Machu Picchu is the temple district, also known as the Acropolis. The Temple of the Three Windows is a hall 35 feet (10.6 metres) long and 14 feet (4.2 metres) wide with three trapezoidal windows (the largest known in Inca architecture) on one wall, which is built of polygonal stones. It stands near the southwestern corner of the Main Plaza. Also near the Main Plaza is the Intihuatana (Hitching Post of the Sun), a uniquely preserved ceremonial sundial consisting of a wide pillar and pedestal that were carved as a single unit and stand 6 feet (1.8 metres) tall. In 2000 this feature was damaged during the filming of a beer commercial. The Princess’s Palace is a bi-level structure of highly crafted stonework that probably housed a member of the Inca nobility. The Palace of the Inca is a complex of rooms with niched walls and a courtyard. At the other end of Machu Picchu, another path leads to the famous Inca Bridge, a rope structure that crosses the Urubamba River. Many other ruined cities—like that atop the dark peak of Huayna Picchu, which is accessible by a lengthy, precipitous stairway and trail—were built in the region; Machu Picchu is only the most extensively excavated of these.

Machu Picchu is the most economically important tourist attraction in Peru, bringing in visitors from around the world. For this reason the Peruvian government wishes to repatriate the materials taken by Bingham to Yale. The ruins are commonly reached in a day trip from Cuzco by first taking a narrow-gauge railway and then ascending nearly 1,640 feet (500 metres) from the Urubamba River valley on a serpentine road. Smaller numbers of visitors arrive by hiking the Inca Trail. The portion of the trail from the “km 88” train stop to Machu Picchu is normally hiked in three to six days. It is composed of several thousand stone-cut steps, numerous high retaining walls, tunnels, and other feats of classical engineering; the route traverses a wide range of elevations between about 8,530 and 13,780 feet (2,600 and 4,200 metres), and it is lined with Inca ruins of various types and sizes. At Machu Picchu there is a hotel with a restaurant, and thermal baths are at the nearby village of Aguas Calientes. The Inca Bridge and other parts of Machu Picchu were damaged by a forest fire in August 1997, but restoration was begun immediately afterward. Concern for the damage caused by tourism was heightened by discussion of the building of a cable-car link to the site.

Best-Places-to-Visit-in-Peru-Machu-Picchu.jpg


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#752 2020-08-04 00:49:36

ganesh
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Re: Miscellany

630) Nose

Nose, the prominent structure between the eyes that serves as the entrance to the respiratory tract and contains the olfactory organ. It provides air for respiration, serves the sense of smell, conditions the air by filtering, warming, and moistening it, and cleans itself of foreign debris extracted from inhalations.

The nose has two cavities, separated from one another by a wall of cartilage called the septum. The external openings are known as nares or nostrils. The roof of the mouth and the floor of the nose are formed by the palatine bone, the mouth part of which is commonly called the hard palate; a flap of tissue, the soft palate, extends back into the nasopharynx, the nasal portion of the throat, and during swallowing is pressed upward, thus closing off the nasopharynx so that food is not lodged in the back of the nose.

The shape of the nasal cavity is complex. The forward section, within and above each nostril, is called the vestibule. Behind the vestibule and along each outer wall are three elevations, running generally from front to rear. Each elevation, called a nasal concha or turbinate, hangs over an air passage. Beside and above the uppermost concha is the olfactory region of the nasal cavity. The rest of the cavity is the respiratory portion. The respiratory area is lined with a moist mucous membrane with fine hairlike projections known as cilia, which serve to collect debris. Mucus from cells in the membrane wall also helps to trap particles of dust, carbon, soot, and bacteria. Sinus cavities are located in the bony skull on both sides of the nose.

In the olfactory (smelling) portion of the nose, most of the lining is mucous membrane. A small segment of the lining contains the nerve cells that are the actual sensory organs. Fibres, called dendrites, which project from the nerve cells into the nasal cavity, are covered only by a thin layer of moisture. The moisture dissolves microscopic particles that the air has carried into the nose from odour-emitting substances, and the particles dissolved in the fluid stimulate the olfactory nerve cells chemically.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#753 2020-08-05 00:59:07

ganesh
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Re: Miscellany

631) Paralysis

Paralysis, also called palsy, loss or impairment of voluntary muscular movement caused by structural abnormalities of nervous or muscular tissue or by metabolic disturbances in neuromuscular function. Paralysis can affect the legs and lower part of the body (paraplegia) or both arms and both legs (quadriplegia). Sometimes the muscles of the lower face, arm, and leg on only one side of the body are involved (hemiplegia).

Most diseases that cause paralysis can be divided into two main groups depending on whether they entail structural alterations in nervous or muscular tissue, or lead to metabolic disturbances in neuromuscular function. Some act systemically and affect one of the three elements in the motor system (upper neuron, lower neuron, or muscle) more or less extensively and exclusively. More often, however, one element or neighbouring portions of two of the three elements are involved over a limited extent by a single focal lesion.

Paralysis From Nervous Tissue Damage

The most common cause of hemiplegia is damage to the corticospinal tracts and associated motor tracts in one hemisphere of the brain from obstruction (blood clot or thrombosis) or rupture (cerebral hemorrhage) of a major cerebral artery. Brain tumour is another but less common cause of hemiplegia and increases in severity gradually over a period of weeks or months. When the lesion is in the left hemisphere of a right-handed person, the resulting right hemiplegia is often associated with one of the various forms of aphasia, the inability to sound words, to write, or to read.

Bilateral hemiplegia with pseudobulbar palsy results from diffuse, bilateral brain diseases such as occurs in severe cerebral arteriosclerosis or cerebral vascular syphilis. The terms cerebral palsy and spastic diplegia refer to bilateral hemiplegia resulting from prenatal developmental brain defects or from injury to the brain at birth.

The spinal cord is rarely the site of vascular obstruction or hemorrhage. Common causes of damage to the corticospinal tracts in the cord include deformities of the spinal column from bone and joint disease and from fracture or dislocation of the spine, spinal cord tumours, multiple sclerosis, strokes, and a number of inflammatory and degenerative diseases associated with pernicious anemia. One of the most common causes of progressive spastic paraplegia in persons past middle age is spinal degenerative arthritis, in which an intervertebral disk protrudes into the lower cervical portion of the spinal canal.

Paralysis From Muscle Tissue Damage

Of the diseases that attack lower motor neurons and result in paralysis with muscular wasting, the most common are poliomyelitis and polyneuritis, the former affecting the cell bodies or the bulbar and spinal motor neurons and the latter affecting their peripheral processes. Bell palsy is a peripheral neuritis of unknown cause affecting the facial nerve and resulting in paralysis of all the muscles of one side of the face.

Diseases that result in paralysis through primary changes in muscle tissue are fewer than the above. Of the conditions belonging in this category, muscular dystrophy is one of the few that are apparently confined to the muscles. Muscular dystrophy is a hereditary disease that results in paralysis through primary changes in muscle tissue. It is characterized by progressive, symmetrical muscular weakness and atrophy. Pseudohypertrophic muscular dystrophy is a rare variety of the disease that begins before puberty, is more common in males, and usually progresses to severe disability within a few years. The other types of dystrophy, in general, begin in adolescence or young adulthood, equally affect males and females, and progress more slowly.

Paralysis From Metabolic Disease

Muscular weakness without structural alteration in nerve or muscle tissue may be a symptom of disturbances in metabolism arising from a wide variety of causes. Among such conditions are diseases of the endocrine glands, certain intoxications, and several metabolic defects. The most common example of a metabolic disorder in neuromuscular function of unknown cause is myasthenia gravis. Myasthenia gravis is characterized by muscular weakness, without atrophy, which may be mild or severe and either generalized or restricted to a few muscle groups. Muscles innervated by cranial nerves usually are affected. Weakness results from a localized defect in the chemical processes involved in the transmission of impulses from motor nerve endings to muscle fibres. Several medications, including neostigmine, may benefit individuals with the disease.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#754 2020-08-06 01:10:33

ganesh
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Re: Miscellany

632) Skull

Skull, skeletal framework of the head of vertebrates, composed of bones or cartilage, which form a unit that protects the brain and some sense organs. The upper jaw, but not the lower, is part of the skull. The human cranium, the part that contains the brain, is globular and relatively large in comparison with the face. In most other animals the facial portion of the skull, including the upper teeth and the nose, is larger than the cranium. In humans the skull is supported by the highest vertebra, called the atlas, permitting nodding motion. The atlas turns on the next-lower vertebra, the axis, to allow for side-to-side motion.

In humans the base of the cranium is the occipital bone, which has a central opening (foramen magnum) to admit the spinal cord. The parietal and temporal bones form the sides and uppermost portion of the dome of the cranium, and the frontal bone forms the forehead; the cranial floor consists of the sphenoid and ethmoid bones. The facial area includes the zygomatic, or malar, bones (cheekbones), which join with the temporal and maxillary bones to form the zygomatic arch below the eye socket; the palatine bone; and the maxillary, or upper jaw, bones. The nasal cavity is formed by the vomer and the nasal, lachrymal, and turbinate bones. In infants the sutures (joints) between the various skull elements are loose, but with age they fuse together. Many mammals, such as the dog, have a sagittal crest down the centre of the skull; this provides an extra attachment site for the temporal muscles, which close the jaws.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#755 2020-08-07 00:45:43

ganesh
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Re: Miscellany

633) Sinusitis

Sinusitis, acute or chronic inflammation of the mucosal lining of one or more paranasal sinuses (the cavities in the bones that adjoin the nose). Sinusitis commonly accompanies upper respiratory viral infections and in most cases requires no treatment. Purulent (pus-producing) sinusitis can occur, however, requiring treatment with antibiotics. Chronic cases caused by irritants in the environment or by impaired immune systems may require more extended treatment, including surgery.

The origin of acute sinus infection is much like that of ear infection. Normally the middle ear and the sinuses are sterile, but the adjacent mouth and nose have a varied bacterial flora. Under normal conditions, very small hairs called cilia move mucus along the lining of the nose and respiratory tract, keeping the sinuses clean. When ciliary function is damaged, infection can be established. Following a common cold, a decrease in ciliary function may permit bacteria to remain on the mucous membrane surfaces within the sinuses and to produce a purulent sinusitis. The organisms usually involved are Haemophilus influenzae, Streptococcus pneumoniae, Staphylococcus aureus, Streptococcus pyogenes, and many other penicillin-sensitive anaerobes. Common symptoms include facial pain, headache, and fever following previous upper respiratory viral illness. On physical examination, persons with sinusitis are usually found to have an elevation in body temperature, nasal discharge, and sinus tenderness. Diagnosis can be confirmed by X-rays of the sinuses and cultures of material obtained from within the sinuses.

Treatment of acute sinusitis is directed primarily at overcoming the infecting organism by the use of systemic antibiotics such as penicillin and at encouraging drainage of the sinuses by the use of vasoconstricting nose drops and inhalations. If the infection persists, the pus localized in any individual sinus may have to be removed by means of a minor surgical procedure known as lavage, in which the maxillary or sphenoidal sinuses are irrigated with water or a saline solution.

Chronic sinusitis may follow repeated or neglected attacks of acute sinusitis, particularly if there is impaired breathing or drainage due to nasal polyps or obstructed sinus openings. It may also be caused by allergy to agents in the environment, such as fungi or pollen. The symptoms of chronic sinusitis are a tendency to colds, purulent nasal discharge, obstructed breathing, loss of smell, and sometimes headache. Pain is not a feature of chronic sinusitis. If antibiotic therapy or repeated lavage do not alleviate the condition, steroidal medications may be given to relieve swelling and antihistamines to relieve allergic reactions. In severe cases endoscopic surgery may be necessary to remove obstructions.

sinus.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#756 2020-08-08 01:00:46

ganesh
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Re: Miscellany

634) Sedimentary Rocks

Sedimentary rocks are one of three main types of rocks, along with igneous and metamorphic. They are formed on or near the Earth’s surface from the compression of ocean sediments or other processes.

Sedimentary rocks are formed on or near the Earth’s surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth. The most important geological processes that lead to the creation of sedimentary rocks are erosion, weathering, dissolution, precipitation, and lithification.

Erosion and weathering include the effects of wind and rain, which slowly break down large rocks into smaller ones. Erosion and weathering transform boulders and even mountains into sediments, such as sand or mud. Dissolution is a form of weathering—chemical weathering. With this process, water that is slightly acidic slowly wears away stone. These three processes create the raw materials for new, sedimentary rocks.

Precipitation and lithification are processes that build new rocks or minerals. Precipitation is the formation of rocks and minerals from chemicals that precipitate from water. For example, as a lake dries up over many thousands of years, it leaves behind mineral deposits; this is what happened in California’s Death Valley. Finally, lithification is the process by which clay, sand, and other sediments on the bottom of the ocean or other bodies of water are slowly compacted into rocks from the weight of overlying sediments.

Sedimentary rocks can be organized into two categories. The first is detrital rock, which comes from the erosion and accumulation of rock fragments, sediment, or other materials—categorized in total as detritus, or debris. The other is chemical rock, produced from the dissolution and precipitation of minerals.

Detritus can be either organic or inorganic. Organic detrital rocks form when parts of plants and animals decay in the ground, leaving behind biological material that is compressed and becomes rock. Coal is a sedimentary rock formed over millions of years from compressed plants. Inorganic detrital rocks, on the other hand, are formed from broken up pieces of other rocks, not from living things. These rocks are often called clastic sedimentary rocks. One of the best-known clastic sedimentary rocks is sandstone. Sandstone is formed from layers of sandy sediment that is compacted and lithified.

Chemical sedimentary rocks can be found in many places, from the ocean to deserts to caves. For instance, most limestone forms at the bottom of the ocean from the precipitation of calcium carbonate and the remains of marine animals with shells. If limestone is found on land, it can be assumed that the area used to be under water. Cave formations are also sedimentary rocks, but they are produced very differently. Stalagmites and stalactites form when water passes through bedrock and picks up calcium and carbonate ions. When the chemical-rich water makes its way into a cave, the water evaporates and leaves behind calcium carbonate on the ceiling, forming a stalactite, or on the floor of the cave, creating a stalagmite.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#757 2020-08-09 00:46:42

ganesh
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Re: Miscellany

635) Igneous Rocks

Igneous rocks are one of three main types of rocks (along with sedimentary and metamorphic), and they include both intrusive and extrusive rocks.

Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust. All magma develops underground, in the lower crust or upper mantle, because of the intense heat there.

Igneous rocks can have many different compositions, depending on the magma they cool from. They can also look different based on their cooling conditions. For example, two rocks from identical magma can become either rhyolite or granite, depending on whether they cool quickly or slowly.

The two main categories of igneous rocks are extrusive and intrusive. Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet.

When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly. Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth. These fine-grained rocks are known as aphaniti - from a Greek word meaning “invisible.” They are given this name because the crystals that form within them are so small that they can be seen only with a microscope. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian. There are many other kinds of extrusive igneous rocks. For example, Pele’s hair is long, extremely thin strands of volcanic glass, while pahoehoe is smooth lava that forms shiny, rounded piles.

Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface. They have large crystals that are usually visible without a microscope. This surface is known as a phaneritic texture. Perhaps the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called pegmatite, found often in the U.S. state of Maine. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#758 2020-08-10 01:28:32

ganesh
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Re: Miscellany

636) Metamorphic Rocks

Metamorphic rocks start as one type of rock an - with pressure, heat, and time - gradually change into a new type of rock.

The term “metamorphosis” is most often used in reference to the process of a caterpillar changing into a butterfly. However, the word “metamorphosis” is a broad term that indicates a change from one thing to another. Even rocks, a seemingly constant substance, can change into a new type of rock. Rocks that undergo a change to form a new rock are referred to as metamorphic rocks.

In the rock cycle, there are three different types of rocks: sedimentary, igneous, and metamorphic. Sedimentary and igneous rocks began as something other than rock. Sedimentary rocks were originally sediments, which were compacted under high pressure. Igneous rocks formed when liquid magma or lav - magma that has emerged onto the surface of the Earth - cooled and hardened. A metamorphic rock, on the other hand, began as a rock - either a sedimentary, igneous, or even a different sort of metamorphic rock. Then, due to various conditions within the Earth, the existing rock was changed into a new kind of metamorphic rock.

The conditions required to form a metamorphic rock are very specific. The existing rock must be exposed to high heat, high pressure, or to a hot, mineral-rich fluid. Usually, all three of these circumstances are met. These conditions are most often found either deep in Earth’s crust or at plate boundaries where tectonic plates collide. In order to create metamorphic rock, it is vital that the existing rock remain solid and not melt. If there is too much heat or pressure, the rock will melt and become magma. This will result in the formation of an igneous rock, not a metamorphic rock.

Consider how granite changes form. Granite is an igneous rock that forms when magma cools relatively slowly underground. It is usually composed primarily of the minerals quartz, feldspar, and mica. When granite is subjected to intense heat and pressure, it changes into a metamorphic rock called gneiss.
Slate is another common metamorphic rock that forms from shale. Limestone, a sedimentary rock, will change into the metamorphic rock marble if the right conditions are met.

Although metamorphic rocks typically form deep in the planet’s crust, they are often exposed on the surface of the Earth. This happens due to geologic uplift and the erosion of the rock and soil above them. At the surface, metamorphic rocks will be exposed to weathering processes and may break down into sediment. These sediments could then be compressed to form sedimentary rocks, which would start the entire cycle anew.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#759 2020-08-11 01:18:10

ganesh
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Re: Miscellany

637) Cavities/tooth decay

Overview

Cavities are permanently damaged areas in the hard surface of your teeth that develop into tiny openings or holes. Cavities, also called tooth decay or caries, are caused by a combination of factors, including bacteria in your mouth, frequent snacking, sipping sugary drinks and not cleaning your teeth well.

Cavities and tooth decay are among the world's most common health problems. They're especially common in children, teenagers and older adults. But anyone who has teeth can get cavities, including infants.

If cavities aren't treated, they get larger and affect deeper layers of your teeth. They can lead to a severe toothache, infection and tooth loss. Regular dental visits and good brushing and flossing habits are your best protection against cavities and tooth decay.

Symptoms

The signs and symptoms of cavities vary, depending on their extent and location. When a cavity is just beginning, you may not have any symptoms at all. As the decay gets larger, it may cause signs and symptoms such as:

•    Toothache, spontaneous pain or pain that occurs without any apparent cause
•    Tooth sensitivity
•    Mild to sharp pain when eating or drinking something sweet, hot or cold
•    Visible holes or pits in your teeth
•    Brown, black or white staining on any surface of a tooth
•    Pain when you bite down

When to see a dentist

You may not be aware that a cavity is forming. That's why it's important to have regular dental checkups and cleanings, even when your mouth feels fine. However, if you experience a toothache or mouth pain, see your dentist as soon as possible.

Causes

Cavities are caused by tooth decay — a process that occurs over time. Here's how tooth decay develops:
•    Plaque forms. Dental plaque is a clear sticky film that coats your teeth. It's due to eating a lot of sugars and starches and not cleaning your teeth well. When sugars and starches aren't cleaned off your teeth, bacteria quickly begin feeding on them and form plaque. Plaque that stays on your teeth can harden under or above your gum line into tartar (calculus). Tartar makes plaque more difficult to remove and creates a shield for bacteria.
•    Plaque attacks. The acids in plaque remove minerals in your tooth's hard, outer enamel. This erosion causes tiny openings or holes in the enamel — the first stage of cavities. Once areas of enamel are worn away, the bacteria and acid can reach the next layer of your teeth, called dentin. This layer is softer than enamel and less resistant to acid. Dentin has tiny tubes that directly communicate with the nerve of the tooth causing sensitivity.
•    Destruction continues. As tooth decay develops, the bacteria and acid continue their march through your teeth, moving next to the inner tooth material (pulp) that contains nerves and blood vessels. The pulp becomes swollen and irritated from the bacteria. Because there is no place for the swelling to expand inside of a tooth, the nerve becomes pressed, causing pain. Discomfort can even extend outside of the tooth root to the bone.

Risk factors

Everyone who has teeth is at risk of getting cavities, but the following factors can increase risk:

•    Tooth location. Decay most often occurs in your back teeth (molars and premolars). These teeth have lots of grooves, pits and crannies, and multiple roots that can collect food particles. As a result, they're harder to keep clean than your smoother, easy-to-reach front teeth.
•    Certain foods and drinks. Foods that cling to your teeth for a long time — such as milk, ice cream, honey, sugar, soda, dried fruit, cake, cookies, hard candy and mints, dry cereal, and chips — are more likely to cause decay than foods that are easily washed away by saliva.
•    Frequent snacking or sipping. When you steadily snack or sip sugary drinks, you give mouth bacteria more fuel to produce acids that attack your teeth and wear them down. And sipping soda or other acidic drinks throughout the day helps create a continual acid bath over your teeth.
•    Bedtime infant feeding. When babies are given bedtime bottles filled with milk, formula, juice or other sugar-containing liquids, these beverages remain on their teeth for hours while they sleep, feeding decay-causing bacteria. This damage is often called baby bottle tooth decay. Similar damage can occur when toddlers wander around drinking from a sippy cup filled with these beverages.
•    Inadequate brushing. If you don't clean your teeth soon after eating and drinking, plaque forms quickly and the first stages of decay can begin.
•    Not getting enough fluoride. Fluoride, a naturally occurring mineral, helps prevent cavities and can even reverse the earliest stages of tooth damage. Because of its benefits for teeth, fluoride is added to many public water supplies. It's also a common ingredient in toothpaste and mouth rinses. But bottled water usually does not contain fluoride.
•    Younger or older age. In the United States, cavities are common in very young children and teenagers. Older adults also are at higher risk. Over time, teeth can wear down and gums may recede, making teeth more vulnerable to root decay. Older adults also may use more medications that reduce saliva flow, increasing the risk of tooth decay.
•    Dry mouth. Dry mouth is caused by a lack of saliva, which helps prevent tooth decay by washing away food and plaque from your teeth. Substances found in saliva also help counter the acid produced by bacteria. Certain medications, some medical conditions, radiation to your head or neck, or certain chemotherapy drugs can increase your risk of cavities by reducing saliva production.
•    Worn fillings or dental devices. Over the years, dental fillings can weaken, begin to break down or develop rough edges. This allows plaque to build up more easily and makes it harder to remove. Dental devices can stop fitting well, allowing decay to begin underneath them.
•    Heartburn. Heartburn or gastroesophageal reflux disease (GERD) can cause stomach acid to flow into your mouth (reflux), wearing away the enamel of your teeth and causing significant tooth damage. This exposes more of the dentin to attack by bacteria, creating tooth decay. Your dentist may recommend that you consult your doctor to see if gastric reflux is the cause of your enamel loss.
•    Eating disorders. Anorexia and bulimia can lead to significant tooth erosion and cavities. Stomach acid from repeated vomiting (purging) washes over the teeth and begins dissolving the enamel. Eating disorders also can interfere with saliva production.

Complications

Cavities and tooth decay are so common that you may not take them seriously. And you may think that it doesn't matter if children get cavities in their baby teeth. However, cavities and tooth decay can have serious and lasting complications, even for children who don't have their permanent teeth yet.
Complications of cavities may include:

•    Pain
•    Tooth abscess
•    Swelling or pus around a tooth
•    Damage or broken teeth
•    Chewing problems
•    Positioning shifts of teeth after tooth loss

When cavities and decay become severe, you may have:

•    Pain that interferes with daily living
•    Weight loss or nutrition problems from painful or difficult eating or chewing
•    Tooth loss, which may affect your appearance, as well as your confidence and self-esteem
•    In rare cases, a tooth abscess — a pocket of pus that's caused by bacterial infection — which can lead to more serious or even life-threatening infections

Prevention

Good oral and dental hygiene can help you avoid cavities and tooth decay. Here are some tips to help prevent cavities. Ask your dentist which tips are best for you.

•    Brush with fluoride toothpaste after eating or drinking. Brush your teeth at least twice a day and ideally after every meal, using fluoride-containing toothpaste. To clean between your teeth, floss or use an interdental cleaner.
•    Rinse your mouth. If your dentist feels you have a high risk of developing cavities, he or she may recommend that you use a mouth rinse with fluoride.
•    Visit your dentist regularly. Get professional teeth cleanings and regular oral exams, which can help prevent problems or spot them early. Your dentist can recommend a schedule that's best for you.
•    Consider dental sealants. A sealant is a protective plastic coating applied to the chewing surface of back teeth. It seals off grooves and crannies that tend to collect food, protecting tooth enamel from plaque and acid. The Centers for Disease Control and Prevention (CDC) recommends sealants for all school-age children. Sealants may last for several years before they need to be replaced, but they need to be checked regularly.
•    Drink some tap water. Most public water supplies have added fluoride, which can help reduce tooth decay significantly. If you drink only bottled water that doesn't contain fluoride, you'll miss out on fluoride benefits.
•    Avoid frequent snacking and sipping. Whenever you eat or drink beverages other than water, you help your mouth bacteria create acids that can destroy tooth enamel. If you snack or drink throughout the day, your teeth are under constant attack.
•    Eat tooth-healthy foods. Some foods and beverages are better for your teeth than others. Avoid foods that get stuck in grooves and pits of your teeth for long periods, or brush soon after eating them. However, foods such as fresh fruits and vegetables increase saliva flow, and unsweetened coffee, tea and sugar-free gum help wash away food particles.
•    Consider fluoride treatments. Your dentist may recommend periodic fluoride treatments, especially if you aren't getting enough fluoride through fluoridated drinking water and other sources. He or she may also recommend custom trays that fit over your teeth for application of prescription fluoride if your risk of tooth decay is very high.
•    Ask about antibacterial treatments. If you're especially vulnerable to tooth decay — for example, because of a medical condition — your dentist may recommend special antibacterial mouth rinses or other treatments to help cut down on harmful bacteria in your mouth.
•    Combined treatments. Chewing xylitol-based gum along with prescription fluoride and an antibacterial rinse can help reduce the risk of cavities.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#760 2020-08-12 01:05:26

ganesh
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Posts: 31,668

Re: Miscellany

638) Monteverde Cloud Forest Reserve

The Monteverde Cloud Forest Reserve (Reserva Biológica Bosque Nuboso Monteverde) is a Costa Rican reserve located along the Cordillera de Tilarán within the Puntarenas and Alajuela provinces. Named after the nearby town of Monteverde and founded in 1972, the Reserve consists of over 10,500 hectares (26,000 acres) of cloud forest, the reserve is visited by roughly 70,000 visitors a year. The Reserve consists of 6 ecological zones, 90% of which are virgin forest. A high biodiversity, consisting of over 2,500 plant species (including the most orchid species in a single place), 100 species of mammals, 400 bird species, 120 reptilian and amphibian species, and thousands of insects, has drawn both scientists and tourists alike.

History

The 1950s, First Farmers in the Area Arrive

In 1951, several dozen Quakers (from 11 families) from Alabama seeking to live as farmers moved to and purchased land in Costa Rica. This was primarily to avoid the Korean War draft, an obligation which contradicted Quaker pacifist ideology. They chose Costa Rica because it had just abolished its army just three years earlier. It was the Quakers who named the place ‘Monteverde’ (“Green Mountain”), for round the year green plants.

The 1960s, Biologists Take Note

Biologists began to take note of Monteverde in the 1960s. Despite the lack of infrastructure and shelter with which to conduct scientific research, these original biologists not only have been continuously documenting, but continue to live in, Monteverde.

In 1968, Dr. Joseph Tosi, who worked for the Tropical Science Center, a foundation for tropical conservation, accompanied Dr. Leslie Holdridge on a journey to Monteverde. The visit was part of a study of the northern region of Costa Rica, requested by the government's National Planning Office. There, they met Mr. Hubert Mendenhall, leader of the Quaker community at the time, who took them to see the primary forests that surrounded the community.

At the end of their visit, Holdridge and Tosi recommended to the Quaker community that the native forests be preserved as much as possible in order to protect their water sources and, given the strong winds that swept though the area, to use the forests as windbreakers to protect their fields and homes.

The 1970s, Conservation Efforts Mount

The years went by, and in 1972 a young graduate student, George Powell, visited the Tropical Science Center (TSC) in San José. He lived in Monteverde while doing doctoral research on the birds of the area, and he found that the fauna and habitats were ideal for research purposes.

Amazed by the extraordinary biological richness of the cloud forests, including the habitat of the endemic golden toad, and alarmed by the depredation caused by hunters and land squatters, Powell received a promise from the Guacimal Land Company that they would donate 328 hectares (810 acres) of land, if he could form or find a civic association that would sponsor him in taking over the property. George used his personal funds to buy out several of the squatters, hoping to establish a small biological preserve in the region.

At the time, there were few national parks in Costa Rica, and the TSC had a program to create private preserves for research and biological education, where each preserve would represent a different ecological area of the country. Immediately, the TSC became interested in Powell's offer and started the process that led to the acquisition of the 328 hectares (810 acres) in April 1973. The cost of the farm was a symbolic 1 colón (less than USD $1).

Along with Powell, Costa Rican biologist Adelaida Chaverri and wildlife specialist Christopher Vaughn promoted the establishment of this private preserve, at the time a less-than-popular idea. In fact, Adelaida Chaverri became one of the sponsors, along with Dr. Joseph Tosi and other TSC members, of what is today the Monteverde Cloud Forest Reserve. They provided continuity to the interest expressed by Dr. George Powell when he obtained the donation of the first piece of land for the Reserve.

In 1975, 431 visitors came to the budding preserve, most of the scientists and bird watchers. Two years later, there still was no lodging available for visitors to the community, but Mrs. Wood, a local Quaker, started a small bed-and-breakfast in her own home, where occasional visitors would stay overnight. In addition to this, overnight the 431 visitors came to the budding preserve

The 1980s, First Days of Tourism

The number of foreign visitors increased from 2,700 in 1980 to more than 40,000 in 1991. The preserve increased in size during these years, but its best-known endemic species, the Golden Toad, as well as 40% of Monteverde's amphibian population became extinct, due to a deadly fungal pandemic Chytridiomycosis.

The 1990s through Today

Currently the Reserve is visited by more than 70,000 people each year, who are eager to get to know the biodiversity found within.

Biology

Flora

Epiphytes, which make up 29% of the flora with 878 species, are the richest life form among species of flora in Monteverde. The Monteverde region is also known as the site with the largest number of orchids in the world. The total number of known species surpasses 500, and of these, 34 species discovered in the Reserve were new to science at the time of their discovery.

Fauna

Herpetofauna of the area is worth noting, with 161 species of amphibians and reptiles. Monteverde is known worldwide as the habitat of the golden toad (Bufo periglenes), a species that disappeared in 1989.

91 (21%) of Monteverde's bird species are long distance migratory birds, which reproduce in North America and pass through Monteverde during their migration or spend the winter in the area. Three of these species, the swallow-tailed kite (Elanoides forficatus), the piratic flycatcher (Legatus leucophaius), and the yellow-green vireo (Vireo flavoviridis), reproduce in Monteverde and migrate to South America during their non-reproductive phase.

The resplendent quetzal (Pharomachrus mocinno) moves seasonally from high elevation nesting sites to lower elevations on both sides of the Continental Divide. The beginning of the migration of the three-wattled bellbird (Procnias tricarunculata) is similar to that of the quetzal, with reproduction occurring close to the Continental Divide, from March to June, and followed by a post-reproductive move downhill on the Pacific slope during the months of August and September.

The majority of the bird species in Monteverde are primarily insectivores, given that the plants in the region offer a wide variety of fruit. The epiphytes are important resources both for frugivores and insectivores in Monteverde. On a global scale, the cloud forests of Monteverde are home to ten species of birds that are considered to be endangered by the organization Birdlife International, due to their very restricted habitat worldwide.

The mammals of Monteverde include representatives from both North and South America as endemic species. The mammalian fauna of the region includes six species of marsupials, three muskrats, at least 58 bats, three primates, seven edentates, two rabbits, one ground hog, three species of squirrels, one species of spiny mouse, at least 15 species of long-tailed rats and mice (family Muridae); one species of porcupine, one species of agouti, one paca, two canids, five mustelids, four procyonids, six felines, two species of wild pigs, two species of deer, and one tapir.

Facilities

Currently, the Reserve has bus service that runs five times per day from Monteverde and Santa Elena; it also has a lodge that hosts up to 47 visitors, a small restaurant, a gift shop, and the Monteverde Nature Center information center, serpentarium, frog pond, bat jungle, and butterfly gardens. There are well maintained trails that run through the reserve, as well as suspension bridges and zip-lines. Horseback tours are sometimes arranged.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#761 2020-08-13 00:38:16

ganesh
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Re: Miscellany

639) Computer program

Computer program, detailed plan or procedure for solving a problem with a computer; more specifically, an unambiguous, ordered sequence of computational instructions necessary to achieve such a solution. The distinction between computer programs and equipment is often made by referring to the former as software and the latter as hardware.

Programs stored in the memory of a computer enable the computer to perform a variety of tasks in sequence or even intermittently. The idea of an internally stored program was introduced in the late 1940s by the Hungarian-born mathematician John von Neumann. The first digital computer designed with internal programming capacity was the EDVAC (acronym for Electronic Discrete Variable Automatic Computer), constructed in 1949.

A program is prepared by first formulating a task and then expressing it in an appropriate computer language, presumably one suited to the application. The specification thus rendered is translated, commonly in several stages, into a coded program directly executable by the computer on which the task is to be run. The coded program is said to be in machine language, while languages suitable for original formulation are called problem-oriented languages. A wide array of problem-oriented languages has been developed, some of the principal ones being COBOL (Common Business-Oriented Language), FORTRAN (Formula Translation), BASIC (Beginner’s All-Purpose Symbolic Instruction Code), and Pascal.

Computers are supplied with various programs designed primarily to assist the user to run jobs or optimize system performance. This collection of programs, called the operating system, is as important to the operation of a computer system as its hardware. Current technology makes it possible to build in some operating characteristics as fixed programs (introduced by customer orders) into a computer’s central processing unit at the time of manufacture. Relative to user programs, the operating system may be in control during execution, as when a time-sharing (q.v.) monitor suspends one program and activates another, or at the time a user program is initiated or terminated, as when a scheduling program determines which user program is to be executed next. Certain operating-system programs, however, may operate as independent units to facilitate the programming process. These include translators (either assemblers or compilers), which transform an entire program from one language to another; interpreters, which execute a program sequentially, translating at each step; and debuggers, which execute a program piecemeal and monitor various circumstances, enabling the programmer to check whether the operation of the program is correct or not.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#762 2020-08-14 00:59:11

ganesh
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Re: Miscellany

640) Anemia

Anemia, also spelled anaemia, condition in which the red blood cells (erythrocytes) are reduced in number or volume or are deficient in hemoglobin, their oxygen-carrying pigment. The most noticeable outward symptom of anemia is usually pallor of the skin, mucous membranes, and nail beds. Symptoms of tissue oxygen deficiency include pulsating noises in the ear, dizziness, fainting, and shortness of breath. Compensatory action of the heart may lead to its enlargement and to a rapid pulse rate. There are close to 100 different varieties of anemia, distinguished by the cause and by the size and hemoglobin content of the abnormal cells.

Anemia results when the destruction of red blood cells exceeds production, production of red blood cells is reduced, or acute or chronic blood loss occurs. Increased destruction of red blood cells (hemolysis) may be caused by hereditary cell defects, as in sickle cell anemia, hereditary spherocytosis, or glucose-6-phosphate dehydrogenase deficiency. Destruction also may be caused by exposure to hemolytic chemicals (substances causing the release of hemoglobin from the red cells) such as the antibiotic drug sulfanilamide, the antimalarial drug primaquine, or naphthalene (mothballs), or it may be caused by development of antibodies against the red blood cells, as in erythroblastosis fetalis. Reduced production of red cells may be caused by disorders of the bone marrow, as in leukemia and aplastic anemia, or by deficiency of one or more of the nutrients, notably vitamin B12, folic acid (folate), and iron, that are necessary for the synthesis of red cells. Lower production may also be caused by deficiency of certain hormones or inhibition of the red-cell-forming processes by certain drugs or by toxins produced by disease, particularly chronic infection, cancer, and kidney failure.

Structurally, the anemias generally fall into the following types: (1) macrocytic anemia, characterized by larger-than-normal red cells (e.g., pernicious anemia), (2) normocytic anemia, characterized by a decrease in the number of red cells, which are otherwise relatively normal (e.g., anemia caused by sudden blood loss, as in a bleeding peptic ulcer, most cases of hemophilia, and purpura), (3) simple microcytic anemia, characterized by smaller-than-normal red cells (encountered in cases of chronic inflammatory conditions and in renal disease), and (4) microcytic hypochromic anemia, characterized by a reduction in red-cell size and hemoglobin concentration (frequently associated with iron-deficiency anemia but also seen in thalassemia).

The treatment of anemia varies greatly, depending on the diagnosis. It includes supplying the missing nutrients in the deficiency anemias, detecting and removing toxic factors, improving the underlying disorder with drugs and other forms of therapy, decreasing the extent of blood destruction by methods that include surgery (e.g., splenectomy), or restoring blood volume with transfusion.

Aplastic anemia

Aplastic anemia, disease in which the bone marrow fails to produce an adequate number of blood cells. There may be a lack of all cell types—white blood cells (leukocytes), red blood cells (erythrocytes), and platelets—resulting in a form of the disease called pancytopenia, or there may be a lack of one or more cell types. Rarely, the disease may be congenital (Fanconi anemia); more commonly, it is acquired by exposure to certain drugs (e.g., the antibiotic chloramphenicol) or chemicals (e.g., benzene) or to ionizing radiation. About half of all cases are idiopathic (cause unknown). Aplastic anemia is most common in persons 15 to 30 years of age. Onset of the disease may be abrupt, becoming quickly severe and possibly fatal; more commonly, it is insidious, running a chronic course of several years. Symptoms of chronic aplastic anemia include weakness and fatigue in the early stages, followed by shortness of breath, headache, fever, and pounding heart. There is usually a waxy pallor, and hemorrhages occur in the mucous membranes, skin, and other organs. If white blood cells (specifically, neutrophils) are lacking, resistance to infection is much lowered and infection becomes the major cause of death. When platelets are very low, bleeding may be severe.

The treatment of choice for severe aplastic anemia is bone marrow transplantation, provided a compatible donor can be found. If transplantation is not practical, treatment involves avoidance of the toxic agent if known, supportive care (administration of fluids, glucose, and proteins, often intravenously), transfusions of blood components, and administration of antibiotics. Spontaneous recovery occurs occasionally.

Pernicious anemia

Pernicious anemia, disease in which the production of red blood cells (erythrocytes) is impaired as a result of the body’s inability to absorb vitamin B12, which is obtained in the diet and is necessary for red blood cells to mature properly in the bone marrow. Pernicious anemia is one of many types of anemia, a disease marked by a reduction in red blood cells or in the oxygen-carrying substance hemoglobin found in those cells.

Pernicious anemia occurs most often in persons over age 30, although a juvenile form of the disease does occur, usually in children younger than 3 years old. The disease shows a familial tendency and is more common in individuals of northern European descent. Pernicious anemia is in most cases associated with an inflammation of the stomach called autoimmune gastritis.

Pathophysiology

In pernicious anemia vitamin B12 is unavailable owing to a lack of intrinsic factor, a substance responsible for intestinal absorption of the vitamin. In a healthy person, intrinsic factor is produced by the parietal cells of the stomach, the cells that also secrete hydrochloric acid. Intrinsic factor forms a complex with dietary vitamin B12 in the stomach. The complex remains intact, preventing degradation of the vitamin by intestinal juices, until it reaches the ileum of the small intestine, where the vitamin is released and absorbed into the body.

When intrinsic factor is prevented from binding with vitamin B12 or when the parietal cells are unable to produce intrinsic factor, the vitamin is not absorbed and pernicious anemia results. This effect is thought to stem from an autoimmune reaction in which the malfunctioning immune system produces antibodies against intrinsic factor and against the parietal cells.

Without an adequate amount of vitamin B12, the body is unable to synthesize DNA properly. This in turn affects red blood cell production: the cells divide, but their nuclei remain immature. These cells, called megaloblasts, are for the most part destroyed in the bone marrow and are not released to the circulation. Some megaloblasts mature to become large red blood cells called macrocytes; they reach the circulation but function abnormally. A deficiency of white blood cells (leukopenia) and platelets (thrombocytopenia) in the blood may occur.

Symptoms

Symptoms of pernicious anemia include fatigue, weakness, waxy pallor, shortness of breath, rapid heartbeat, unsteady gait, smooth tongue, gastrointestinal disturbances, and neurological problems. Weight loss, depressed mood, and memory loss are common in affected persons. Levels of the substances homocysteine and methylmalonic acid typically are high in persons with pernicious anemia, while gastric secretions lack hydrochloric acid (achlorhydria). The anemia may become severe before the disorder is diagnosed, since the vitamin deficiency develops very gradually.

Treatment

Treatment involves a monthly intramuscular injection of vitamin B12 that must be continued for life. Most patients improve quickly, although neurological damage is seldom fully reversible and atrophy of the parietal cells and achlorhydria persist. Before the discovery of treatment in the 1920s, the modifier pernicious, although something of a misnomer today, was appropriate, since the disease was usually fatal.

Folic acid deficiency anemia

Folic acid deficiency anemia, also called folate deficiency anemia, type of anemia resulting from a deficient intake of the vitamin folic acid (folate). Folic acid, a B vitamin, is needed for the formation of heme, the pigmented, iron-containing portion of the hemoglobin in red blood cells (erythrocytes). A deficient intake of folic acid impairs the maturation of young red blood cells, which results in anemia. The disease also is characterized by leukopenia (a deficiency of white blood cells, or leukocytes), by thrombocytopenia (a deficiency of platelets), by ineffective blood formation in the bone marrow, and by progressive gastrointestinal symptoms, such as sore tongue, fissures at the corners of the mouth, diarrhea, inflammation of the pharynx or esophagus, and ulceration of the stomach and intestine. Folic acid deficiency develops over a period of several months and may result from a diet that is low or lacking in foods containing folic acid. The deficiency may also be brought on by poor absorption of folic acid in the intestine due to celiac disease or anticonvulsant drug therapy or by faulty metabolism of the vitamin in the liver due to cirrhosis. It can also occur in pregnant women and in persons with severe hemolytic anemia (dissolution of red blood cells by hemolysin). The oral administration of folic acid produces quick improvement in all symptoms; an adequate diet results in cure in cases due to simple malnutrition.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#763 2020-08-15 13:10:47

ganesh
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Re: Miscellany

641) Nightingale

Nightingale, any of several small Old World thrushes, belonging to the family Turdidae (order Passeriformes), renowned for their song. The name refers in particular to the Eurasian nightingale (Erithacus, or Luscinia, megarhynchos), a brown bird, 16 centimetres (6 1/2 inches) long, with a rufous tail. Its strong and varied song, in which crescendo effects are prominent, is uttered by day or night from perches in shrubbery.

The thrush nightingale, or sprosser (E. luscinia), is a closely related, somewhat more northerly species with slightly darker plumage. Its song lacks the crescendo.

The term nightingale is also applied to other birds with rich songs, such as members of the neotropical nightingale thrush group, the Chinese nightingale, and, in the West Indies, the mockingbird.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#764 2020-08-17 00:53:20

ganesh
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Re: Miscellany

642) Cuckoo

Cuckoo, any of numerous birds of the family Cuculidae (order Cuculiformes). The name usually designates some 60 arboreal members of the subfamilies Cuculinae and Phaenicophaeinae. In western Europe “cuckoo,” without modifiers, refers to the most common local form, elsewhere called the common, or European, cuckoo (Cuculus canorus). Many cuckoos have specialized names, such as ani, coua, coucal, guira, and roadrunner. Members of the subfamily Neomorphinae are called ground cuckoos.

The family Cuculidae is worldwide, found in temperate and tropical regions but is most diverse in the Old World tropics. Cuculids tend to be shy inhabitants of thick vegetation, more often heard than seen. Many species are named for the sounds they make—e.g., brain-fever bird (a hawk cuckoo, Cuculus varius), koel (Eudynamys scolopacea), and cuckoo itself, the latter two names being imitations of the bird’s song.

Cuculids range in length from about 16 cm (6.5 inches) in the glossy cuckoos (Chrysococcyx and Chalcites) to about 90 cm (36 inches) in the larger ground cuckoos. Most are coloured in drab grays and browns, but a few have striking patches of rufous (reddish) or white, and the glossy cuckoos are largely or partially shining emerald green. Some of the tropical cuckoos have strongly iridescent bluish plumage on their backs and wings. With the exception of a few strongly migratory species, most cuckoos are short-winged. All have long (sometimes extremely long), graduated tails, usually with the individual feathers tipped with white. The legs vary from medium to rather long (in the terrestrial forms) and the feet are zygodactyl; i.e., the outer toe is reversed, pointing backward. The bill is rather stout and somewhat downcurved.

The attribute for which the cuckoos are best known is the habit of brood parasitism, found in all of the Cuculinae and three species of Phaenicophaeinae. It consists of laying the eggs singly in the nests of certain other bird species to be incubated by the foster parents, who rear the young cuckoo. Among the 47 species of cuculines, various adaptations enhance the survival of the young cuckoo: egg mimicry, in which the cuckoo egg resembles that of the host, thus minimizing rejection by the host; removal of one or more host eggs by the adult cuckoo, reducing both the competition from host nestlings and the danger of recognition by the host that an egg has been added to the nest; and nest-mate ejection, in which the young cuckoo heaves from the nest the host’s eggs and nestlings. Some species of Cuculus resemble certain bird-eating hawks (Accipiter) in appearance and mannerisms, apparently frightening the potential host and allowing the cuckoo to approach the nest unmolested.

The nonparasitic phaenicophaeine cuckoos are represented in North America by the widespread yellow-billed and black-billed cuckoos (Coccyzus americanus and C. erythropthalmus) and the mangrove cuckoo (C. minor), which is restricted in the United States to coastal southern Florida (also found in the West Indies and Mexico to northern South America); they are represented in Central and South America by about 12 other species, some placed in the genera Piaya (squirrel cuckoos) and Saurothera (lizard cuckoos). The 13 Old World phaenicophaeine species are divided among nine genera.

The phaenicophaeine cuckoos build flimsy stick nests in low vegetation. Both parents share in incubation and feeding the young.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#765 2020-08-18 00:52:29

ganesh
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Re: Miscellany

643) Wasp

Wasp, any member of a group of insects in the order Hymenoptera, suborder Apocrita, some of which are stinging. Wasps are distinguished from the ants and bees of Apocrita by various behavioral and physical characteristics, particularly their possession of a slender, smooth body and legs with relatively few hairs. Wasps also generally are predatory or parasitic and have stingers with few barbs that can be removed easily from their victims. Similar to other members of Apocrita, wasps have a narrow petiole, or “waist,” which attaches the abdomen to the thorax.

Wasps have biting mouthparts and antennae with 12 or 13 segments. They are normally winged. In stinging species, only the females are provided with a formidable sting, which involves use of a modified ovipositor (egg-laying structure) for piercing and venom-producing glands. Adult wasps may feed on nectar and, in some species, on the secretions produced by larvae. Larvae of predatory wasp species typically feed on insects, while larvae of parasitic species feed on their hosts.

Wasps are subdivided into two groups: solitary wasps, which live alone, and social wasps, which live in colonies. Of the tens of thousands of species of wasps that have been described, the vast majority are solitary in habit. The social wasps are confined to about 1,000 species within the family Vespidae (superfamily Vespoidea) and include the hornets and yellow jackets (yellowjackets). They differ from other wasp families in having their wings folded longitudinally when at rest.

Solitary wasps are distributed in the superfamilies Chrysidoidea, Vespoidea, and Apoidea. Most species build isolated nests, which they provision with paralyzed insects or spiders. The female wasp deposits an egg in each cell of the nest, and the wasp larva hatching from that egg feeds to maturity upon the food with which its cell has been provisioned. The vast majority of solitary wasps nest in the ground, digging tunnels in the soil in which to lay their eggs. But the Sphecidae, or thread-waisted wasps (superfamily Apoidea), contain forms of more diverse habits, with some nesting in wood, pithy plant stems, or in nests made of mud. Spider wasps (Pompilidae) usually build nests in rotten wood or in rock crevices and provision them with spiders. The potter, or mason, wasps (subfamily Eumeninae) of the Vespidae build nests of mud, which are sometimes vaselike or juglike and may be found attached to twigs or other objects.

The social wasps within the family Vespidae are among the best-known species of wasps. Most of them belong to the subfamilies Vespinae or Polistinae. In their societies they have a caste system consisting of one or several queens, a few drones (males), and sterile females called workers. The queen, a fertilized female, begins the colony in the spring by building a small nest and laying eggs that hatch into workers. The latter enlarge the paperlike nest, which is composed of chewed dry plant material, usually wood, that has been mixed with saliva and regurgitated. The nest consists of one or more layers of cells that are arranged vertically with the openings downward. Depending on the species, the nest may be found in cavities in the soil, in tree trunks, or hanging from leaves, branches, or the eaves of buildings.

The most familiar social wasps in northern temperate regions are species of the genera Polistes, Vespa, and Vespula. Many are large and aggressive and are equipped with formidable stings. Some Vespula species are called yellow jackets owing to the black and yellow bands on their abdomen. Species of Vespa are called hornets, which are mostly black, with yellowish markings on the face, thorax, and the tip of the abdomen. The Asian giant hornet (Vespa mandarinia) is the largest known hornet in the world, with some workers growing to nearly 4 cm (1.6 inches) in body length and queens typically exceeding that size.

Four major groups of solitary wasps are parasitic and do not construct nests. These are the cuckoo wasps (family Chrysididae) in the superfamily Chrysidoidea, and the tiphiid wasps (family Tiphiidae), scoliid wasps (family Scoliidae), and velvet ants (family Mutillidae) in the superfamily Vespoidea. Cuckoo wasps are mostly brilliant metallic-green or -blue in colour and have intricate sculpturing on the exoskeleton. They lay their eggs in the nests of solitary bees or wasps. The larvae hatching from those eggs feed on the bee or wasp larvae or on the food provisioned by the latter’s parents. The velvet ants have bodies clothed with long thick hair of contrasting colours, often black and red. The females are wingless and antlike in appearance. Most of them are parasitic on the larvae and pupae of solitary bees and wasps. Most species of tiphiid and scoliid wasps parasitize beetle grubs that live in the soil.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#766 2020-08-19 01:10:34

ganesh
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Re: Miscellany

644) Radish

Radish is the common name for herbaceous plant, Raphanus sativus, of the mustard family (Brassicaceae), grown as an annual or biennial, and characterized by a large, fleshy root and white to purple hermaphrodite flowers clustered in a terminal raceme. The term turnip also refers to this edible, succulent, pungent root, which is commercially popular.

As a root vegetable, the radish has been cultivated since pre-Roman times. Its sharp taste offers a unique culinary experience and today radishes are grown and consumed throughout the world. Radishes have numerous varieties, varying in size, color, shape, and duration of required cultivation time. They are generally consumed raw, such as in salads or as an appetizer. There are some radishes that are grown for their seeds; oilseed radishes are grown, as the name implies, for oil production.

Overview and description

Radishes belong to the flowering plant family Brassicaceae (or Cruciferae), also known as the crucifers, the mustard family, or the cabbage family. (Cruciferae is an older name for the family. It means "cross-bearing," because the four petals of their flowers are reminiscent of a cross.) The family contains species of great economic importance, providing much of the world's winter vegetables. In addition to radish, these include cabbage, broccoli, cauliflower, brussels sprouts, collards, mustard, and kale.

Brassicaceae consists only of herbaceous plants with annual, biennial, or perennial lifespans. The leaves are alternate (rarely opposite), sometimes organized in basal rosettes. They do not have stipules. The structure of the flowers is extremely uniform throughout the family. They have four free saccate sepals and four clawed free petals, staggered, and with a typical cross-like arrangement. They have six stamens, four of which are longer (as long as the petals, so relatively short in fact) and are arranged in a cross like the petals and the other two are shorter (tetradynamous flower). The pistil is made up of two fused carpels and the style is very short, with two lobes.

Brassicaceae fruit is a peculiar kind of capsule named siliqua (plural siliquae, American English silique/siliques). It opens by two valves, which are the modified carpels, leaving the seeds attached to a framework made up of the placenta and tissue from the junction between the valves (replum).

The radish, Raphanus sativus, is characterized by white to purple flowers that form ebracteate racemose inflorescences (floral clustera comprising blossoms attached along a central axis and without underlying bracts). They are hermaphrodite (have both male and female organs) and are pollinated by bees and flies. Nectar is produced at the base of the stamens and stored on the sepals.

The fleshy, edible root varies in shape (round, oval, or elongated), size (small globes, one-half inch in diameter to those carrot-like giants one and one-half feet in length), and color (white to pink to red to purple to black to various combinations).

Radish comes from the Latin radix, meaning "root". The descriptive Greek name of the genus Raphanus means "quickly appearing" and refers to the rapid germination of these plants. Raphanistrum from the same Greek root is an old name once used for this genus.

Varieties

There are numerous varieties of radishes, differentiated according to flavor, size, color, and shape. In American markets, the most common variety is the globular or oval-shaped red-skinned radish, which can vary in size from a small cherry to a tiny orange. It may vary from mild to peppery in flavor, depending on age and variety.  The following are some common varieties.

Spring or summer radishes

Sometimes referred to as European radishes, or as spring radishes if they are typically planted in cooler weather, summer radishes are generally small and have a relatively short 3-4 week cultivation time.

•    The April Cross is a giant white radish hybrid that bolts very slowly.
•    Cherry Belle is a bright red-skinned round variety with a white interior (Faust 1996). It is familiar in North American supermarkets.
•    Champion is round and red-skinned like the Cherry Belle, but with slightly larger roots, up to about 5 cm, and a milder flavor (Faust 1996).
•    Red King has a mild flavor, with good resistance to club foot, a problem that can arise from poor drainage (Faust 1996).
•    Snow Belle is an all-white variety of radish, also round like the Cherry Belle (Faust 1996).
•    White Icicle or just Icicle is a white carrot-shaped variety, around 10-12 cm long, dating back to the 16th century. It slices easily, and has better than average resistance to pithiness (Faust 1996; Peterson 1999).
•    French Breakfast is an elongated red-skinned radish with a white splash at the root end. It is typically slightly milder than other summer varieties, but is among the quickest to turn pithy (Peterson 1999).
•    Plum Purple a purple-fuchsia radish that tends to stay crisp longer than the average radish (Peterson 1999).
•    Gala and Roodbol are two varieties popular in the Netherlands in a breakfast dish, thinly sliced on buttered bread (Faust 1996).
•    Easter Egg is not an actual variety, but a mix of varieties with different skin colors (Peterson 1999), typically including white, pink, red, and purple radishes. Sold in markets or seed packets under the name, the seed mixes can extend harvesting duration from a single planting, as different varieties may mature at different times (Peterson 1999).

Winter varieties

Black Spanish or Black Spanish Round are occur in both round and elongated forms, and is sometimes simply called the black radish or known by the French Gros Noir d'Hiver. It dates in Europe to 1548 (Aiton 1812), and was a common garden variety in England and France the early 19th century (Lindley 1831). It has a rough black skin with hot-flavored white flesh, is round or irregularly pear shaped (McIntosh 1828), and grows to around 10 centimeters in diameter.

Daikon

Daikon refers to a wide variety of winter radishes from east Asia. While the Japanese name daikon has been adopted in English, it is also sometimes called the Japanese radish, Chinese radish, or Oriental radish (AMHER 2004). In areas with a large South Asian population, it is marketed as mooli. Daikon commonly have elongated white roots, although many varieties of daikon exist. One well known variety is April Cross, with smooth white roots (Faust 1996; Peterson 1999). Faust (1996) describes Masato Red and Masato Green varieties as extremely long, well suited for fall planting and winter storage. The Sakurajima daikon is a hot flavored variety, which is typically grown to around four to five pounds (1.8 to 2.2 kilograms) when harvested, but which is reputed to grow as heavy as 70 pounds (32 kilograms) when left in the ground (Faust 1996).

Seed pod varieties

The seeds of radishes grow in pods, following flowering that happens when left to grow past their normal harvesting period. The seeds are edible, and are sometimes used as a crunchy, spicy addition to salads (Peterson 1999). Some varieties are grown specifically for their seeds or seed pods, rather than their roots. The Rat-tailed radish, an old European variety thought to have come from East Asia centuries ago, has long, thin, curly pods, which can exceed 20 centimeters in length. In the seventeenth century, the pods were often pickled and served with meat (Peterson 1999). The München Bier variety supplies spicy seeds that are sometimes served raw as an accompaniment to beer in Germany (Williams 2004).

Cultivation

Harvested summer radishes
Although the radish was a well-established crop in Hellenistic and Roman times, which leads to the assumption that it was brought into cultivation at an earlier time, Zohary and Hopf (2000) note that "there are almost no archaeological records available" to help determine its earlier history and domestication. Wild forms of the radish and its relatives the mustards and turnip can be found over west Asia and Europe, suggesting that their domestication took place somewhere in that area. However, Zohary and Hopf conclude, "Suggestions as to the origins of these plants are necessarily based on linguistic considerations."

Growing radishplants

Summer radishes mature rapidly, with many varieties germinating in three to seven days, and reaching maturity in three to four weeks (Faust 1996; Peterson 1999). A common garden crop in the United States, the fast harvest cycle makes them a popular choice for children's gardens (Faust 1996). Harvesting periods can be extended through repeated plantings, spaced a week or two apart (Beattie and Beattie 1938).

Radishes grow best in full sun (Cornell 2006) and light, sandy loams with pH 6.5 to 7.0 (Dainello 2003). They are in season from April to June and from October to January in most parts of North America; in Europe and Japan they are available year-round (due to the plurality of varieties grown). As with other root crops, tilling the soil helps the roots grow (Beattie and Beattie 1938) Most soil types will work, though sandy loams are particularly good for winter and spring crops, while soils that form a hard crust can impair growth (Beattie and Beattie 1938). The depth at which seeds are planted affects the size of the root, from 1 centimeter deep recommended for small radishes to 4 centimeters for large radishes (Peterson 1999).

Uses

Radishes are used for food, for medicinal purposes, and in industry for their oil.

Radishes (the root) are rich in ascorbic acid, folic acid, and potassium. They are a good source of vitamin B6, riboflavin, magnesium, copper, and calcium. They are low in calories. One cup of sliced red radish bulbs provides approximately 20 calories or less, coming largely from carbohydrates, making radishes, relative to their size, a very filling food for their caloric value.

The most popular part for eating is the napiform taproot, although the entire plant is edible and the tops can be used as a leaf vegetable. The bulb of the radish is usually eaten raw, most often in salads (Herbst 2001), but tougher specimens can be steamed. The raw flesh has a crisp texture and a pungent, peppery flavor, caused by chewing glucosinolates and the enzyme myrosinase in the radish, that, when brought together form allyl isothiocyanates, also present in mustard, horseradish and wasabi. Radishes can be made more crisp by soaking in icewater for a couple of hours (Herbst 2001).

Radishes are suggested as an alternative treatment for a variety of ailments including whooping cough, cancer, coughs, gastric discomfort, liver problems, constipation, dyspepsia, gallbladder problems, arthritis, gallstones, kidney stones, and intestinal parasites (Adams; PFAF 2008).

The seeds of the Raphanus sativus species can be pressed to extract seed oil. Wild radish seeds contain up to 48 percent oil content, and while not suitable for human consumption the oil has promise as a source of biofuel. The oilseed radish grows well in cool climates.

Radish_DSC3978-(1).jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#767 2020-08-20 01:03:27

ganesh
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Registered: 2005-06-28
Posts: 31,668

Re: Miscellany

645) Superconductor

A superconductor is a material that exhibits zero resistance to the flow of electrical current and becomes diamagnetic (opaque to magnetic fields) when cooled to a sufficiently low temperature .

An electrical current will persist indefinitely in a ring of superconducting material; also, a magnet can be levitated (suspended in space) by the magnetic field produced by a superconducting, diamagnetic object. Because of these unique properties, superconductors have found wide applications in the generation of powerful magnetic fields, magnetometry, magnetic shielding, and other technologies. Many researchers are seeking to devise "high-temperature" superconductors—materials that superconduct at or above the boiling point of nitrogen (N2), 77 K—that can carry large amounts of current without lapsing from the superconducting state. Such materials are already increasingly useful in power transmission and other applications.

Superconductivity history and theory

Superconductivity was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes (1853–1926). After succeeding in liquefying helium (He), Onnes observed that the electrical resistance of a mercury filament dropped abruptly to an experimentally undetectable value at a temperature near -451.84°F (-268.8°C, 4.2K), the boiling point of helium. Onnes wrote: "Mercury has passed into a new state, which, because of its extraordinary electrical properties, may be called the superconductive state."

The temperature below which the resistance of a material = zero is referred to as the superconducting transition temperature or the critical temperature of that material, Tc. Another unique characteristic of superconductors is their diamagnetic property, which was discovered by German physicist W. Meissner (1882–1974), working with a graduate student, in 1933. When a superconducting object is placed in a weak magnetic field, a persistent super-current or "screening current" is set up on its surface. This persistent current induces a magnetic field that exactly mirrors or cancels the external field, and the interior of the superconductor remains field-free. This phenomenon is called the Meissner effect and is the basis of the ability to of superconducting objects to levitate magnets. (Levitation only occurs when the force of repulsion of the magnetic field, which is a function of the field's intensity, exceeds the weight of the magnet itself.)

Superconductors are categorized as type I (soft) and type II (hard). For type I superconductors (e.g., most pure superconducting elements, including lead , tin, and mercury), diamagnetism and superconductivity break down together when the material is subjected to an external magnetic field whose strength is above a certain critical threshold Hc, the thermodynamic critical field. For type II superconductors (e.g., some superconducting alloys and compounds such as Mb3Sn), diamagnetism (but not superconductivity) breaks down at a first threshold field strength Hc1 and superconductivity persists until a higher threshold Hc2 is reached. These properties arise from differences in the ways in which microscopic swirls or vortices of current tend to arise in each particular material in response to an external magnetic field.

No unified or complete theory of superconductivity yet exists. However, the basic underlying mechanism for superconductivity has been suggested to be an electron-lattice interaction. U.S. physicists John Bardeen (1908–1991), Leon Cooper (1930–), and Robert Schrieffer (1931–) derived a theory (termed the BCS theory, after their initials) in 1957, proposing that in the lattice of atoms comprising the material, pairing occurs between electrons with opposite momentum and spin. These electron pairs are called Cooper pairs, and as described by Schrieffer, they condense into a single state and flow as a totally frictionless "fluid." BCS theory also predicts that an energy gap—energy levels a discrete amount below those of normal electrons—exists in superconductors. English Brian Josephson (1940–), in 1962, proposed that Cooper pairs could tunnel from one superconductor to another through a thin insulating layer. Such a structure, called a Josephson junction, has for years been fabricated widely for superconducting electronic devices.

High-temperature superconductors

Before 1986, although a variety of superconductors had been discovered and synthesized, all had critical temperatures at or below the boiling point of He (e.g., Pb at -446.5°F [-265.8°C, 7.19K] and Nb3Sn at -426.91°F [-254.95°C, 18.05K]). Since expensive refrigeration units are required to produce liquid He, this strictly limited the circumstances under which it was economical to apply superconductivity.
The first superconductor to be discovered having Tc > -320.8°F (-196.0°C, 77K) (the boiling point of liquid N2, which is much cheaper to produce than liquid He) was YBa2Cu3O7 (Tc ~ -294°F [-181°C, 92K]). The Y-Ba-Cu-O compound was discovered by U.S. physicist C. W. Chu (1948–), working with a graduate student, in 1987 following the 1986 discovery by German physicists G. Bednorz (1950–) and K. A. Müller (1927–) of the La-Ba-Cu-O oxide superconductor (Tc = -394.6°F [-237.0°C, 36K). One year later, in 1988, bismuth-based (e.g., (Bi, Pb)2Sr2Ca2Cu3O10, Tc = -261.4°F [-163.0°C, 110K) and thallium-based (e.g.,Tl2Ba2Ca2Cu3O10, Tc = -234.4°F [-148.0°C, 125K]) superconductors were successfully synthesized; their Tc's were some 20K higher than that of Y-Ba-Cu-O. Very recently, mercury-based cuprates (HgBa2Can-1CunO2n+2+Δ) have been shown to have Tc values higher than 130K. These oxide superconductors are now classified as the high-temperature (or high-Tc) superconductors (HTSCs).

All HTSCs so far discovered have an atomic structure that consists of thin planes of atoms, many of which consist of the compound copper dioxide (CuO2). This compound is, so far, uniquely important to producing the property of high-temperature conductivity. Ironically, CuO2 is a Mott insulator, meaning that at temperatures approaching absolute zero it begins to behave as an insulator (a substance having very high resistance) rather than as a conductor: yet at higher temperatures, embedded in an appropriate crystal matrix, it is key to the production of zero resistance (superconduction).

Current flow in the CuO2 family of HTSCs has directional properties. That is, the critical current density , Jc—the largest current density that a superconductor can carry without lapsing into finite resistivity—along the CuO2 plane direction is orders of magnitude higher than at right angles to it. For HTSCs to carry a large amount of current, this implies that individual crystalline grains in bulk conductors (e.g., wires or tapes) of HTSC material should be well aligned with the current transport direction. Significant grain misorientation and chemistry inhomogeneity at grain boundaries can form weak links between neighboring grains and thus lower local Jc values. Several bulk material manufacture technologies, such as the melt-powder-melt-growth (MPMG) method for Y-Ba-Cu-O and the oxide-powder-in-tube (OPIT) process for Bi- and Tlbased superconductors, have been demonstrated to develop textured bulk structures. J values of 106–108 c amperes per square centimeter at 77K have been achieved over small distances. For thin-film growth, dual ion beam sputtering (DIBS), molecular beam epitaxy (MBE), pulsed-laser deposition (PLD), and metal-organic chemical vapor deposition (MOCVD) have been shown to be successful methods. By optimizing processing temperature and pressure and using proper buffer layers, epitaxial HTSC films can be deposited on templates of other crystalline materials such as Si and MgO. With the integration of Si-based microelectronics processes, HTSC thin-film devices can be fabricated for a variety of applications.

Superconductivity applications

Superconductivity applications fall into two main areas—electromagnets (magnets whose magnetism depends on an externally-powered current passing through a winding) and electronics . In electromagnets, superconducting windings have much lower power consumption than do conventional copper windings, and thus are particularly attractive for high-field applications. Superconducting magnets can be used in magnetic resonance imaging, magnetic sorting of metals, magnetic levitation trains, and magnetic shielding. For power utility applications, superconductors are promising for magnetic energy storage, electrical power transmission, motors, and generators. They are also useful as the coatings for radio-frequency cavities. In electronic applications, thin-film interconnections and Josephson junctions are two key elements. Superconductors offer fast switching speeds and reduced wiring delays so that they are applicable for logic devices and memory cells. Superconducting field-effect transistors and Josephson junction integrated circuits have been demonstrated. At the temperature of liquid nitrogen, 77K, superconductors can be further integrated with semiconductors to form hybrid devices. For sensor operation, superconducting quantum interference devices (SQUIDs), based on the Josephson junction technology, are the most sensitive detector for measuring changes in magnetic field. For example, they can detect they very faint signals (on the order of 10-15 Tesla) produced by the human brain and heart . Also, SQUID-based gradiometry is a very powerful instrument for non-destructive evaluation of nonliving materials. The increased energy gap in HTSCs allows the fabrication of superconducting electromagnetic radiation detectors used for over the spectrum from x ray to the far infrared.

As time goes by, superconductors will find more and more applications. Recently, Y-Ba-Cu-O has been shown to be a good material for the top and bottom electrodes of oxide ferroelectric thin-film capacitors which exhibit fatigue resistance superior to that of capacitors with conventional Pt electrodes (used in dynamic random-access computer memories). This suggests that when the microstructures and the properties of HTSC materials can be well controlled and tailored, oxide superconductors are promising for many hybrid designs—designs incorporating both conventional and superconducting materials. We can also expect upcoming hybrid fabrication technologies. Processes for thin films, thick films, wires, and tapes may all be needed for the integration of a single superconductor-based instrument. Future growth in superconductors technology in electronic components, medical sensing, geology , military technology, transportation, and power transmission and storage is very promising, especially if, as researchers believe, transition temperatures and critical current densities can be significantly increased.

Superconductors-levitation.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#768 2020-08-22 00:40:58

ganesh
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Registered: 2005-06-28
Posts: 31,668

Re: Miscellany

646) pH Value

pH, quantitative measure of the acidity or basicity of aqueous or other liquid solutions. The term, widely used in chemistry, biology, and agronomy, translates the values of the concentration of the hydrogen ion—which ordinarily ranges between about 1 and {10}^{−14} gram-equivalents per litre—into numbers between 0 and 14. In pure water, which is neutral (neither acidic nor alkaline), the concentration of the hydrogen ion is {10}^{−7} gram-equivalents per litre, which corresponds to a pH of 7. A solution with a pH less than 7 is considered acidic; a solution with a pH greater than 7 is considered basic, or alkaline.

The measurement was originally used by the Danish biochemist S.P.L. Sørensen to represent the hydrogen ion concentration, expressed in equivalents per litre, of an aqueous solution: pH = −log[H+] (in expressions of this kind, enclosure of a chemical symbol within square brackets denotes that the concentration of the symbolized species is the quantity being considered).

Because of uncertainty about the physical significance of the hydrogen ion concentration, the definition of the pH is an operational one; i.e., it is based on a method of measurement. The U.S. National Institute of Standards and Technology has defined pH values in terms of the electromotive force existing between certain standard electrodes in specified solutions.

The pH is usually measured with a pH meter, which translates into pH readings the difference in electromotive force (electrical potential or voltage) between suitable electrodes placed in the solution to be tested. Fundamentally, a pH meter consists of a voltmeter attached to a pH-responsive electrode and a reference (unvarying) electrode. The pH-responsive electrode is usually glass, and the reference is usually a mercury-mercurous chloride (calomel) electrode, although a silver-silver chloride electrode is sometimes used. When the two electrodes are immersed in a solution, they act as a battery. The glass electrode develops an electric potential (charge) that is directly related to the hydrogen-ion activity in the solution, and the voltmeter measures the potential difference between the glass and reference electrodes. The meter may have either a digital or an analog (scale and deflected needle) readout. Digital readouts have the advantage of exactness, while analog readouts give better indications of rates of change. Battery-powered portable pH meters are widely used for field tests of the pH of soils. Tests of pH may also be performed, less accurately, with litmus paper or by mixing indicator dyes in liquid suspensions and matching the resulting colours against a colour chart calibrated in pH.

In agriculture, the pH is probably the most important single property of the moisture associated with a soil, since that indication reveals what crops will grow readily in the soil and what adjustments must be made to adapt it for growing any other crops. Acidic soils are often considered infertile, and so they are for most conventional agricultural crops, although conifers and many members of the family Ericaceae, such as blueberries, will not thrive in alkaline soil. Acidic soil can be “sweetened,” or neutralized, by treating it with lime. As soil acidity increases so does the solubility of aluminum and manganese in the soil, and many plants (including agricultural crops) will tolerate only slight quantities of those metals. Acid content of soil is heightened by the decomposition of organic material by microbial action, by fertilizer salts that hydrolyze or nitrify, by oxidation of sulfur compounds when salt marshes are drained for use as farmland, and by other causes.

ph-chart.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#769 2020-08-24 01:01:37

ganesh
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Registered: 2005-06-28
Posts: 31,668

Re: Miscellany

647) Hydrogen

The most abundant element in the universe, hydrogen is also a promising source of "clean" fuel on Earth.

Named after the Greek words hydro for "water" and genes for "forming," hydrogen makes up more than 90 percent of all of the atoms, which equals three quarters of the mass of the universe, according to the Los Alamos National Laboratory. Hydrogen is essential for life, and it is present in nearly all the molecules in living things, according to the Royal Society of Chemistry. The element also occurs in the stars and powers the universe through the proton-proton reaction and carbon-nitrogen cycle. Stellar hydrogen fusion processes release huge amounts of energy as they combine hydrogen atoms to form helium, according to Los Alamos.

Pure hydrogen gas is scarce in Earth's atmosphere and any hydrogen that actually enters the atmosphere rapidly escapes Earth's gravity, according to the Royal Society. On our planet, hydrogen occurs mainly in combination with oxygen and water, as well as in organic matter such as living plants, petroleum and coal, Los Alamos reports.

Just the facts

Atomic number (number of protons in the nucleus): 1
Atomic symbol (on the Periodic Table of Elements): H
Atomic weight (average mass of the atom): 1.00794
Density: 0.00008988 grams per cubic centimeter
Phase at room temperature: Gas
Melting point: minus 434.7 degrees Fahrenheit (minus 259.34 degrees Celsius)
Boiling point: minus 423.2 F (minus 252.87 C)
Number of isotopes (atoms of the same element with a different number of neutrons): 3 common isotopes, including 2 stable ones
Most common isotope: 1H, natural abundance 99.9885 percent

Hydrogen discovery

Robert Boyle produced hydrogen gas in 1671 while he was experimenting with iron and acids, but it wasn't until 1766 that Henry Cavendish recognized it as a distinct element, according to Jefferson Lab. The element was named hydrogen by the French chemist Antoine Lavoisier.

Hydrogen has three common isotopes: protium, which is just ordinary hydrogen; deuterium, a stable isotope discovered in 1932 by Harold C. Urey; and tritium, an unstable isotope discovered in 1934, according to Jefferson Lab. The difference between the three isotopes lies in the number of neutrons each of them has. Hydrogen has no neutrons at all; deuterium has one, while tritium has two neutrons, according to Lawrence Berkeley National Laboratory. Deuterium and tritium are used as fuel in nuclear fusion reactors, according to Los Alamos.

Hydrogen combines with other elements, forming a number of compounds, including common ones such as water (H2O), ammonia (NH3), methane (CH4), table sugar (C12H22O11), hydrogen peroxide (H2O2) and hydrochloric acid (HCl), according to Jefferson Lab.

Hydrogen is typically produced by heating natural gas with steam to form a mixture of hydrogen and carbon monoxide called syngas, which is then separated to produce hydrogen, according to the Royal Society.

Hydrogen is used to make ammonia for fertilizer, in a process called the Haber process, in which it is reacted with nitrogen. The element is also added to fats and oils, such as peanut oil, through a process called hydrogenation, according to Jefferson Lab. Other examples of hydrogen use include rocket fuel, welding, producing hydrochloric acid, reducing metallic ores and filling balloons, according to Los Alamos. Researchers have been working on developing the hydrogen fuel cell technology that allows significant amounts of electrical power to be obtained using hydrogen gas as a pollution-free source of energy that can be used as fuel for cars and other vehicles.

Hydrogen is also used in the glass industry as a protective atmosphere for making flat glass sheets, while the electronics industry, it is used as a flushing gas in the process of manufacturing silicon chips, according to the Royal Society.

Who knew?

(i) Hydrogen is the main component of Jupiter and the other gas giant planets, according to Los Alamos.
(ii) The first gas balloon flight was launched in Paris in 1783 and the gas used in the balloon was hydrogen, according to the National Balloon Museum. Its use in filling airships ended when the Hindenburg caught on fire, according to the Royal Society.
(iii) NASA uses hydrogen as rocket fuel to deliver crew to space.
(iv) Liquefied hydrogen is extremely cold and it can cause severe frostbite when it comes into contact with skin.
(v) Hydrogen is about 14 times lighter than air, according to "The Principles of Chemistry."
(vi) Lavoisier, the French chemist who gave hydrogen its name, served as a financier and public administrator before the French Revolution and was executed during the revolution, according to Encyclopedia Britannica.
(vii) About 3 billion cubic feet of hydrogen are produced in the United States per year, according to Los Alamos.
(viii) Hydrogen has the lowest density of all gases, according to the Royal Society.
(ix) Hydrogen is the only element whose three common isotopes – protium, deuterium and tritium – have been given different names, Los Alamos reports.

Current research

Researchers have been studying hydrogen with great interest for years because of its potential as a pollution-free fuel. "Hydrogen is an energy carrier with no carbon in it, so when you burn it, you only produce water," which makes it a clean fuel, with no emissions at all, said Richard Chahine, the director of the Hydrogen Research Institute at University of Québec at Trois-Rivières in Canada. However, there is a major problem with hydrogen fuel: it is more expensive than gas. In fact, last year, Toyota’s senior vice president Bob Carter announced that, according to the Department of Energy's estimates, a full tank of compressed hydrogen would initially cost around $50, Ecomento.com reported. In general, costs associated with hydrogen fuel technology are "a very challenging barrier because, as of now, people would prefer to have better technologies at the ongoing price," Chahine told Live Science.

Another issue with hydrogen fuel is that the process of hydrogen production itself is in fact not so "clean" or pollution free. "As of today, most of the hydrogen produced comes from natural gas," a process that generates carbon dioxide (CO2), Chahine said.

Researchers have therefore been looking for alternative and more environmentally friendly ways of producing hydrogen that would ideally eliminate CO2 emissions from the process. Last year, for instance, scientists at the U.S. Department of Energy’s Argonne National Laboratory developed a small-scale "nano-sized hydrogen generator," a device that produces pure hydrogen using light and graphene and without burning fossil fuels. The current version of the generator is really small, but if it turns out it can be expanded, it could allow scientists to produce enough hydrogen to provide fuel for cars and generators.

Another way of producing hydrogen, called "biological water splitting," would involve the use of certain photosynthetic microbes that use light energy to produce hydrogen from water as part of their metabolic processes, according to the National Renewable Energy Laboratory, where researchers are currently investigating this process. Another potential method to produce hydrogen involves the fermentation of renewable biomass materials, the NREL reports. Researchers at the NREL have also been working on converting agricultural residues (such as peanut shells) and consumer wastes (such as plastics and waste grease) into a liquid product called bio-oil whose components can then be separated into fuels, including hydrogen. The cleanest way to produce hydrogen, however, is through splitting water into hydrogen and oxygen by using sunlight, the NREL reports.

KmpifM7nQyaAbdZDhh3NK6-650-80.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#770 2020-08-25 01:37:44

ganesh
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Posts: 31,668

Re: Miscellany

648) Aerobics

Aerobics, system of physical conditioning that increases the efficiency of the body’s intake of oxygen, thereby stimulating the cardiovascular system, developing endurance, and reducing body fat. Increased energy, lower blood pressure and cholesterol, greater suppleness, stronger bones, better posture, and decreased stress levels are other benefits that may accrue from aerobic activity. To be effective, aerobic training must include a minimum of three sessions per week. During each session, usually lasting an hour, the exerciser’s heart rate must be raised to a training level for at least 20 minutes.

The concept of aerobics was pioneered in the United States by physician Kenneth H. Cooper and popularized in his books ‘Aerobics’ (1968) and ‘The Aerobics Way’ (1977). Cooper’s system uses point charts to rate the aerobic value of various exercises for different age-groups. As individuals progressively upgrade the quantity and quality of their exercise, they can gauge the improvement in their physical condition through the point system. In the 1980s aerobics was popularized by Jane Fonda and Richard Simmons through workout videotapes and instructional programs. Sometimes called group fitness, aerobics is most often practiced in health and fitness clubs where groups of one to two dozen exercisers follow the lead of an instructor whose movements are synchronized to up-tempo popular music.

Uplift-Your-Mood-And-Get-Fit-With-Aerobics-Feel-Good-Factor-1.png


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#771 2020-08-26 01:00:12

ganesh
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Registered: 2005-06-28
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Re: Miscellany

649) TGV

Overview

Locale : France, with services extending to Belgium, Luxembourg, Germany, Switzerland, Italy, Spain and the Netherlands
Dates of operation : 1981–present
Technical

Track gauge : 1,435 mm (4 ft 8 1⁄2 in) (standard gauge)

High-speed lines in France

The TGV (French: Train à Grande Vitesse, "high-speed train") is France's intercity high-speed rail service, operated by the SNCF, the state-owned national rail operator. The SNCF started working on a high-speed rail network in 1966 and later presented the project to President Valéry Giscard d'Estaing who approved it. Originally designed as turbotrains to be powered by gas turbines, TGV prototypes evolved into electric trains with the 1973 oil crisis. In 1976 the SNCF ordered 87 high-speed trains from GEC-Alsthom. Following the inaugural service between Paris and Lyon in 1981 on the LGV Sud-Est (LGV for Ligne à Grande Vitesse; "high-speed line"), the network, centered on Paris, has expanded to connect major cities across France (including the likes of Marseille, Lille, Bordeaux, Strasbourg, Rennes, Montpellier) and in neighbouring countries on a combination of high-speed and conventional lines. The TGV network in France carries about 110 million passengers a year.

Like the Shinkansen in Japan, the TGV has never experienced a fatal accident during its operational history; the onboard security systems are among the world's most advanced. The high-speed tracks, maintained by SNCF Réseau, are also subject to heavy regulation. Confronted with the fact that train drivers would not be able to see signals along the track-side when trains reach full speed, engineers developed the TVM cab-signalling technology, which would later be exported worldwide. It allows for a train engaging in an emergency braking to request within seconds all following trains to reduce their speed; if a driver does not react within 1.5 km (0.93 mi), the system overrides the controls and reduces the train's speed automatically. The TVM safety mechanism enables TGVs using the same line to depart every three minutes.

A TGV test train set the world record for the fastest wheeled train, reaching 574.8 km/h (357.2 mph) on 3 April 2007. Conventional TGV services operate up to 320 km/h (200 mph) on the LGV Est, LGV Rhin-Rhône and LGV Méditerranée. In 2007, the world's fastest scheduled rail journey was a start-to-stop average speed of 279.4 km/h (173.6 mph) between the Gare de Champagne-Ardenne and Gare de Lorraine on the LGV Est, not surpassed until the 2013 reported average of 283.7 km/h (176.3 mph) express service on the Shijiazhuang to Zhengzhou segment of China's Shijiazhuang–Wuhan high-speed railway.

The TGV was conceived at the same period as other technological projects sponsored by the Government of France, including the Ariane 1 rocket and Concorde supersonic airliner; those funding programmes were known as champion national policies (literal translation: national champion). The commercial success of the first high-speed line led to a rapid development of services to the south (LGV Rhône-Alpes, LGV Méditerranée, LGV Nîmes–Montpellier), west (LGV Atlantique, LGV Bretagne-Pays de la Loire, LGV Sud Europe Atlantique), north (LGV Nord, LGV Interconnexion Est) and east (LGV Rhin-Rhône, LGV Est). Neighbouring countries Italy, Spain and Germany developed their own high-speed rail services.[citation needed]

The TGV system itself extends to neighbouring countries, either directly (Italy, Spain, Belgium, Luxembourg and Germany) or through TGV-derivative networks linking France to Switzerland (Lyria), to Belgium, Germany and the Netherlands (Thalys), as well as to the United Kingdom (Eurostar). Several future lines are planned, including extensions within France and to surrounding countries. Cities such as Tours and Le Mans have become part of a "TGV commuter belt" around Paris; the TGV also serves Charles de Gaulle Airport and Lyon–Saint-Exupéry Airport. A visitor attraction in itself, it stops at Disneyland Paris and in tourist cities such as Avignon and Aix-en-Provence as well. Brest, Chambéry, Nice, Toulouse and Biarritz are reachable by TGVs running on a mix of LGVs and modernised lines. In 2007, SNCF generated profits of €1.1 billion (approximately US$1.75 billion, £875 million) driven largely by higher margins on the TGV network.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#772 2020-08-27 00:51:51

ganesh
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Re: Miscellany

650) Gear and Differential gear

Gear

Gear, machine component consisting of a toothed wheel attached to a rotating shaft. Gears operate in pairs to transmit and modify rotary motion and torque (turning force) without slip, the teeth of one gear engaging the teeth on a mating gear. If the teeth on a pair of mating gears are arranged on circles, i.e., if the gears are toothed wheels, the ratios of the rotary speeds and torques of the shafts are constant. If the teeth are arranged on noncircular bodies the speed and torque ratios vary.

Most gears are circular. To transmit motion smoothly and with a nonvarying speed ratio at every instant, the contacting surfaces of gear teeth must be carefully shaped to a specific profile. If the smaller of a gear pair (the pinion) is on the driving shaft, the pair acts to reduce speed and to amplify torque; if the pinion is on the driven shaft the pair acts as a speed increaser and a torque reducer. If the driven gear has twice as many teeth as the pinion, for example, the torque of the driven gear is twice the pinion torque, whereas the pinion speed is twice the speed of the driven gear.
The shafts that gears connect must be relatively close, but they may have practically any spatial relationship with respect to one another; they may be parallel or nonparallel and intersecting or nonintersecting. For each of these arrangements of the shafts, gears having appropriate capabilities can be made. Parallel shafts can be connected by gears with teeth that are straight lengthwise and parallel to the shaft axes (spur gears) or by gears with twisted, screwlike teeth (helical gears). Intersecting shafts are connected by gears with tapered teeth arranged on truncated cones (bevel gears). Nonparallel, nonintersecting shafts are usually connected by a worm and gear. The worm resembles a screw, and the gear resembles a quarter section of a long nut that has been bent around a cylinder. The commonest angle between nonparallel shafts, either intersecting or nonintersecting, is a right angle (90°).

Because it is basically a screw, a worm gear may have only one thread (tooth), whereas to maintain continuous contact with parallel shaft gears (spur and helical), the pinion must have at least five teeth. For this reason, to obtain a large speed ratio in a single gear pair, a worm and gear are well suited. If the shafts must be parallel, it may be necessary to use several gear pairs in series (a train) to obtain a large ratio.

Differential gear

Differential gear, in automotive mechanics, gear arrangement that permits power from the engine to be transmitted to a pair of driving wheels, dividing the force equally between them but permitting them to follow paths of different lengths, as when turning a corner or traversing an uneven road. On a straight road the wheels rotate at the same speed; when turning a corner the outside wheel has farther to go and will turn faster than the inner wheel if unrestrained.

The conventional automobile differential was invented in 1827 by a Frenchman, Onésiphore Pecqueur. It was used first on steam-driven vehicles and was a well-known device when internal-combustion engines appeared at the end of the 19th century.

The elements of the Pecqueur differential are shown in the Figure. The power from the transmission is delivered to the bevel ring gear by the drive-shaft pinion, both of which are held in bearings (not shown) in the rear-axle housing. The case is an open boxlike structure that is bolted to the ring gear and contains bearings to support one or two pairs of diametrically opposite differential bevel pinions. Each wheel axle is attached to a differential side gear, which meshes with the differential pinions. On a straight road the wheels and the side gears rotate at the same speed, there is no relative motion between the differential side gears and pinions, and they all rotate as a unit with the case and ring gear. If the vehicle turns to the left, the right-hand wheel will be forced to rotate faster than the left-hand wheel, and the side gears and the pinions will rotate relative to one another. The ring gear rotates at a speed that is equal to the mean speed of the left and right wheels. If the wheels are jacked up with the transmission in neutral and one of the wheels is turned, the opposite wheel will turn in the opposite direction at the same speed.

The torque (turning moment) transmitted to the two wheels with the Pecqueur differential is the same. Consequently, if one wheel slips, as in ice or mud, the torque to the other wheel is reduced. This disadvantage can be overcome somewhat by the use of a limited-slip differential. In one version a clutch connects one of the axles and the ring gear. When one wheel encounters low traction, its tendency to spin is resisted by the clutch, thus providing greater torque for the other wheel.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#773 2020-08-28 00:42:03

ganesh
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Re: Miscellany

651) Gearbox

When it comes to gearing components, things can get confusing quickly. There are many terms that gear manufacturers as well as engineers and designers use to talk about what sometimes is essentially the same thing. The term “gearbox” is one of those terms, often times used interchangeably with gearhead or gear reducer … even though they sometimes refer to slightly different physical arrangements of gears.

The most basic definition of a gearbox is that it is a contained gear train, or a mechanical unit or component consisting of a series of integrated gears within a housing. In fact, the name itself defines what it is — a box containing gears. In the most basic sense, a gearbox functions like any system of gears; it alters torque and speed between a driving device like a motor and a load.

The gears inside of a gearbox can be any one of a number of types from bevel gears and spiral bevel gears to wormgears and others such as planetary gears. The gears are mounted on shafts, which are supported by and rotate via rolling element bearings. The gearbox is a mechanical method of transferring energy from one device to another and is used to increase torque while reducing speed.

Gearboxes are used in many applications including machine tools, industrial equipment, conveyors, and really any rotary motion power transmission application that requires changes to torque and speed requirements.

So it’s clear — a gearbox is always a fully integrated mechanical component consisting of a series of mating gears contained in a housing with shafts and bearings (to support and resolve loads) and in many cases a flange for motor mounting. Most of the motion industry makes no differentiation between the terms gearhead and gearbox. But in a few contexts, the term gearbox specifically refers to housed gearing as described above while the more general term gearhead refers to assemblies otherwise open gearing that installs within some existing machine frame. The latter are targeted to compact or battery-powered mobile designs necessitating especially tight integration and omission of extra subcomponents. Here, a series of parallel plates might support the gear-train shafts (and their bearings) and allow bolting to a motor face.

Though beyond the scope of this FAQ, other open gearing simply mounts to the electric-motor output and operates exposed to the environment. Some such open gearing is self-lubricating — constructed of dimensionally stable polyamides or similar materials engineered to meet stringent cleanliness, vibration, weight, and cost requirements.

Power Transmission

The function of a gear is to mesh with other gears to transmit altered torque and rotation. In fact, gearing can change the speed, torque and direction of motion from a drive source.

Geometry & general gear design

When two gears with an unequal number of teeth engage, the mechanical advantage makes their rotational speeds and torques different.

In the simplest setups, gears are flat with spur teeth (with edges parallel to the shaft) and the input gear’s shaft is parallel to that of the output. Spur gears mostly roll through meshing, so can be 98% or more efficient per reduction stage. However, there is some sliding between tooth surfaces, and initial tooth-to-tooth contact occurs along the whole tooth width at once, causing small shock loads that induce noise and wear. Sometimes lubrication helps mitigate these issues.

In slightly more complex setups, parallel-axis gearsets have helical gears that engage at an angle between 90° and 180° for more tooth contact and higher torque capacity. Helical reducers are suitable for higher-horsepower applications where long-term operational efficiency is more important than initial cost. Helical gear teeth engage gradually over the tooth faces for quieter and smoother operation than spur gearsets. They also tend to have higher load capacities.

No matter the subtype, most parallel-axis gearsets have gear teeth with tailored involute profiles—customized versions of the rolled trace off a circle with an imaginary string. Here, mating gears have tangent pitch circles for smooth rolling engagement that minimizes slipping. A related value, the pitch point, is where one gear initially contacts its mate’s pitch point.

Involute gearsets also have an action path that passes through the pitch point tangent to a base circle.

Besides parallel-axis gearsets, there are non-parallel and right-angle gearsets. These have input and output shafts that protrude in different directions to give engineers more mounting and design options. The gear teeth of such gearsets are either bevel (straight, spiral or zerol), worm, hypoid, skew or crossed-axis helical gears. The most common are bevel gearsets with teeth cut on an angular or conical shape.

Hypoid gears are much like spiral-bevel gearsets, but the input and output shaft axes don’t intersect, so it’s easier to integrate supports. In contrast, zerol gearsets have curved teeth that align with the shaft to minimize thrust loads.

General speed reducers, shaft-mount sets & worm drives

Gear reducers, called speed reducers, are a component of many mechanical, electrical and hydraulic motors. Essentially, it is a gear or series of gears combined in such a manner as to alter the torque of a motor. Typically, the torque increases in direct proportion to the reduction of rotations per unit of time.

Speed reducers come in two varieties: base mounted and shaft mounted. Shaft-mounted types come in two versions. One is truly shaft mounted in that the input shaft of the drive motor supports it … with a special coupling to address torque reactions. The other mounts to the machine housing so the input shaft doesn’t support the reducer’s weight or address torque reactions.

By the American Gear Manufacturers Association (AGMA) definition, engineers apply the term “speed reducer” to units operating at pinion speeds below 3,600 rpm or pitch-line velocities below 5,000 fpm. (The AGMA is an international group of gear manufacturers, consultants, academics, users and suppliers.)

Reducers operating at speeds higher than these are called high-speed units. Manufacturers base catalog ratings and engineering specifications for speed reducers on these AGMA standards.

There are as many types of speed reducers as there are gear types. Consider reducers in which the input and output shafts are at different angles. The most common of these are worm-gear reducers.

Worm-gear reducers are used in low- to moderate-horsepower applications. They offer low initial cost, high ratios and high output torque in a small package, along with a higher tolerance for shock loading than helical gear reducers. In a traditional setup, a cylindrical toothed worm engages a disk-shaped wheel gear with teeth on its circumference or face.

Most worm gears are cylindrical with teeth of consistent size. Some worm-gear reducers use a double-enveloping tooth geometry—with a pitch diameter that goes from deep into short and back to deep—so more teeth engage. No matter the version, most wheel gears in worm-based reducers sport cupped teeth edges that wrap around the worm shaft during engagement. In many cases, the sliding engagement lowers efficiency but extends life, as worm-gear mating holds a film of lubricant during operation. The ratio of a worm-gear is the number of wheel teeth to the number of threads (starts or leads) on the worm.

A few words on gearheads

A gearhead is similar to a gear reducer, but a gearhead doesn’t just reduce speed. Engineers use them wherever an application calls for high torque at low speed. It reduces a load’s reflected mass inertia, which makes accelerating heavy loads easier, enabling designs to run off smaller motors. Gearheads come in a variety of styles from basic spur gearheads to more complex planetary gearheads and harmonic type gearheads, each with their own characteristics and suitable applications. One caveat: In some applications, gearhead backlash is an issue. In this case, consider using a gearhead with low or zero backlash.

Gearboxes, specialty gearheads & servo gearsets … including planetary sets

Servo systems are precision-motion setups with feedback and (in most cases) fairly stringent accuracy demands. So for these designs, engineers should pick servogear reducers with good torsional stiffness, reliable output torque and minimal backlash. OEMs tasked with integrating servo systems should look for quiet reducers that easily mount to the motor and require little or (if possible) no maintenance.

In fact, a lot of advanced machinery integrates servogears into application-specific electromechanical arrangements, and several of these arrangements are common enough to have specific labels. Here is a look at some of the most widespread.

Gearmotor: This complete motion component is a gear reducer integrated with an ac or dc electric motor. Usually the motor includes the gears on its output (typically in the form of an assembled gearbox) to reduce speed and boost available output torque. Engineers use gearmotors in machines that must move heavy objects. Speed specifications for gearmotors are normal speed and stall-speed torque.

Gearbox: This is a contained gear train … a mechanical unit or component consisting of a series of integrated gears. Planetary gears are common in integrated gearboxes.
Planetary gears: Particularly common in servo systems, these gearsets consist of one or more outer planet gears that revolve about a central, or sun, gear. Typically, the planet gears mount on a movable arm or carrier that rotates relative to the sun gear. The sets often use an outer ring gear, or annulus, that meshes with the planet gears.

The gear ratio of a planetary set requires calculation, because there are several ways they can convert an input rotation to an output rotation. Typically, one of these three gear wheels stays stationary; another is an input that provides power to the system, and the last acts as an output that receives power from the driving motor. The ratio of input rotation to output rotation depends on the number of teeth in each gear and on which component is held stationary.

Planetary gearsets offer several advantages over other gearsets. These include high power density, the ability to get large reductions from a small volume, multiple kinematic combinations, pure torsional reactions and coaxial shafting. Another advantage to planetary gearbox arrangements is power-transmission efficiency. Losses are typically less than 3% per stage, so rather than waste energy on mechanical losses inside the gearbox, these gearboxes transmit a high proportion of the energy for productive motion output.

Planetary gearbox arrangements distribute load efficiently, too.

Multiple planets share transmitted load between them, which greatly increases torque density. The more planets in the system, the greater load ability and the higher the torque density. This arrangement is also very stable due to the even distribution of mass and increased rotational stiffness. Disadvantages include high bearing loads, inaccessibility and design complexity.

In servo systems, besides boosting output torque, gearboxes impart another benefit—reducing settling time. Settling time is a problem when motor inertia is low compared to load inertia … an issue that’s the source of constant debate (and regular improvement) in the industry. Gearboxes reduce the reflected inertia at the controls by a factor equal to the gear reduction squared.

Strain-wave gearing

Strain-wave gearing is a special gear design for speed reduction. It uses the metal elasticity (deflection) of a gear to reduce speed. (Strain-wave gearing sets are also known as Harmonic Drives, a registered trademark term of Harmonic Drive Systems Inc.) Benefits of using strain-wave gearing include zero backlash, high torque, compact size and positional accuracy.

A strain-wave gearset consists of three components: wave generator, flexspline and circular spline.

The wave generator is an assembly of a bearing and a steel disk called a wave generator plug. The outer surface of the wave generator plug has an elliptical shape machined to a precise specification. A specially designed ball bearing is pressed around this bearing plug causing the bearing to conform to the same elliptical shape of the wave generator plug. Designers typically use the wave generator as the input member, usually attached to a servomotor.

The flexspline is a thin-walled steel cup. Its geometry lets the walls of the cup be radially compliant but remain torsionally stiff (because the cup has a large diameter). Manufacturers machine the gear teeth into the outer surface near the open end of the cup (near the brim). The flexspline is usually the output member of the mechanism.

The cup has a rigid boss at one end to provide a rugged mounting surface. The wave generator is inserted inside the flexspline so the bearing is at the same axial location as the flexspline teeth. The flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. This conforms the teeth on the outer surface of the flexspline to the elliptical shape. That way, the flexspline effectively has an elliptical gear-pitch diameter on its outer surface.

The circular spline is a rigid circular steel ring with teeth on the inside diameter. It is usually attached to the housing and does not rotate. Its teeth mesh with those of the flexspline. The tooth pattern of the flexspline engages the tooth profile of the circular spline along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within a circle. Mathematically, an inscribed ellipse contacts a circle at two points. However, the gear teeth have a finite height. So there are actually two regions (instead of two points) of tooth engagement. Roughly 30% of the teeth are engaged at all times.

The elastic radial deformation acts like a very stiff spring to compensate for space between the teeth that would otherwise increase backlash.

The pressure angle of the gear teeth transforms the output torque’s tangential force into a radial force acting on the wave-generator bearing. The teeth of the flexspline and circular spline engage near the ellipse’s major axis and disengage at the ellipse’s minor axis. Note that the flexspline has two less teeth than the circular spline, so every time the wave generator rotates one revolution, the flexspline and circular spline shift by two teeth. The gear ratio is calculated:

number of flexspline teeth ÷ (number of flexspline teeth – number of circular spline teeth)

The tooth engagement motion (kinematics) of the strain wave gear is different than that of planetary or spur gearing. The teeth engage in a manner that lets up to 30% of the teeth (60 for a 100:1 gear ratio) engage at all times. This contrasts with maybe six teeth for a planetary gear, and one or two teeth for a spur gear.

In addition, the kinematics enable the gear teeth to engage on both sides of the tooth flank. Backlash is the difference between the tooth space and tooth width, and this difference is zero in strain-wave gearing.

Industrial-Gearbox.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#774 2020-08-30 01:02:46

ganesh
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Re: Miscellany

652) Jeans

Jeans, also called Blue Jeans, Dungarees, Denims, or Levi’s, trousers originally designed in the United States by Levi Strauss in the mid-19th century as durable work clothes, with the seams and other points of stress reinforced with small copper rivets. They were eventually adopted by workingmen throughout the United States and then worldwide.

Jeans are particularly identified as a standard item of “Western” apparel worn by the American cowboy. After the mid-20th century, various adaptations became internationally a characteristic part of clothing for both men and women.

Denim

Denim, durable twill-woven fabric with coloured (usually blue) warp and white filling threads; it is also woven in coloured stripes. The name is said to have originated in the French serge de Nîmes. Denim is yarn-dyed and mill-finished and is usually all-cotton, although considerable quantities are of a cotton-synthetic fibre mixture. Decades of use in the clothing industry, especially in the manufacture of overalls and trousers worn for heavy labour, have demonstrated denim’s durability. This quality also made denim serviceable for leisure wear in the late 20th century.

Levi Strauss & Co.

Levi Strauss & Co., world’s largest maker of pants, noted especially for its blue denim jeans called Levi’s (registered trademark). Its other products include tailored slacks, jackets, hats, shirts, skirts, and belts, and it licenses the manufacture of novelty items. The company is headquartered in San Francisco.

The company traces its origin to Levi Strauss (1829–1902), a Bavarian immigrant who arrived in San Francisco in 1850 during the Gold Rush, bringing dry goods for sale to miners. Hearing of the miners’ need for durable pants, Strauss hired a tailor to make garments out of tent canvas. Later, denim was substituted, and copper riveting was added to pocket seams. A merchandising partnership of Strauss and his two brothers, Jonas and Louis, was formed in 1853.

After Strauss’s death in 1902, leadership of the company passed to four nephews and, after 1918, to in-laws, the Haas family. The company’s most spectacular growth occurred after 1946, when it decided to abandon wholesaling and concentrate on manufacturing clothing under its own label. By the 1960s, Levi’s and other jeans—once worn chiefly by American cowboys—had become popular worldwide. When the company went public in 1971, it was operating in 50 countries.

In 1985 the Haas family, along with other descendants of Levi Strauss, staged a leveraged buyout that returned the company to private ownership. In 1986 Levi Strauss & Co. introduced in the United States a new line of casual pants called Dockers; the brand was released in Europe in 1994.

During the 1980s, because of increasing competition and financial difficulties, Levi Strauss closed nearly 60 of its U.S. manufacturing plants and began shifting production overseas. In 1990 a class-action lawsuit against the company alleged that it had closed its plant in San Antonio, Texas, and relocated it to Costa Rica to avoid paying pension, disability, and other benefits to its workers; the case was eventually dismissed. The last two U.S. manufacturing plants were closed in 2004. However, in the following decade some production of Levi Strauss items returned to the United States.

In 1991 an investigation revealed that some products that Levi Strauss had represented as made in the United States were actually manufactured in the Northern Mariana Islands (a U.S. commonwealth) by Chinese labourers working in illegal sweatshop conditions. The subcontractor managing the island factories was fined nearly $10 million by the U.S. government, and Levi Strauss subsequently took steps to improve labour standards and inspection practices for its offshore suppliers.

In 1996 Levi’s Vintage Clothing (LVC)—a line of reproductions of clothing items from the Levi Strauss Archives—was introduced worldwide. The company later launched (2003) the Signature by Levi Strauss & Co. brand, a more affordable line of jeans and casual wear, and in 2007 Levi Strauss partnered with the French company ModeLabs Group to develop a series of Levi’s-branded mobile telephones.

Despite these moves, sales stagnated, and in 2011 Levi Strauss hired Chip Bergh as CEO. He was credited with turning the company around as he instituted various changes, such as modernizing its e-commerce division and expanding overseas markets. In March 2019 Levi Strauss went public again, and its IPO raised more than $620 million.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#775 2020-09-01 00:42:21

ganesh
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Re: Miscellany

653) Magna Carta

Magna Carta, English Great Charter, charter of English liberties granted by King John on June 15, 1215, under threat of civil war and reissued, with alterations, in 1216, 1217, and 1225. By declaring the sovereign to be subject to the rule of law and documenting the liberties held by “free men,” the Magna Carta provided the foundation for individual rights in Anglo-American jurisprudence.

Origin Of The Magna Carta

With his conquest of England in 1066, William I secured for himself and his immediate successors a position of unprecedented power. He was able to dominate not only the country but also the barons who had helped him win it and the ecclesiastics who served the English church. He forced Pope Alexander II to be content with indirect control over the church in a land that the papacy hitherto had regarded as bound by the closest ties to Rome. William’s son Henry I—whose accession (1100) was challenged by his eldest brother, Robert, duke of Normandy—was compelled to make concessions to the nobles and clergy in the Charter of Liberties, a royal edict issued upon his coronation. His successor, Stephen (1135), whose hold on the throne was threatened by Henry I’s daughter Matilda, again issued a solemn charter (1136) with even more generous promises of good government in church and state. Matilda’s son Henry II also began his reign (1154) by issuing a solemn charter promising to restore and confirm the liberties and free customs that King Henry, his grandfather, had granted “to God and holy church and all his earls, barons and all his men.” There developed, in fact, through the 12th century a continuous tradition that the king’s coronation oath should be strengthened by written promises stamped with the king’s seal.

Although the volume of common law increased during that period, in particular during Henry II’s reign (which ended in 1189), no converse definition had been secured in regard to the financial liabilities of the baronage to the crown. The baronage also had no definition of the rights of justice that they held over their own subjects. As the Angevin administration became ever more firmly established with learned judges, able financiers, and trained clerks in its service, the baronage as a whole became ever more conscious of the weakness of its position in the face of the agents of the crown. Compounding discontent among the nobility were tax increases during Richard I’s reign (1189–99), which resulted from his Crusade, his ransom, and his war with France. John was confronted with those myriad challenges upon his rise to the throne in 1199. His position, already precarious, was made even weaker because of the rival claim of his nephew Arthur of Brittany and the determination of Philip II of France to end the English hold on Normandy.

Unlike his predecessors, John did not issue a general charter to his barons at the beginning of his reign. At Northampton, however, Archbishop of Canterbury Hubert Walter, royal adviser William Marshal, and justiciar Geoffrey Fitzpeter summoned the nobility and promised, on behalf of the king (who was still in France), that he would render to each his rights if they would keep faith and peace with him. As early as 1201, however, the earls were refusing to cross the English Channel in the king’s service unless he first promised them “their rights.” In 1205, in the face of a threat of invasion from France, the king was compelled to swear that he would preserve the rights of the kingdom unharmed. After the loss of Normandy in 1204, John was forced to rely on English resources alone, and the crown began to feel a new urgency in the matter of revenue collection. Royal demands for scutage (money paid in lieu of military service) became more frequent. The quarrel with Pope Innocent III over the election of Stephen Langton to the see of Canterbury resulted in a papal interdict (1208–13) and left the English church defenseless in the face of John’s financial demands. The excommunication of the king in 1209 deprived him of some of his ablest administrators. It is not surprising then that when peace with the church was made and Langton became archbishop of Canterbury, he emerged as a central figure in the baronial unrest. Indeed, it was Langton who advised that the demand for a solemn grant of liberties from the king be founded on the coronation charter of Henry I.

Great Charter Of 1215

A detailed account of the months preceding the sealing of the Magna Carta has been preserved by the historians of St. Albans abbey, where an initial draft of the charter was read in 1213. Many, although not all, of the documents issued immediately before the charter have survived either in the original or as official transcripts. From those records, it is clear that King John had already realized that he would have to grant free election to ecclesiastical offices and meet the barons’ general demands. It is equally clear that Langton and the most-influential earl, William Marshal, earl of Pembroke, had considerable difficulty in bringing the most-extreme members of the baronage to a frame of mind in which they would negotiate. Those nobles wanted to fight, although it is not clear what use they would have made of a military victory in 1215.

On June 15, 1215, the document known as the Articles of the Barons was at last agreed upon, and to it the king’s great seal was set. It became the text from which the draft of the charter was hammered out in the discussions at Runnymede (beside the River Thames, between Windsor and Staines, now in the county of Surrey), and the final version of the Magna Carta was accepted by the king and the barons on June 19. The charter was a compromise, but it also contained important clauses designed to bring about reforms in judicial and local administration.

Much explosive material is set out in the Magna Carta, which was sealed by King John “in the meadow called Ronimed between Windsor and Staines on the fifteenth day of June in the seventeenth year of our reign.” The remarkable fact is not that war broke out between John and his barons in the following months but that the king had ever been brought to agree to the sealing of such a document at all. That the king genuinely wished to avoid civil war, that he was prepared to accede to reasonable demands for a statement of feudal law, and that he had a basic desire to give good government to his subjects are all strikingly shown by his submission to clauses that, in effect, authorized his subjects to declare war on their king.

Clause 61 of the 1215 charter called upon the barons to choose 25 representatives from their number to serve as a “form of security” to ensure the preservation of the rights and liberties that had been enumerated. John’s dissatisfaction with that clause and its implementation was recorded by chronicler Matthew Paris, and historians since that time have questioned its genesis. Was clause 61 proposed by Langton as a method of progressing toward a limited monarchy, or did it come from the barons as a way of expressing their feudal right of formal defiance in the face of a lord who had broken a contract? Whatever its origin, that clause is of interest because it illustrates the way that the western European elite were talking and thinking about kingship in 1215. Although clause 61 was omitted from reissued versions of the charter, after the deposing of King Henry III during the Barons’ War (1264), it served as the model for an even harsher attempt to control the king.

Reissues Of 1216, 1217, And 1225

King John died on October 18/19, 1216, while Louis of France (afterward Louis VIII), supported by rebellious English barons, was trying to gain control of England. One of the first acts of the council of John’s young successor, Henry III, was to reissue the Magna Carta on November 12 in the hope of recalling men to their allegiance to the rightful king. The charter of 1216 was considerably shorter than its predecessor—42 clauses versus 63 in the 1215 document—as the council had omitted clauses dealing with purely temporary and political matters as well as those that might limit its own power to raise money or forces to carry on the war. The church, while keeping a general promise of freedom, lost its specific guarantee of free election to office. Even in that moment of danger, the council did not forget one main purpose of the charter: to provide a definitive statement of feudal law. It tried to address points in doubt, such as specific matters of inheritance law and the precise year at which an heir should attain his majority (age 21). Instead of the “form of security,” the council stated that all omissions were postponed for future consideration. They were never replaced.

When the charter was reissued for the second time, in the autumn of 1217, the council had reconsidered it clause by clause. They made further verbal changes for the sake of clarity and accuracy. They modified the promise of assize justices’ visiting every shire four times a year to the more-practical suggestion of an annual visit. More difficult cases would be heard by the bench judges. A widow’s rights in issues of inheritance were more clearly set out. The vexed question of scutage, omitted altogether in 1216, was in 1217 glossed over by a promise to revert to the practice of Henry II. That the council in 1217 still wished to make the charter an authoritative statement of English law and practice is shown by the inclusion of three new clauses, each dealing with a question of the day: the possibility of a land owner’s giving away so much of his holdings that he could not perform his service to his lord from the remainder (clause 39); the conduct of the shire court and view of frankpledge, a mutual responsibility to keep the peace (clause 42); and a first attempt at mortmain legislation (clause 43). Section VII of the 1215 document was set aside in favour of a separate forest charter that dealt with the use and scope of royal land holdings. That attempt at compartmentalization demonstrates that the council was beginning to realize that a full statement of the law on a single important subject could not be compressed into a general charter of liberties, no matter how long that charter might be.

In 1223 Pope Honorius III declared that Henry III was of age to make valid grants, and the young king reissued the Magna Carta two years later. That version reflected only minor changes from the 1217 document, and it seems probable that the council had concluded that maintaining the charter as an evolving code of law was impracticable.

Historical Significance Of The Magna Carta

By the time of the 1225 reissue, the Magna Carta had become more than a sober statement of the common law; it was a symbol in the battle against oppression. It had been read so many times in shire courts throughout the land that memorable phrases would be invoked in later documents, and whenever liberty seemed in danger, men spoke of the charter as their defense. The influence of the Magna Carta in England—and, later, in its colonies—had come not from the detailed expression of the feudal relationship between lord and subject but from the more-general clauses in which every generation could see its own protection. In England the Petition of Right in 1628 and the Habeas Corpus Act of 1679 looked directly back to clause 39 of the 1215 charter, which read:

‘No free man shall be arrested or imprisoned or disseised or outlawed or exiled or in any way victimised, neither will we attack him or send anyone to attack him, except by the lawful judgment of his peers or by the law of the land.’

Indeed, this passage would serve as the foundational expression of the concept of due process in Anglo-American jurisprudence. In the 17th century, when England’s North American colonies were shaping their own fundamental laws, the words of the Magna Carta were worked into them. The basic rights embodied in the Constitution of the United States of America (1789) and the Bill of Rights (1791) echo the charter, and the Fourteenth Amendment (1868) can trace its ancestry to the Magna Carta as well.

The essential virtue of the Magna Carta, which has made it comparable in historical importance to the Twelve Tables of ancient Rome, lies not in any individual clause or group of clauses but in the solemn circumstances of its first granting and the comprehensive nature of that grant. Thus, the Magna Carta that is commonly remembered is the Magna Carta of King John, and the date that always has been commemorated as its granting is 1215.

That many clauses were omitted from the charter as it finally appeared on the statute rolls and that new ones had been inserted and some original clauses redrafted have made no difference in the collective memory of this venerable document. Nevertheless, in trying to estimate the influence of the charter on constitutional development in England and elsewhere, it should be borne in mind that, while the drama has never faded from the field of Runnymede, the actual phrases studied by those who fought oppression in 17th-century England or 18th-century America came immediately from the 1217 charter.

Surviving Copies Of The Magna Carta

There still exist four original copies of the charter of 1215, two of them held by the cathedral churches in which they were originally deposited—Lincoln and Salisbury—and the other two in the British Library. The Lincoln charter was considered the most nearly perfect and was reproduced in the Statutes of the Realm in 1810. Lincoln also possesses the forest charter of 1225. Durham Cathedral holds the 1216, 1217, and 1225 versions of the Magna Carta as well as the forest charters of 1217 and 1225. The Wiltshire copy of the 1225 charter was deposited at Lacock Abbey and survives. The four extant “originals” of the 1215 Magna Carta were assembled in one place for the first time in February 2015 as part of a British Library commemoration of the 800th anniversary of the charter’s issue.

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