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#901 2021-01-10 00:20:46

ganesh
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Re: Miscellany

879) Guinea pig

Guinea pig, (Cavia porcellus), a domesticated species of South American rodent belonging to the cavy family (Caviidae). It resembles other cavies in having a robust body with short limbs, large head and eyes, and short ears. The feet have hairless soles and short sharp claws. There are four toes on the forefeet and three on the hind feet. Several breeds of domesticated guinea pigs exist, which are sometimes grouped by coat texture and hair length. The term guinea pig is also used colloquially to refer to a person who serves as a test subject in an experiment.

Among rodents, domestic guinea pigs are fairly large, weighing 500 to 1,500 grams (roughly 1 to 3 pounds) and having a body 20 to 40 cm (8 to 16 inches) long. The tail is not visible externally. There is a crest of longer hairs at the neck, but length and texture of the fur vary from smooth (short or long) to coarse and short or long and silky. Coloration is extremely variable: the coat may be white, cream, tan, reddish or chocolate brown, black, or a combined pattern.

Guinea pigs eat vegetation and do not require water to drink if supplied with sufficiently moist food, but they must have water if fed dry commercial food. They breed all year in captivity. Females bear up to 13 young per litter (4 is average); gestation takes 68 days. Although the young can scamper about and eat solid food the day they are born, they are not fully weaned for about three weeks. Females mature in two months, males in three, and captive guinea pigs live up to eight years, although three to five is typical.

No natural population of this species exists in the wild. Guinea pigs were apparently domesticated more than 3,000 years ago in Peru, coinciding with humans’ transition from a nomadic to an agricultural lifestyle. The Incas kept guinea pigs, and the animals were bred during the same period by various people who lived along the Andes Mountains from northwestern Venezuela to central Chile. These rodents remain a sustainable food source for the native peoples of Ecuador, Peru, and Bolivia, who either keep them in their homes or allow them to scavenge freely both indoors and out. Guinea pigs were taken to Europe in the 16th century, and since the 1800s they have been popular as pets. They are also used internationally as laboratory animals for studies of anatomy, nutrition, genetics, toxicology, pathology, serum
development, and other research programs.

The origin of the colloquial name guinea pig is a subject of much debate. The first part of the name may have been derived from the price of the animal in 16th- and 17th-century England—that is, possibly one guinea—or it may have arisen from the animals’ being carried to European markets after first being transferred to ships in ports in Guinea. The moniker could also have originated with a mispronounced form of the word Guiana, the name of the region where some guinea pigs were collected. Another possible etymology is from the name of the class of ships—the Guineamen—that transported the animal. These were vessels that made port in West Africa as part of the transatlantic slave trade. The second part of the name also originated with Europeans, who compared the squealing sound the animal made (as well as the taste of its cooked flesh) to that of a pig.

There are five nondomesticated members of the genus Cavia that are also called guinea pigs: the Brazilian guinea pig (C. aperea) found from Colombia, Venezuela, and the Guianas south to northern Argentina; the shiny guinea pig (C. fulgida), inhabiting eastern Brazil; the montane guinea pig (C. tschudii), ranging from Peru to northern Chile and northwestern Argentina; the greater guinea pig (C. magna), occurring in southeastern Brazil and Uruguay; and the Moleques do Sul guinea pig (C. intermedia), which is limited to an island in the Moleques do Sul archipelago off the southern coast of Brazil. Breeding and molecular studies suggest that the domestic guinea pig was derived from one of the wild Brazilian, shiny, or montane species.

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#902 2021-01-11 00:16:44

ganesh
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Re: Miscellany

880) Sheep

Sheep, (Ovis aries), species of domesticated ruminant (cud-chewing) mammal, raised for its meat, milk, and wool. The sheep is usually stockier than its relative the goat (genus Capra); its horns, when present, are more divergent; it has scent glands in its face and hind feet; and the males lack the beards of goats. Sheep usually have short tails. In all wild species of sheep, the outer coat takes the form of hair, and beneath this lies a short undercoat of fine wool that has been developed into the fleece of domesticated sheep. Male sheep are called rams, the females ewes, and immature animals lambs. Mature sheep weigh from about 35 to as much as 180 kg (80 to 400 pounds).

A sheep regurgitates its food and chews the cud, thus enabling its four separate stomach compartments to thoroughly digest the grasses and other herbage that it eats. The animals prefer grazing on grass or legume vegetation that is short and fine, though they will also consume high, coarse, or brushy plants as well. They graze plants closer to the root than do cattle, and so care must be taken that sheep do not overgraze a particular range. Sheep are basically timid animals who tend to graze in flocks and are almost totally lacking in protection from predators. They mature at about one year of age, and many breed when they reach the age of about one and a half years. Most births are single, although sheep do have twins on occasion. The lambs stop suckling and begin to graze at about four or five months of age.

Sheep were first domesticated from wild species of sheep at least 5000 BCE, and their remains have been found at numerous sites of early human habitation in the Middle East, Europe, and Central Asia. Domesticated sheep are raised for their fleece (wool), for milk, and for meat. The flesh of mature sheep is called mutton; that of immature animals is called lamb. There were estimated to be more than one billion sheep in the world in the early 21st century. The major national producers are Australia, New Zealand, China, India, the United States, South Africa, Argentina, and Turkey. Countries that have large areas of grassland are the major producers.

Domestic sheep differ from their wild progenitors and among themselves in conformation, quantity and quality of fleece, colour, size, milk production, and other characteristics. Most breeds of domesticated sheep produce wool, while a few produce only hair, and wild sheep grow a combination of wool and hair. Several hundred different breeds of sheep have been developed to meet environmental conditions influenced by latitudes and altitudes and to satisfy human needs for clothing and food. Breeds of sheep having fine wool are generally raised for wool production alone, while breeds with medium or long wool or with only hair are generally raised for meat production. Several crossbreeds have been developed that yield both wool and meat of high quality, however. Of the more than 200 breeds of sheep in the world, the majority are of limited interest except in local areas.

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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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#903 2021-01-12 00:44:16

ganesh
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Re: Miscellany

881) Marathon

Marathon, long-distance footrace first held at the revival of the Olympic Games in Athens in 1896. It commemorates the legendary feat of a Greek soldier who, in 490 BC, is supposed to have run from Marathon to Athens, a distance of about 40 km (25 miles), to bring news of the Athenian victory over the Persians and then expired. The story of this messenger from the Battle of Marathon was later conflated with the story of another Greek soldier, Pheidippides, who ran from Athens to Sparta in advance of the fighting. Appropriately, in 1896 the first modern marathon winner was a Greek, Spyridon Louis.

In 1924 the Olympic marathon distance was standardized at 42,195 metres (26 miles 385 yards). This was based on a decision of the British Olympic Committee to start the 1908 Olympic race from Windsor Castle and finish it in front of the royal box in the stadium at London. The marathon was added to the women’s Olympic program in 1984.

After the Olympic Games championship, one of the most coveted honours in marathon running is victory in the Boston Marathon, held annually since 1897. It draws athletes from all parts of the world and in 1972 became the first major marathon to officially allow women to compete. Other premiere marathons are held in London, Chicago, Berlin, New York City, Tokyo, and Amsterdam. Marathons are not held on the track but on roads, and, despite the fact that courses are not of equal difficulty, the International Association of Athletics Federations (IAAF) does list world records for the marathon and also for the half-marathon. World-record times in the marathon steadily declined over the course of the 20th century from slightly under three hours to slightly more than two hours.

It was long considered necessary for a runner to prepare for a marathon by training over that distance. At the 1952 Olympic Games, however, Czech Emil Zátopek set an Olympic record of 2 hours 23 minutes 3.2 seconds, even though he had never run the distance before. In the decades following, other first-time marathoners also won premiere events and set records at the distance. By the late 20th century, road racing, and marathon running in particular, had grown to become a recreational activity with broad appeal. Ultramarathons, which are neither Olympic nor IAAF events, are longer races based on a specific distance or an allotted time period for competition, such as a 12-hour race.

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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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#904 2021-01-13 00:02:54

ganesh
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Re: Miscellany

882) Cornea

Cornea, dome-shaped transparent membrane about 12 mm (0.5 inch) in diameter that covers the front part of the eye. Except at its margins, the cornea contains no blood vessels, but it does contain many nerves and is very sensitive to pain or touch. It is nourished and provided with oxygen anteriorly by tears and is bathed posteriorly by aqueous humour. It protects the pupil, the iris, and the inside of the eye from penetration by foreign bodies and is the first and most powerful element in the eye’s focusing system. As light passes through the cornea, it is partially refracted before reaching the lens. The curvature of the cornea, which is spherical in infancy but changes with age, gives it its focusing power; when the curve becomes irregular, it causes a focusing defect called astigmatism, in which images appear elongated or distorted.

The cornea itself is composed of multiple layers, including a surface epithelium, a central, thicker stroma, and an inner endothelium. The epithelium (outer surface covering) of the cornea is an important barrier to infection. A corneal abrasion, or scratch, most often causes a sensation of something being on the eye and is accompanied by intense tearing, pain, and light sensitivity. Fortunately, the corneal epithelium is able to heal quickly in most situations.

The collagen fibres that make up the corneal stroma (middle layer) are arranged in a strictly regular, geometric fashion. This arrangement has been shown to be the essential factor resulting in the cornea’s transparency. When the cornea is damaged by infection or trauma, the collagen laid down in the repair processes is not regularly arranged, with the result that an opaque patch or scar may occur. If the clouded cornea is removed and replaced by a healthy one (i.e., by means of corneal transplant), usually taken from a deceased donor, normal vision can result.

The innermost layer of the cornea, the endothelium, plays a critical role in keeping the cornea from becoming swollen with excess fluid. As endothelial cells are lost, new cells are not produced; rather, existing cells expand to fill in the space left behind. Once loss of a critical number of endothelial cells has occurred, however, the cornea can swell, causing decreased vision and, in severe cases, surface changes and pain. Endothelial cell loss can be accelerated via mechanical trauma or abnormal age-related endothelial cell death (called Fuchs endothelial dystrophy). Treatment may ultimately require corneal transplant.

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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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#905 2021-01-14 00:34:12

ganesh
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Re: Miscellany

883) Iris

Iris, in anatomy, the pigmented muscular curtain near the front of the eye, between the cornea and the lens, that is perforated by an opening called the pupil. The iris is located in front of the lens and ciliary body and behind the cornea. It is bathed in front and behind by a fluid known as the aqueous humour. The iris consists of two sheets of smooth muscle with contrary actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina. The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, whereas the dilator muscle of the iris expands the opening when it contracts. The amount of pigment contained in the iris determines eye colour. When there is very little pigment, the eye appears blue. With increased pigment, the shade becomes deep brown to black. Inflammation of the iris is termed iritis or anterior uveitis, a condition that commonly has no determinable cause. As a result of inflammation, the iris sticks to the lens or the cornea, blocking the normal flow of fluid in the eye. Complications of iritis include secondary glaucoma and blindness; treatment usually involves topical steroid eyedrops.

The-human-eye-with-the-cornea-lens-optic-nerve-retina-and-fovea-all-indicated.png


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

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#906 2021-01-15 00:45:47

ganesh
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Re: Miscellany

884) Placenta

Placenta, in zoology, the vascular (supplied with blood vessels) organ in most mammals that unites the fetus to the uterus of the mother. It mediates the metabolic exchanges of the developing individual through an intimate association of embryonic tissues and of certain uterine tissues, serving the functions of nutrition, respiration, and excretion.

All of the fetal membranes function by adapting the developing fetus to the uterine environment. Lying in the chorionic cavity (a thin liquid-filled space) between two membranous envelopes (chorion and amnion) is a small balloon-like sac, yolk sac, or vitelline sac, attached by a delicate strand of tissue to the region where the umbilical cord (the structure connecting the fetus with the placenta) leaves the amnion. Two large arteries in the umbilical cord radiate from the attachment of the cord on the inner surface of the placenta and divide into small arteries that penetrate outward into the depths of the placenta through hundreds of branching and interlacing strands of tissue known as villi. The chorionic villi cause the mother’s blood vessels in their vicinity to rupture, and the villi become bathed directly in maternal blood. The constant circulation of fetal and maternal blood and the very thin tissue separation of fetal blood in the capillaries from maternal blood bathing the villi provide a mechanism for efficient interchange of blood constituents between the maternal and fetal bloodstreams without (normally) allowing any opportunity for the blood of one to pour across into the blood vessels of the other.

Nutrients, oxygen, and antibodies (proteins formed in response to a foreign substance, or antigen), as well as other materials in the mother’s blood, diffuse into the fetal blood in the capillaries of the villi, and nitrogenous wastes and carbon dioxide diffuse out of these capillaries into the maternal blood circulation. The purified and enriched blood in the capillaries of the villi is collected into fetal veins, which carry it back to the inner surface of the placenta and collect at the attachment of the cord to form the umbilical vein. This vein enters the cord alongside the two arteries and carries the blood back to the fetus, thus completing the circuit to and from the placenta.

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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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#907 2021-01-16 00:25:37

ganesh
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Re: Miscellany

885) Plasma

Plasma, also called blood plasma, the liquid portion of blood. Plasma serves as a transport medium for delivering nutrients to the cells of the various organs of the body and for transporting waste products derived from cellular metabolism to the kidneys, liver, and lungs for excretion. It is also a transport system for blood cells, and it plays a critical role in maintaining normal blood pressure. Plasma helps to distribute heat throughout the body and to maintain homeostasis, or biological stability, including acid-base balance in the blood and body.

Plasma is derived when all the blood cells—red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes)—are separated from whole blood. The remaining straw-coloured fluid is 90–92 percent water, but it contains critical solutes necessary for sustaining health and life. Important constituents include electrolytes such as sodium, potassium, chloride, bicarbonate, magnesium, and calcium. In addition, there are trace amounts of other substances, including amino acids, vitamins, organic acids, pigments, and enzymes. Hormones such as insulin, corticosteroids, and thyroxine are secreted into the blood by the endocrine system. Plasma concentrations of hormones must be carefully regulated for good health. Nitrogenous wastes (e.g., urea and creatinine) transported to the kidney for excretion increase markedly with renal failure.

Plasma contains 6–8 percent proteins. One critical group is the coagulation proteins and their inhibitors, synthesized primarily in the liver. When blood clotting is activated, fibrinogen circulating in the blood is converted to fibrin, which in turn helps to form a stable blood clot at the site of vascular disruption. Coagulation inhibitor proteins help to prevent abnormal coagulation (hypercoagulability) and to resolve clots after they are formed. When plasma is allowed to clot, fibrinogen converts to fibrin, trapping the cellular elements of blood. The resulting liquid, devoid of cells and fibrinogen, is called serum. Biochemical testing of plasma and serum is an important part of modern clinical diagnosis and treatment monitoring. High or low concentrations of glucose in the plasma or serum help to confirm serious disorders such as diabetes mellitus and hypoglycemia. Substances secreted into the plasma by cancers may indicate an occult malignancy; for instance, an increased concentration of prostate-specific antigen (PSA) in a middle-aged asymptomatic man may indicate undiagnosed prostate cancer.

Serum albumin, another protein synthesized by the liver, constitutes approximately 60 percent of all of the plasma proteins. It is very important in maintaining osmotic pressure in the blood vessels; it is also an important carrier protein for a number of substances, including hormones. Other proteins called alpha and beta globulins transport lipids such as cholesterol as well as steroid hormones, sugar, and iron.

The gamma globulins, or immunoglobulins, are an important class of proteins that are secreted by B lymphocytes of the immune system. They include most of the body’s supply of protective antibodies produced in response to specific viral or bacterial antigens. Cytokines are proteins synthesized by cells of various organs and by cells found in the immune system and bone marrow in order to maintain normal blood cell formation (hematopoiesis) and regulate inflammation. For example, one cytokine called erythropoietin, synthesized by specialized kidney cells, stimulates bone marrow blood progenitor cells to produce red blood cells. Other cytokines stimulate the production of white blood cells and platelets. Another protein system in the plasma, called complement, is important in mediating appropriate immune and inflammatory responses to a variety of infectious agents.

The electrolytes and acid-base system found in the plasma are finely regulated. For example, potassium is normally present in plasma in a concentration of only 4 milliequivalents per litre. A slight rise in plasma potassium (to 6–7 milliequivalents per litre) can result in death. Likewise, sodium, chloride, bicarbonate, calcium, and magnesium levels in the plasma must be precisely maintained within a narrow range. Smaller molecules such as sodium, potassium, glucose, and calcium are primarily responsible for the concentration of dissolved particles in the plasma. However, it is the concentration of much larger proteins (especially albumin) on either side of semipermeable membranes such as the endothelial cells lining the capillaries that creates crucial pressure gradients necessary to maintain the correct amount of water within the intravascular compartment and, therefore, to regulate the volume of circulating blood. So, for example, patients who have kidney dysfunction or low plasma protein concentrations (especially low albumin) may develop a migration of water from the vascular space into the tissue spaces, causing edema (swelling) and congestion in the extremities and vital organs, including the lungs.

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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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#908 2021-01-17 00:33:29

ganesh
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Re: Miscellany

886) Eyeball

Eyeball, spheroidal structure containing sense receptors for vision, found in all vertebrates and constructed much like a simple camera. The eyeball houses the retina—an extremely metabolically active layer of nerve tissue made up of millions of light receptors (photoreceptors)—and all of the structures needed to focus light onto it. The sclera, the tough protective outer shell of the eyeball, is composed of dense fibrous tissue that covers four-fifths of the eyeball and provides attachments for the muscles that move the eye. The sclera is itself covered anteriorly by the conjunctiva, a transparent mucous membrane that prevents the eye from drying out. At the front of the eye, the tear film covers the transparent cornea, the “window” through which light passes into the eye. Working in concert with the aqueous humour behind it, the cornea provides the greatest focusing power of the eye. However, unlike the lens, the shape and focusing power of the cornea are not adjustable. Other important structures in the eyeball include the iris and the lens. Much of the eyeball is filled with a transparent gel-like material, called the vitreous humour, that helps to maintain the spheroidal shape.

Immediately beneath the sclera is an underlying vascular layer, called the uvea, that supplies nutrients to many parts of the eye. One component of the uvea is the ciliary body, a muscular structure located behind the iris that alters the shape of the lens during focusing and produces the aqueous humour that bathes the anterior chamber. The other components of the uvea are the iris and the choroid. The choroid is a highly vascular tissue layer that provides blood to the outer layers of the retina that lie over it.

The cornea, where the focusing process begins, is curved to a much greater extent than the rest of the eyeball. Defects in corneal curvature cause a distortion of vision known as astigmatism. Behind the cornea is the anterior chamber, which extends posteriorly to the plane of the iris and pupil. It is filled with a watery fluid called the aqueous humour. The iris is a doughnut-shaped, muscular curtain that opens and closes to regulate the amount of light entering the eye through the pupil, the opening at the iris’s centre. The aqueous humour flows through the pupil from the posterior chamber (a small space between the iris and the lens) to the anterior chamber and out of the eye through the trabecular meshwork and Schlemm’s canal, which encircles the peripheral iris. Some aqueous humour also exits the eye directly through the ciliary body. The ciliary muscle attachments and the lens separate the aqueous humour in front from the vitreous humour behind.

The shape of the lens is controlled by the action of the ciliary body, altering the focusing power of the lens as needed. The cornea and lens focus an image onto the retina at the back of the eye. If the image is projected too far in front of the retina, it causes the visual defect called myopia, or nearsightedness. If the image is theoretically focused “behind” the retina, the result is hyperopia, or farsightedness. If no deformation of the lens is present, the image is projected onto the fovea, a structure near the centre of the retina that contains a large number of cone photoreceptors and that provides the sharpest vision. When stimulated by light, retinal photoreceptor cells send signals to neighbouring cells in the retina that then relay the signals through the optic nerve to the visual centres of the brain.

eye-diagram-2.png


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

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#909 2021-01-18 00:23:13

ganesh
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Re: Miscellany

887) Blind spot

Blind spot, small portion of the visual field of each eye that corresponds to the position of the optic disk (also known as the optic nerve head) within the retina. There are no photoreceptors (i.e., rods or cones) in the optic disk, and, therefore, there is no image detection in this area. The blind spot of the right eye is located to the right of the centre of vision and vice versa in the left eye. With both eyes open, the blind spots are not perceived because the visual fields of the two eyes overlap. Indeed, even with one eye closed, the blind spot can be difficult to detect subjectively because of the ability of the brain to “fill in” or ignore the missing portion of the image.

The optic disk can be seen in the back of the eye with an ophthalmoscope. It is located on the nasal side of the macula lutea, is oval in shape, and is approximately 1.5 mm (0.06 inch) in diameter. It is also the entry point into the eye for major blood vessels that serve the retina. The optic disk represents the beginning of the optic nerve (second cranial nerve) and the point where axons from over one million retinal ganglion cells coalesce. Clinical evaluation of the optic nerve head is critical in the diagnosis and monitoring of glaucoma and other optic neuropathies that may lead to vision loss.

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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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#910 2021-01-19 00:03:28

ganesh
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Re: Miscellany

888) Retina

Retina, layer of nervous tissue that covers the inside of the back two-thirds of the eyeball, in which stimulation by light occurs, initiating the sensation of vision. The retina is actually an extension of the brain, formed embryonically from neural tissue and connected to the brain proper by the optic nerve.

The retina is a complex transparent tissue consisting of several layers, only one of which contains light-sensitive photoreceptor cells. Light must pass through the overlying layers to reach the photoreceptor cells, which are of two types, rods and cones, that are differentiated structurally by their distinctive shapes and functionally by their sensitivity to different kinds of light. Rods predominate in nocturnal animals and are most sensitive to reduced light intensities; in humans they provide night vision and aid in visual orientation. Cones are more prominent in humans and those animals that are active during the day and provide detailed vision (as for reading) and colour perception. In general, the more cones per unit area of retina, the finer the detail that can be discriminated by that area. Rods are fairly well distributed over the entire retina, but cones tend to concentrate at two sites: the fovea centralis, a pit at the rear of the retina, which contains no rods and has the densest concentration of cones in the eye, and the surrounding macula lutea, a circular patch of yellow-pigmented tissue about 5 to 6 mm (0.2 to 0.24 inch) in diameter.

When light enters the eye, it passes through the cornea and the lens and is refracted, focusing an image onto the retina. Light-sensitive molecules in the rods and cones react to specific wavelengths of light and trigger nerve impulses. Complex interconnections (synapses) between and within retinal cell layers assemble these impulses into a coherent pattern, which in turn is carried through the optic nerve to the visual centres of the brain, where they are further organized and interpreted.

eye-anatomy-retina.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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#911 2021-01-20 00:11:05

ganesh
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Re: Miscellany

889) Embryo

Human And Animal

Embryo, the early developmental stage of an animal while it is in the egg or within the uterus of the mother. In humans the term is applied to the unborn child until the end of the seventh week following conception; from the eighth week the unborn child is called a fetus.

A brief treatment of embryonic development follows.

In organisms that reproduce mating, the union of a male gamete and female gamete results in a zygote, or fertilized egg, which undergoes a series of divisions called cleavages as it passes down the fallopian tube. After several cleavages have taken place, the cells form a hollow ball called a blastula. In most mammals the blastula attaches itself to the uterine lining, thus stimulating the formation of a placenta, which will transfer nutrients from the mother to the growing embryo. In lower animals the embryo is nourished by the yolk.

By the process of gastrulation, the embryo differentiates into three types of tissue: the ectoderm, producing the skin and nervous system; the mesoderm, from which develop connective tissues, the circulatory system, muscles, and bones; and the endoderm, which forms the digestive system, lungs, and urinary system. Mesodermal cells migrate from the surface of the embryo to fill the space between the other two tissues through an elongated depression known as the primitive streak. As the embryo develops, the cell layers fold over so that the endoderm forms a long tube surrounded by mesoderm, with an ectodermal layer around the whole.

Nutrients pass from the placenta through the umbilical cord, and the amnion, a fluid-filled membrane, surrounds and protects the embryo. The division of the body into head and trunk becomes apparent, and the brain, spinal cord, and internal organs begin to develop. All of these changes are completed early in embryonic development, by about the fourth week, in humans.

Between the head and the heart, a series of branchial arches, cartilaginous structures that support the gills of fishes and larval amphibians, begin to form. In higher vertebrates these structures form part of the jaw and ear. Limb buds also appear, and by the end of the embryonic stage, the embryo is distinguishable as a representative of its species.

2-Photographs-of-human-embryos-at-five-stages-of-gestation-reproduced-from-the.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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#912 2021-01-21 00:53:31

ganesh
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Re: Miscellany

890) Serum albumin

Serum albumin, protein found in blood plasma that helps maintain the osmotic pressure between the blood vessels and tissues. Serum albumin accounts for 55 percent of the total protein in blood plasma. Circulating blood tends to force fluid out of the blood vessels and into the tissues, where it results in edema (swelling from excess fluid). The colloid nature of albumin—and, to a lesser extent, of other blood proteins called globulins—keeps the fluid within the blood vessels. Albumin also acts as a carrier for two materials necessary for the control of blood clotting: (1) antithrombin, which keeps the clotting enzyme thrombin from working unless needed, and (2) heparin cofactor, which is necessary for the anticlotting action of heparin. The serum albumin level falls and rises in such liver disorders as cirrhosis or hepatitis. Transfusions of serum albumin are used to combat shock and whenever it is necessary to remove excess fluid from the tissues. Similar albumin compounds with other functions occur in plants, animal tissues, egg whites, and milk.

Human serum albumin is the serum albumin found in human blood. It is the most abundant protein in human blood plasma; it constitutes about half of serum protein. It is produced in the liver. It is soluble in water, and it is monomeric.

Albumin transports hormones, fatty acids, and other compounds, buffers pH, and maintains oncotic pressure, among other functions.

Albumin is synthesized in the liver as preproalbumin, which has an N-terminal peptide that is removed before the nascent protein is released from the rough endoplasmic reticulum. The product, proalbumin, is in turn cleaved in the Golgi apparatus to produce the secreted albumin.

The reference range for albumin concentrations in serum is approximately 35–50 g/L (3.5–5.0 g/dL). It has a serum half-life of approximately 21 days. It has a molecular mass of 66.5 kDa.

The gene for albumin is located on chromosome 4 in locus 4q13.3 and mutations in this gene can result in anomalous proteins. The human albumin gene is 16,961 nucleotides long from the putative 'cap' site to the first poly(A) addition site. It is split into 15 exons that are symmetrically placed within the 3 domains thought to have arisen by triplication of a single primordial domain.

Prolonged release of drugs

Cancer is the uncontrolled growth of cells with loss of differentiation and commonly with metastasis. Anticancer drugs are used to control the growth of cancerous cells. Oxaliplatin is a third-generation platinum-derived antineoplastic agent that has been proved effective mainly against advanced colorectal cancer (CRC). Oxaliplatin administration has an acute excitatory and sensitizing effect, including unpleasant cold allodynia in the distal extremities, mouth, and throat and usually associated with muscle cramps. However, the major side effect, a dose-limiting toxicity of the peripheral nerves, affects the sustainability of the planned treatment. Although these acute symptoms resolve within 1 week, severe chronic sensory neuropathy develops with the increasing cumulative dose and is characterized by distal paresthesia and numbness. Drug  delivery  systems  aim to deliver the drugs over an extended duration or at a specific time during treatment. Therefore, the improvement of the efficiency and convenience of a drug release system is of paramount importance. In the past two decades, numerous nanoparticles (NPs) have been extensively explored in order to achieve precisely controlled drug delivery. However, the application and development of high-efficiency drug release system are still limited due to a lacking of simple, stable and efficient drug delivery vehicle. Human serum albumin (HSA) is a highly water-soluble globular monomeric plasma protein with a relative molecular weight of 67 KDa, consisting of 585 amino acid residues, one sulfhydryl group and 17 disulfide bridges. Among nanoparticulate carriers, HSA nanoparticles have long been the center of attention in the pharmaceutical industry due to their ability to bind to various drug molecules, great stability during storage and in vivo usage, no toxicity and antigenicity, biodegradability, reproducibility, scale up of the production process and a better control over release properties. In addition, significant amounts of drug can be incorporated into the particle matrix because of the large number of drug binding sites on the albumin molecule.

Function

i) Maintains oncotic pressure
ii) Transports thyroid hormones
iii) Transports other hormones, in particular, ones that are fat-soluble
iv) Transports fatty acids ("free" fatty acids) to the liver and to myocytes for utilization of energy
v) Transports unconjugated bilirubin
vi) Transports many drugs; serum albumin levels can affect the half-life of drugs. Competition between drugs for albumin binding sites may cause drug interaction by increasing the free fraction of one of the drugs, thereby affecting potency.
vii) Competitively binds calcium ions (Ca2+)
viii) Serum albumin, as a negative acute-phase protein, is down-regulated in inflammatory states. As such, it is not a valid marker of nutritional status; rather, it is a marker of an inflammatory state
ix) Prevents photodegradation of folic acid
x) Prevent pathogenic effects of Clostridium difficile toxins

Measurement

Serum albumin is commonly measured by recording the change in absorbance upon binding to a dye such as bromocresol green or bromocresol purple.

Reference ranges
The normal range of human serum albumin in adults (> 3 y.o.) is 3.5–5.0  g/dL (35–50 g/L). For children less than three years of age, the normal range is broader, 2.9–5.5 g/dL.

Low albumin (hypoalbuminemia) may be caused by liver disease, nephrotic syndrome, burns, protein-losing enteropathy, malabsorption, malnutrition, late pregnancy, artefact, genetic variations and malignancy.

High albumin (hyperalbuminemia) is almost always caused by dehydration. In some cases of retinol (Vitamin A) deficiency, the albumin level can be elevated to high-normal values (e.g., 4.9 g/dL). This is because retinol causes cells to swell with water (this is also the reason too much Vitamin A is toxic). This swelling also likely occurs during treatment with 13-cis retinoic acid (isotretnoin), a pharmaceutical for treating severe acne, amongst other conditions. In lab experiments it has been shown that all-trans retinoic acid down regulates human albumin production.

Pathology

i) Hypoalbuminemia

Hypoalbuminemia means low blood albumin levels. This can be caused by:

a) Liver disease; cirrhosis of the liver is most common
b) Excess excretion by the kidneys (as in nephrotic syndrome)
c) Excess loss in bowel (protein-losing enteropathy, e.g., Ménétrier's disease)
d) Burns (plasma loss in the absence of skin barrier)
e) Redistribution (hemodilution [as in pregnancy], increased vascular permeability or decreased lymphatic clearance)
f) Acute disease states (referred to as a negative acute-phase protein)
g) Malnutrition and wasting
i) Mutation causing analbuminemia (very rare)
j) Anorexia nervosa (most common cause in adolescents)

ii) Hyperalbuminemia

Hyperalbuminemia is an increased concentration of albumin in the blood. Typically, this condition is due to dehydration. Hyperalbuminemia has also been associated with high protein diets.

Medical use

Human albumin solution (HSA) is available for medical use, usually at concentrations of 5–25%.

Human albumin is often used to replace lost fluid and help restore blood volume in trauma, burns and surgery patients. There is no strong medical evidence that albumin administration (compared to saline) saves lives for people who have hypovolaemia or for those who are critically ill due to burns or hypoalbuminaemia. It is also not known if there are people who are critically ill that may benefit from albumin.[20] Therefore, the Cochrane Collaboration recommends that it should not be used, except in clinical trials.

In acoustic droplet vaporization (ADV), albumin is sometimes used as a surfactant. ADV has been proposed as a cancer treatment by means of occlusion therapy.

Human serum albumin may be used to potentially reverse drug/chemical toxicity by binding to free drug/agent.

Human albumin may also be used in treatment of decompensated cirrhosis.

Human serum albumin has been used as a component of a frailty index.

It has not been shown to give better results than other fluids when used simply to replace volume, but is frequently used in conditions where loss of albumin is a major problem, such as liver disease with ascites.

Glycation

It has been known for a long time that human blood proteins like hemoglobin and serum albumin may undergo a slow non-enzymatic glycation, mainly by formation of a Schiff base between ε-amino groups of lysine (and sometimes arginine) residues and glucose molecules in blood (Maillard reaction). This reaction can be inhibited in the presence of antioxidant agents. Although this reaction may happen normally, elevated glycoalbumin is observed in diabetes mellitus.

Glycation has the potential to alter the biological structure and function of the serum albumin protein.

Moreover, the glycation can result in the formation of Advanced Glycation End-Products (AGE), which result in abnormal biological effects. Accumulation of AGEs leads to tissue damage via alteration of the structures and functions of tissue proteins, stimulation of cellular responses, through receptors specific for AGE-proteins, and generation of reactive oxygen intermediates. AGEs also react with DNA, thus causing mutations and DNA transposition. Thermal processing of proteins and carbohydrates brings major changes in allergenicity. AGEs are antigenic and represent many of the important neoantigens found in cooked or stored foods. They also interfere with the normal product of nitric oxide in cells.

Although there are several lysine and arginine residues in the serum albumin structure, very few of them can take part in the glycation reaction.

Oxidation

The albumin is the predominant protein in most body fluids, its Cys34 represents the largest fraction of free thiols within body. The albumin Cys34 thiol exists in both reduced and oxidized forms. In plasma of healthy young adults, 70–80% of total HSA contains the free sulfhydryl group of Cys34 in a reduced form or mercaptoalbumin (HSA-SH). However, in pathological states characterized by oxidative stress and during the aging process, the oxidized form, or non-mercaptoalbumin (HNA), could predominate. The albumin thiol reacts with radical hydroxyl (.OH), hydrogen peroxide (H2O2) and the reactive nitrogen species as peroxynitrite (ONOO.), and have been shown to oxidize Cys34 to sulfenic acid derivate (HSA-SOH), it can be recycled to mercapto-albumin; however at high concentrations of reactive species leads to the irreversible oxidation to sulfinic (HSA-SO2H) or sulfonic acid (HSA-SO3H) affecting its structure. Presence of reactive oxygen species (ROS), can induce irreversible structural damage and alter protein activities.

Loss via kidneys

In the healthy kidney, albumin's size and negative electric charge exclude it from excretion in the glomerulus. This is not always the case, as in some diseases including diabetic nephropathy, which can sometimes be a complication of uncontrolled or of longer term diabetes in which proteins can cross the glomerulus. The lost albumin can be detected by a simple urine test. Depending on the amount of albumin lost, a patient may have normal renal function, microalbuminuria, or albuminuria.

Interactions

Human serum albumin has been shown to interact with FCGRT.

It might also interact with a yet-unidentified albondin (gp60), a certain pair of gp18/gp30, and some other proteins like osteonectin, hnRNPs, calreticulin, cubilin, and megalin.

Crystal-structure-of-human-serum-albumin-The-illustration-shows-the-tertiary-structure.png


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#913 2021-01-22 00:10:34

ganesh
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Re: Miscellany

891) Nephron

Nephron, functional unit of the kidney, the structure that actually produces urine in the process of removing waste and excess substances from the blood. There are about 1,000,000 nephrons in each human kidney. The most primitive nephrons are found in the kidneys (pronephros) of primitive fish, amphibian larvae, and embryos of more advanced vertebrates. The nephrons found in the kidneys (mesonephros) of amphibians and most fish, and in the late embryonic development of more advanced vertebrates, are only slightly more advanced in structure. The most advanced nephrons occur in the adult kidneys, or metanephros, of land vertebrates, such as reptiles, birds, and mammals.

Each nephron in the mammalian kidney is a long tubule, or extremely fine tube, about 30–55 mm (1.2–2.2 inches) long. At one end this tube is closed, expanded, and folded into a double-walled cuplike structure. This structure, called the renal corpuscular capsule, or Bowman’s capsule, encloses a cluster of microscopic blood vessels—capillaries—called the glomerulus. The capsule and glomerulus together constitute the renal corpuscle. Blood flows into and away from the glomerulus through tiny arteries called arterioles, which reach and leave the glomerulus through the open end of the capsule. In the renal corpuscle, fluid filters out of the blood in the glomerulus through the inner wall of the capsule and into the nephron tubule. As this filtrate passes through the tubule, its composition is altered by the secretion of certain substances into it and by the selective reabsorption of water and other constituents from it. The final product is urine, which is conveyed through the collecting tubules into the renal pelvis.

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#914 2021-01-23 00:43:34

ganesh
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Re: Miscellany

892) Truss bridge

Truss bridge, bridge with its load-bearing structures composed of a series of wooden or metal triangles, known as trusses. Given that a triangle cannot be distorted by stress, a truss gives a stable form capable of supporting considerable external loads over a large span. Trusses are popular for bridge building because they use a relatively small amount of material for the amount of weight they can support. They commonly are used in covered bridges, railroad bridges, and military bridges.

Form And Mechanics

The individual pieces of a truss bridge intersect at truss joints, or panel points. The connected pieces forming the top and bottom of the truss are referred to respectively as the top and bottom chords. The sloping and vertical pieces connecting the chords are collectively referred to as the web of the truss.

The component parts of a truss bridge are stressed primarily in axial tension or compression. A single-span truss bridge is like a simply supported beam because it carries vertical loads by bending. Bending leads to compression, in the top chords (or horizontal members), tension in the bottom chords, and either tension or compression in the vertical and diagonal members, depending on their orientation.

Early trusses were built without precise knowledge of how the loads are carried by each part of the truss. The first engineer to analyze correctly the stresses in a truss was Squire Whipple, an American who designed hundreds of small truss bridges and published his theories in 1869. Understanding precisely how loads were carried led to a reduction in materials, which by then were shifting from wood and stone to iron and steel.

History And Uses

There is no evidence of truss bridges in the ancient world, but the 13th-century sketchbook of the French architect Villard de Honnecourt depicts a species of truss bridge, and the Italian Andrea Palladio’s “Treatise on Architecture” (1570) describes four designs. Several notable covered bridges, which are enclosed truss bridges, were constructed in Switzerland. The Kappel Bridge (1333) of Luzern has been decorated since 1599 with 112 paintings in the triangular spaces between the roof and the crossbeams, depicting the history of the town and the lives of its two patron saints.

In the 18th century, designs with timber trusses reached new span lengths. In 1755 a Swiss builder, Hans Grubenmann, used trusses to support a covered timber bridge with spans of 51 and 58 metres (171 and 193 feet) over the Rhine at Schaffhausen. He and his brother also built a notable arch-truss bridge over the Limmat River in Baden with a clear span of 61 metres (200 feet).

In North America the covered truss bridges underwent further evolution. From simple king-post trusses, in which the roadway was supported by a pair of heavy timber triangles, American carpenters in the 18th and 19th centuries developed bridges combining simplicity of construction with their other economic advantages. The first long covered bridge in America, with a 55-metre (180-foot) centre span, was built by Timothy Palmer, a Massachusetts millwright, over the Schuylkill River at Philadelphia in 1806. A New Haven architect named Ithiel Town patented the Town lattice, in which a number of relatively light pieces, diagonally crisscrossed, took the place of the heavy timbers of Palmer’s design and of the arch. Another highly successful type was designed by Theodore Burr, of Torrington, Connecticut, combining a Palladio truss with an arch. Burr’s McCall’s Ferry Bridge (1815; on the Susquehanna River near Lancaster, Pennsylvania) had a record-breaking span of 108 metres (360 feet). Numerous Town and Burr designs remained standing throughout North America into the early 21st century, some dating back to the early 19th century.

With the increasing importance of locomotive transportation in the 19th century, iron was adopted for covered bridges to carry the heavy loadings of the railroad. At first metal was used for only part of the truss, in either vertical or diagonal members, and later for the whole truss. Cast iron and wrought iron were soon replaced by steel, and a principal form of the modern railroad bridge rapidly evolved. The metal truss did not require protection from the weather and consequently was not covered. The two systems most commonly used are the Pratt and the Warren; in the former the sloping web members are parallel to each other, while in the latter they alternate in direction of slope.

Steel truss bridges have also been built as part of highway systems around the world. The longest continuous-truss bridge in the world is the Ikitsuki Bridge (1991) in Japan, with a main span of 400 metres (1,300 feet). The Astoria Bridge (1966), spanning the mouth of the Columbia River between the states of Oregon and Washington in the United States, consists of three spans reaching a total length of 6,545 metres (21,474 feet) and including a main span of 376 metres (1,232 fee); it is the second longest continuous-truss bridge.

Truss bridges have been used in military operations, particularly where riverbanks are steep or navigation must be kept open. They are usually made up in panels that can be readily transported and quickly bolted together. Such military truss bridges were pioneered in World War II by the highly successful British-invented Bailey bridge, which played an especially important role in the Allied campaign in Italy.

truss-bridge-designs.jpg


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#915 2021-01-24 00:09:19

ganesh
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Re: Miscellany

893) Suspension bridge

Suspension bridge, bridge with overhead cables supporting its roadway. One of the oldest of engineering forms, suspension bridges were constructed by primitive peoples using vines for cables and mounting the roadway directly on the cables. A much stronger type was introduced in India about the 4th century AD that used cables of plaited bamboo and later of iron chain, with the roadway suspended.

In modern times, the suspension bridge provided an economical solution to the problem of long spans over navigable streams or at other sites where it is difficult to found piers in the stream. British, French, American, and other engineers of the late 18th and early 19th centuries encountered serious problems of stability and strength against wind forces and heavy loads; failures resulted from storms, heavy snows, and droves of cattle. Credit for solving the problem belongs principally to John Augustus Roebling, a German-born American engineer who added a web truss to either side of his roadways and produced a structure so rigid that he successfully bridged the Niagara Gorge at Niagara Falls, New York, the Ohio River at Cincinnati, and, finally, in his masterpiece, the East River between Brooklyn and Manhattan at New York City.

The technique of cable spinning for suspension bridges was invented by the French engineer Louis Vicat, a contemporary of Roebling. Vicat’s method employed a traveling wheel to carry the continuous cable strand from the anchorage on one side up over the tower, down on a predetermined sag (catenary) to the midpoint of the bridge, up and over the tower on the farther side to the farther anchorage, where a crew received the wheel, anchored the strand, and returned the wheel, laying a fresh strand. From these successive parallel strands a cable was built up.

Another major development in the modern suspension bridge was the pneumatic caisson, which permitted pier foundation at great depths. It was used initially by French, British, and American engineers, including Washington Roebling, who completed his father’s Brooklyn Bridge.

For a time in the 1930s, American engineers experimented with a narrow solid girder in place of the web truss to stiffen the roadway, but the failure of the Tacoma Narrows Bridge in 1940 under aerodynamic forces instigated a return to the web truss. Later, aerodynamically stable box girders replaced the web truss.

By the late 1980s, three suspension bridges (the Golden Gate, in San Francisco, the Verrazano-Narrows, in New York City, and the Humber Bridge, near Hull, England) had main-span lengths of more than 4,000 feet (1,200 metres). Modern steel alloys are considered capable of much greater spans. Though suspension bridges can be made strong enough to support freight trains, they have nearly all been designed for automobile traffic.

A cable-braced bridge was developed by German engineers at Cologne, Düsseldorf, and elsewhere in the 1950s and ’60s; in this form a single tower at the midpoint supports the roadway by means of a number of cables. Another development of the 1960s, aimed at reducing time of construction, was cable fabricated in the shop.

how-bridges-work-6.jpg


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#916 2021-01-25 00:37:22

ganesh
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Re: Miscellany

894) Movable bridge

Movable bridge, either a drawbridge, a vertical-lift bridge, a transporter bridge, or a swing (pivot) bridge. The drawbridge, or bascule, is the best known; it may be single- or double-leafed. It originated in medieval Europe, probably Normandy, as a defensive feature of castles and towns. It was operated by a counterweight and winch. The drawbridge that formed one span of Old London Bridge was occasionally raised to permit passage of a ship having masts too tall to pass under at this point. In the late 19th century drawbridges began to be built specifically to aid navigation; the Tower Bridge, London, and the Van Buren Street Bridge, Chicago, were built almost simultaneously. Both were double-leaf bascules, and their success led to wide imitation; more than 20 were built to span the Chicago River alone.

At the same time, another movable bridge was pioneered in Chicago: the vertical lift, designed by J.A.L. Waddell. For several years it was unimitated; later, when its great strength for railroad loading was appreciated, it was repeated widely, in increasing span lengths, many exceeding 500 feet (152 metres). The vertical lift also relies on counterweights; the entire bridge roadway is elevated by counterweights and machinery in two towers. The transporter bridge consists of a car suspended from a trolley traveling along an overhead bridge superstructure. It carries passengers and vehicles across a waterway.

For exceptionally long spans, the pivot, or swing bridge, which turns on a table, is suitable. Several of more than 500 feet have been built in the United States, but the turntable obstructs the river, limiting its use.

wttw_1340905516.jpg


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#917 2021-01-26 00:14:31

ganesh
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Re: Miscellany

895) Beam

Beam, in engineering, originally a solid piece of timber, as a beam of a house, a plow, a loom, or a balance. In building construction, a beam is a horizontal member spanning an opening and carrying a load that may be a brick or stone wall above the opening, in which case the beam is often called a lintel. The load may be a floor or roof in a building, in which case the beam is called a floor joist or a roof joist. In a bridge deck the lightly loaded longitudinal beams are the stringers; the heavier, transverse members are called floor beams.

Large beams carrying the ends of other beams perpendicular to them are usually called girders. Metal girders may be single rolled pieces or, to permit greater stiffness and longer spans, may be built up in the form of an I by rivetting or welding plates and angles. Concrete girders are also widely used.

Beams may be of wood, steel or other metals, reinforced or prestressed concrete, plastics, and even brickwork with steel rods in the bond between bricks. For weight reduction, beams of metal are formed as an I or other shape having a thin vertical web and thicker horizontal flanges where most of the strain appears.

beam-2.jpg


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#918 2021-01-27 00:02:50

ganesh
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Re: Miscellany

896) Viaduct

Viaduct, type of long bridge or series of bridges, usually supported by a series of arches or on spans between tall towers. The purpose of a viaduct is to carry a road or railway over water, a valley, or another road. The viaduct is both functionally and etymologically related to the aqueduct, which carries water; both were developed by Roman engineers.

The long spans of Roman viaducts were supported by semicircular arches resting on piers of stone or masonry. A well-preserved example is the span over the Tagus River at Alcantara, Spain (c. AD 105). The next advance in viaduct construction did not occur until the late 18th-century development of iron bridges and the 19th-century introduction of steel.

In the early 20th century the spread of reinforced-concrete construction led to the building of concrete arch structures such as the Colorado Street viaduct over the Pasadena Freeway in California (1938). A recent method used on long viaducts is segmental construction. The sections are precast and jacked forward from one end of the viaduct to form the extension.

IMG_20190101_152816-885x664.jpg


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#919 2021-01-28 00:16:57

ganesh
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Re: Miscellany

897) Cantilever bridge

Cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. A balcony protruding from a building would be an example of a cantilever. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 460m (1,500 ft), and can be more easily constructed at difficult crossings by virtue of using little or no falsework (temporary supports). The Hassfurt Bridge over the Main river in Germany with a central span of 38m (124 ft) was completed in 1867 and is recognized as the first modern cantilever bridge.

A simple cantilever span is formed by two cantilever arms extending from opposite sides of an obstacle to be crossed, meeting at the centre. In a common variant, the suspended span, the cantilever arms do not meet in the centre; instead, they support a central truss bridge which rests on the ends of the cantilever arms. The suspended span may be built off-site and lifted into place, or constructed in place using special travelling supports.

A common way to construct steel truss and prestressed concrete cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction, forming a balanced cantilever; when they attach to a solid foundation, the counterbalancing arms are called anchor arms. The parallel bridges which form the West-Link in West Dublin were built in this manner. In a bridge built on two foundation piers, there are four cantilever arms: two which span the obstacle, and two anchor arms which extend away from the obstacle. Because of the need for more strength at the balanced cantilever’s supports, the bridge superstructure often takes the form of towers above the foundation piers. The Commodore Barry Bridge is an example of this type of cantilever bridge.

The most famous example of this type of bridge is the Forth Railway Bridge in Scotland, which was the longest span in the world from 1890 until 1919 when the Quebec Bridge in Canada was built.

forthbridge2.jpg


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#920 2021-01-29 00:37:14

ganesh
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Re: Miscellany

898) Potter's wheel

In pottery, a potter's wheel is a machine used in the shaping (known as throwing) of round ceramic ware. The wheel may also be used during the process of trimming the excess body from dried ware, and for applying incised decoration or rings of colour. Use of the potter's wheel became widespread throughout the Old World but was unknown in the Pre-Columbian New World, where pottery was handmade by methods that included coiling and beating.

A potter's wheel may occasionally be referred to as a "potter's lathe". However, that term is better used for another kind of machine that is used for a different shaping process, turning, similar to that used for shaping of metal and wooden articles.

The techniques of jiggering and jolleying can be seen as extensions of the potter's wheel: in jiggering, a shaped tool is slowly brought down onto the plastic clay body that has been placed on top of the rotating plaster mould. The jigger tool shapes one face, the mould the other. The term is specific to the shaping of flat ware, such as plates, whilst a similar technique, jolleying, refers to the production of hollow ware, such as cups.

History

Much early ceramic ware was hand-built using a simple coiling technique in which clay was rolled into long threads that were then pinched and smoothed together to form the body of a vessel. In the coiling method of construction, all the energy required to form the main part of a piece is supplied indirectly by the hands of the potter. Early ceramics built by coiling were often placed on mats or large leaves to allow them to be worked more conveniently. The evidence of this lies in mat or leaf impressions left in the clay of the base of the pot. This arrangement allowed the potter to rotate the vessel during construction, rather than walk around it to add coils of clay.

The earliest forms of the potter's wheel (called tourneys or slow wheels) were probably developed as an extension to this procedure. Tournettes, in use around 4500 BC in the Near East, were turned slowly by hand or by foot while coiling a pot. Only a small range of vessels were fashioned on the tournette, suggesting that it was used by a limited number of potters. The introduction of the slow wheel increased the efficiency of hand-powered pottery production.

In the mid to late 3rd millennium BC the fast wheel was developed, which operated on the flywheel principle. It utilised energy stored in the rotating mass of the heavy stone wheel itself to speed the process. This wheel was wound up and charged with energy by kicking, or pushing it around with a stick, providing a centrifugal force. The fast wheel enabled a new process of pottery-making to develop, called throwing, in which a lump of clay was placed centrally on the wheel and then squeezed, lifted and shaped as the wheel turned. The process tends to leave rings on the inside of the pot and can be used to create thinner-walled pieces and a wider variety of shapes, including stemmed vessels, so wheel-thrown pottery can be distinguished from handmade. Potters could now produce many more pots per hour, a first step towards industrialization.

Many modern scholars suggest that the first potter's wheel was first developed by the ancient Sumerians in Mesopotamia. A stone potter's wheel found at the Sumerian city of Ur in modern-day Iraq has been dated to about 3129 BC, but fragments of wheel-thrown pottery of an even earlier date have been recovered in the same area. However, southeastern Europe and China have also been claimed as possible places of origin. Furthermore, the wheel was also in popular use by potters starting around 3500 BC in major cities of the Indus Valley civilization in South Asia, namely Harappa and Mohenjo-daro (Kenoyer, 2005). Others consider Egypt as "being the place of origin of the potter's wheel. It was here that the turntable shaft was lengthened about 3000 BC and a flywheel added. The flywheel was kicked and later was moved by pulling the edge with the left hand while forming the clay with the right. This led to the counterclockwise motion for the potter's wheel which is almost universal. Hence the exact origin of the wheel is not wholly clear yet.

In the Iron Age, the potter's wheel in common use had a turning platform about one metre (3 feet) above the floor, connected by a long axle to a heavy flywheel at ground level. This arrangement allowed the potter to keep the turning wheel rotating by kicking the flywheel with the foot, leaving both hands free for manipulating the vessel under construction. However, from an ergonomic standpoint, sweeping the foot from side to side against the spinning hub is rather awkward. At some point, an alternative solution was invented that involved a crankshaft with a lever that converted up-and-down motion into rotary motion.

The use of the motor-driven wheel has become common in modern times, particularly with craft potters and educational institutions, although human-powered ones are still in use and are preferred by some studio potters.

Techniques of throwing

There are many techniques in use for throwing ceramic shapes, although this is a typical entry-level procedure:

A recently wedged, slightly lumpy clump of plastic throwing clay is slapped, thrown or otherwise affixed to the wheel-head or a bat. A bat serves as a proxy wheel-head that can be removed with the finished pot. The wedged clay is centered by the speed of the wheel and the steadiness of the potter's hands. Water is used as a lubricant to control the clay and should be used sparingly as it also weakens the clay as it get thinner. It is important to ease onto and off of the clay so that the entire circumference receives the same treatment. A high speed on the wheel (240-300 rpm) makes this operation much easier with less physical exertion needed by the potter. The potter will sit or stand with the wheel-head as close to their waist as possible, allowing them more stability and strength. The wheel is sped up and the potter brings steady, controlled pressure onto the clay starting with the blades of the hands where the clay meets the wheel, working your way up. When the clay is centered the clay needs to be homogenized. The more shear (engineering definition) energy that is applied to the clay, the more strength it has later in pulling up the walls and allows the potter to throw faster and with thinner walls. The operation is sometimes called exercising or wheel wedging the clay and consists of thinning and applying shear energy to as much of the clay as possible while keeping the clay whole and centered. After wheel wedging and centering the clay the next step is to open the clay and set the floor of the pot. This is still done at high speed so that the clay in the floor of the pot receives enough shear energy. To open the clay, softly feel for the center of the clay, having your finger in the center will require the least amount of work. Once you have found center push down towards the wheel-head to set the floor thickness of the pot. When you have established the floor thickness, pull the clay out to establish the floor width. The ring of clay surrounding the floor is now ready to be pulled up into the walls of the pot. The first pull is started at full or near full speed to thin the walls. For right-handed potters working on a wheel going counter-clockwise the left hand is on the inside of the ring on the right hand on the outside at the right tangent of the wheel. The second and third pulls establish the thickness and shape.

The process of throwing around pot

A skilled potter can quickly throw a vessel from up to 15 kg (30 lb) of clay. Alternatively, by throwing and adding coils of clay then throwing again, pots up to four feet high may be made, the heat of a blowlamp being used to firm each thrown section before adding the next coil. In Chinese manufacture, very large pots are made by two throwers working simultaneously.

The potter's wheel in myth and legend

In Ancient Egyptian mythology, the deity Khnum was said to have formed the first humans on a potter's 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. 

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

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#921 2021-01-30 00:09:38

ganesh
Administrator
Registered: 2005-06-28
Posts: 33,015

Re: Miscellany

899) Wheelchair

Wheelchair, any seating surface (e.g., a chair) that has wheels affixed to it in order to help an individual move from one place to another. Wheelchairs range from large, bulky, manually powered models to high-tech electric-powered models that can climb stairs. The modern standard wheelchair design is based on the so-called cross-frame design that was introduced in 1932 by disabled American mining engineer Herbert A. Everest and American mechanical engineer Harry C. Jennings. Together, Everest and Jennings patented the cross-frame wheelchair, which uses a cross brace to attach the two sides of the chair, allowing it to be folded when not in use. They later formed Everest & Jennings, Inc., which subsequently became one of the world’s major wheelchair manufacturers.

Basic Features

Typically, a wheelchair consists of four wheels: two large wheels in the rear, which are used for propelling the wheelchair, and two small wheels in the front, which swivel and are called casters. The large wheels support the majority of the individual’s weight and provide the primary means of propulsion. The casters facilitate maneuverability. Traditionally, wheelchairs are divided into two categories: manual and electric-powered. Those categories are defined by the mechanism used to propel the wheelchair. A manual wheelchair is propelled by human power, and an electric-powered wheelchair is propelled by an electrically based power source (typically a battery and electrical motor).

A manual wheelchair is powered either by the individual using the wheelchair or by an assistant. The most commonly recognized manual wheelchairs are seen at hospitals and nursing homes. Individuals who have the strength and endurance to independently propel the wheelchair typically use manual wheelchairs. They can propel the wheelchair in different ways. For instance, individuals with a spinal cord injury can use their upper extremities. Individuals who have had a stroke that affects only one side of the body can use one upper extremity and one lower extremity. Individuals who have neither the strength to walk without a walker or cane nor the endurance to walk with one can use their lower extremities. An assistant or attendant propels the manual wheelchair when individuals cannot do it themselves.

Style, Material, And Weight

Manual wheelchairs can be divided into numerous categories based on their intended use and design. The most basic characteristic that distinguishes manual wheelchairs is the frame design, but wheelchairs are also categorized by style, material, and weight.

Style

A standard folding wheelchair has a cross-brace design (X-frame), which allows the wheelchair to fold laterally via a scissor-like action. These wheelchairs are very popular because they can be easily folded for transportation. The limitation to folding-frame wheelchairs is that they tend to be heavy and have reduced performance characteristics compared with a rigid-frame manual wheelchair. A rigid frame does not incorporate a folding mechanism into the design, thereby significantly improving aesthetics, performance, strength, and weight.

Material

Another feature that distinguishes wheelchairs is the type of material used. Initially, manufacturers used steel in all manual wheelchairs (primarily mild steel) because of its low cost and ease of machinability. Later there were numerous advances in the materials used to manufacture wheelchairs. Most modern wheelchairs are made using primarily steel, aluminum, and titanium. Steel is limited to standard wheelchairs that have folding-frame mechanisms. Aluminum is used throughout the wheelchair industry, primarily in ultralight wheelchairs and some lightweight wheelchairs. Aluminum has a higher strength-to-weight ratio than mild steel, thereby reducing the overall weight of the wheelchair, and it has the added advantage of being resistant to corrosion. Titanium has been used in ultralight manual wheelchairs, further reducing the weight because of its high strength-to-weight ratio. Titanium also is resistant to corrosion. The key limitations of titanium are its relatively high material costs compared with steel and aluminum and the greater difficulty in machining or welding titanium.

Weight

In general, there are three wheelchair weight categories: standard, lightweight, and ultralight.

Standard wheelchairs are typically folding-frame wheelchairs that are manufactured using mild steel. They are the heaviest of manual wheelchairs, usually weighing more than 18 kg (39.6 pounds) with limited adjustability in components. These wheelchairs are designed most often for temporary use and are usually found in medical facilities (e.g., hospitals and nursing homes).

Lightweight wheelchairs also are typically folding-frame wheelchairs. They have many adjustable components and are available with many features. They tend to be lighter than standard wheelchairs (typically between 13 and 18 kg [28.6 and 39.6 pounds]) because they are usually manufactured using aluminum.

Ultralight wheelchairs have the best performance characteristics of the three weight categories. As the name suggests, these are the lightest-weight wheelchairs (typically less than 13 kg [28.6 pounds]), because they are manufactured using aluminum, high-performance steel, or titanium. However, the key difference between lightweight and ultralight wheelchairs, besides weight, is an adjustable rear-wheel axle. A horizontally adjustable rear wheel (i.e., one that can be moved forward or backward) allows for the optimal placement of the rear wheel on the frame. This makes it easier for the individual to reach the rear wheels during propulsion, reducing stress and strain on the upper extremities.

Components

The frame is the most basic unit of a manual wheelchair and the most influential in terms of performance. However, the components that are attached to the frame to generate a functional manual wheelchair are significant as well. The key components are the tires, the wheels, the axles, the casters, the leg rests, and the armrests.

Tires

Wheelchair tires are either solid rubber or pneumatic (air-filled). Solid rubber tires are almost always used with standard wheelchairs and sometimes with lightweight wheelchairs. Those tires provide a hard ride and have a high rolling resistance, but they have low wear rates and are low maintenance. Pneumatic tires are almost always used with ultralight wheelchairs and sometimes with lightweight wheelchairs. Those tires provide a softer ride, lower rolling resistance, and are lower in weight, but they have high wear rates and are high-maintenance (particularly in maintaining appropriate air pressure).

Wheels

The wheels are usually spoked (wired) or molded (mag). Wheel sizes usually range from about 30 to 66 cm (12 to 26 inches) in diameter, depending on the purpose of the wheelchair. Molded wheels have low maintenance requirements. However, they are significantly heavier and less responsive than spoked wheels.

Axles

Rear-wheel axles are either fixed or quick-release. Fixed axles are almost always used on standard wheelchairs. Quick-release axles are almost always used with ultralight wheelchairs, and either fixed or quick-release are used with lightweight wheelchairs. Fixed axles are a bolt and locknut that require tools to remove and attach the rear wheel to the frame. A quick-release mechanism has a button on the end of the axle that allows for easy removal of the tire without any tools. That may be critical for disassembling a wheelchair when transporting it in an automobile. The fixed axle is low-maintenance, whereas the quick-release axle requires frequent monitoring.

Casters

The casters range in size from about 7.6 to 23.8 cm (3 to 9 inches) in diameter, with the majority falling in the 12.7- to 20.3-cm (5- to 8-inch) range. The caster tires can be solid rubber or pneumatic but are limited to either mag or solid hub wheels.

Leg rests

The leg rests are fixed, swing-away, or elevating. They consist of a hanger that is attached to the frame and a footplate that supports the individual’s feet. Fixed leg rests are integral to the frame; they produce a lighter-weight system since there are fewer components. Swing-away leg rests allow for the removal of the leg rests from the frame in order to facilitate transfers into and out of the wheelchair. Elevating leg rests allow the lower extremities to be positioned at different angles with relation to the seat surface, thereby raising and lowering the leg position. This is often critical to address an individual’s specific physiologic issues (e.g., swelling in the lower extremities).

Armrests

The armrests are either fixed-height or adjustable-height. Armrests facilitate transfers by providing a handhold for the individual. They support the upper extremities when the individual is not propelling the wheelchair, and they provide a means for weight shifting if the individual has the strength to lift his or her body weight using the upper extremities.

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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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#922 2021-01-31 00:07:20

ganesh
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Posts: 33,015

Re: Miscellany

900) Adhesive

Adhesive, any substance that is capable of holding materials together in a functional manner by surface attachment that resists separation. “Adhesive” as a general term includes cement, mucilage, glue, and paste—terms that are often used interchangeably for any organic material that forms an adhesive bond. Inorganic substances such as portland cement also can be considered adhesives, in the sense that they hold objects such as bricks and beams together through surface attachment, but this article is limited to a discussion of organic adhesives, both natural and synthetic.

Natural adhesives have been known since antiquity. Egyptian carvings dating back 3,300 years depict the gluing of a thin piece of veneer to what appears to be a plank of sycamore. Papyrus, an early nonwoven fabric, contained fibres of reedlike plants bonded together with flour paste. Bitumen, tree pitches, and beeswax were used as sealants (protective coatings) and adhesives in ancient and medieval times. The gold leaf of illuminated manuscripts was bonded to paper by egg white, and wooden objects were bonded with glues from fish, horn, and cheese. The technology of animal and fish glues advanced during the 18th century, and in the 19th century rubber- and nitrocellulose-based cements were introduced. Decisive advances in adhesives technology, however, awaited the 20th century, during which time natural adhesives were improved and many synthetics came out of the laboratory to replace natural adhesives in the marketplace. The rapid growth of the aircraft and aerospace industries during the second half of the 20th century had a profound impact on adhesives technology. The demand for adhesives that had a high degree of structural strength and were resistant to both fatigue and severe environmental conditions led to the development of high-performance materials, which eventually found their way into many industrial and domestic applications.

This article begins with a brief explanation of the principles of adhesion and then proceeds to a review of the major classes of natural and synthetic adhesives.

Adhesion

In the performance of adhesive joints, the physical and chemical properties of the adhesive are the most important factors. Also important in determining whether the adhesive joint will perform adequately are the types of adherend (that is, the components being joined—e.g., metal alloy, plastic, composite material) and the nature of the surface pretreatment or primer. These three factors—adhesive, adherend, and surface—have an impact on the service life of the bonded structure. The mechanical behaviour of the bonded structure in turn is influenced by the details of the joint design and by the way in which the applied loads are transferred from one adherend to the other.

Implicit in the formation of an acceptable adhesive bond is the ability of the adhesive to wet and spread on the adherends being joined. Attainment of such interfacial molecular contact is a necessary first step in the formation of strong and stable adhesive joints. Once wetting is achieved, intrinsic adhesive forces are generated across the interface through a number of mechanisms. The precise nature of these mechanisms have been the object of physical and chemical study since at least the 1960s, with the result that a number of theories of adhesion exist. The main mechanism of adhesion is explained by the adsorption theory, which states that substances stick primarily because of intimate intermolecular contact. In adhesive joints this contact is attained by intermolecular or valence forces exerted by molecules in the surface layers of the adhesive and adherend.

In addition to adsorption, four other mechanisms of adhesion have been proposed. The first, mechanical interlocking, occurs when adhesive flows into pores in the adherend surface or around projections on the surface. The second, interdiffusion, results when liquid adhesive dissolves and diffuses into adherend materials. In the third mechanism, adsorption and surface reaction, bonding occurs when adhesive molecules adsorb onto a solid surface and chemically react with it. Because of the chemical reaction, this process differs in some degree from simple adsorption, described above, although some researchers consider chemical reaction to be part of a total adsorption process and not a separate adhesion mechanism. Finally, the electronic, or electrostatic, attraction theory suggests that electrostatic forces develop at an interface between materials with differing electronic band structures. In general, more than one of these mechanisms play a role in achieving the desired level of adhesion for various types of adhesive and adherend.

In the formation of an adhesive bond, a transitional zone arises in the interface between adherend and adhesive. In this zone, called the interphase, the chemical and physical properties of the adhesive may be considerably different from those in the noncontact portions. It is generally believed that the interphase composition controls the durability and strength of an adhesive joint and is primarily responsible for the transference of stress from one adherend to another. The interphase region is frequently the site of environmental attack, leading to joint failure.

The strength of adhesive bonds is usually determined by destructive tests, which measure the stresses set up at the point or line of fracture of the test piece. Various test methods are employed, including peel, tensile lap shear, cleavage, and fatigue tests. These tests are carried out over a wide range of temperatures and under various environmental conditions. An alternate method of characterizing an adhesive joint is by determining the energy expended in cleaving apart a unit area of the interphase. The conclusions derived from such energy calculations are, in principle, completely equivalent to those derived from stress analysis.

Adhesive Materials

Virtually all synthetic adhesives and certain natural adhesives are composed of polymers, which are giant molecules, or macromolecules, formed by the linking of thousands of simpler molecules known as monomers. The formation of the polymer (a chemical reaction known as polymerization) can occur during a “cure” step, in which polymerization takes place simultaneously with adhesive-bond formation (as is the case with epoxy resins and cyanoacrylates), or the polymer may be formed before the material is applied as an adhesive, as with thermoplastic elastomers such as styrene-isoprene-styrene block copolymers. Polymers impart strength, flexibility, and the ability to spread and interact on an adherend surface—properties that are required for the formation of acceptable adhesion levels.

Natural adhesives

Natural adhesives are primarily of animal or vegetable origin. Though the demand for natural products has declined since the mid-20th century, certain of them continue to be used with wood and paper products, particularly in corrugated board, envelopes, bottle labels, book bindings, cartons, furniture, and laminated film and foils. In addition, owing to various environmental regulations, natural adhesives derived from renewable resources are receiving renewed attention. The most important natural products are described below.

Animal glue

The term animal glue usually is confined to glues prepared from mammalian collagen, the principal protein constituent of skin, bone, and muscle. When treated with acids, alkalies, or hot water, the normally insoluble collagen slowly becomes soluble. If the original protein is pure and the conversion process is mild, the high-molecular-weight product is called gelatin and may be used for food or photographic products. The lower-molecular-weight material produced by more vigorous processing is normally less pure and darker in colour and is called animal glue.

Animal glue traditionally has been used in wood joining, book bindery, sandpaper manufacture, heavy gummed tapes, and similar applications. In spite of its advantage of high initial tack (stickiness), much animal glue has been modified or entirely replaced by synthetic adhesives.

Casein glue

This product is made by dissolving casein, a protein obtained from milk, in an aqueous alkaline solvent. The degree and type of alkali influences product behaviour. In wood bonding, casein glues generally are superior to true animal glues in moisture resistance and aging characteristics. Casein also is used to improve the adhering characteristics of paints and coatings.

Blood albumen glue

Glue of this type is made from serum albumen, a blood component obtainable from either fresh animal blood or dried soluble blood powder to which water has been added. Addition of alkali to albumen-water mixtures improves adhesive properties. A considerable quantity of glue products from blood is used in the plywood industry.

Starch and dextrin

Starch and dextrin are extracted from corn, wheat, potatoes, or rice. They constitute the principal types of vegetable adhesives, which are soluble or dispersible in water and are obtained from plant sources throughout the world. Starch and dextrin glues are used in corrugated board and packaging and as a wallpaper adhesive.

Natural gums

Substances known as natural gums, which are extracted from their natural sources, also are used as adhesives. Agar, a marine-plant colloid (suspension of extremely minute particles), is extracted by hot water and subsequently frozen for purification. Algin is obtained by digesting seaweed in alkali and precipitating either the calcium salt or alginic acid. Gum arabic is harvested from acacia trees that are artificially wounded to cause the gum to exude. Another exudate is natural rubber latex, which is harvested from Hevea trees. Most gums are used chiefly in water-remoistenable products.

Synthetic adhesives

Although natural adhesives are less expensive to produce, most important adhesives are synthetic. Adhesives based on synthetic resins and rubbers excel in versatility and performance. Synthetics can be produced in a constant supply and at constantly uniform properties. In addition, they can be modified in many ways and are often combined to obtain the best characteristics for a particular application.

The polymers used in synthetic adhesives fall into two general categories—thermoplastics and thermosets. Thermoplastics provide strong, durable adhesion at normal temperatures, and they can be softened for application by heating without undergoing degradation. Thermoplastic resins employed in adhesives include nitrocellulose, polyvinyl acetate, vinyl acetate-ethylene copolymer, polyethylene, polypropylene, polyamides, polyesters, acrylics, and cyanoacrylics.

Thermosetting systems, unlike thermoplastics, form permanent, heat-resistant, insoluble bonds that cannot be modified without degradation. Adhesives based on thermosetting polymers are widely used in the aerospace industry. Thermosets include phenol formaldehyde, urea formaldehyde, unsaturated polyesters, epoxies, and polyurethanes. Elastomer-based adhesives can function as either thermoplastic or thermosetting types, depending on whether cross-linking is necessary for the adhesive to perform its function. The characteristics of elastomeric adhesives include quick assembly, flexibility, variety of type, economy, high peel strength, ease of modification, and versatility. The major elastomers employed as adhesives are natural rubber, butyl rubber, butadiene rubber, styrene-butadiene rubber, nitrile rubber, silicone, and neoprene.

An important challenge facing adhesive manufacturers and users is the replacement of adhesive systems based on organic solvents with systems based on water. This trend has been driven by restrictions on the use of volatile organic compounds (VOC), which include solvents that are released into the atmosphere and contribute to the depletion of ozone. In response to environmental regulation, adhesives based on aqueous emulsions and dispersions are being developed, and solvent-based adhesives are being phased out.

The polymer types noted above are employed in a number of functional types of adhesives.

Contact cements

Contact adhesives or cements are usually based on solvent solutions of neoprene. They are so named because they are usually applied to both surfaces to be bonded. Following evaporation of the solvent, the two surfaces may be joined to form a strong bond with high resistance to shearing forces. Contact cements are used extensively in the assembly of automotive parts, furniture, leather goods, and decorative laminates. They are effective in the bonding of plastics.

Structural adhesives

Structural adhesives are adhesives that generally exhibit good load-carrying capability, long-term durability, and resistance to heat, solvents, and fatigue. Ninety-five percent of all structural adhesives employed in original equipment manufacture fall into six structural-adhesive families: (1) epoxies, which exhibit high strength and good temperature and solvent resistance, (2) polyurethanes, which are flexible, have good peeling characteristics, and are resistant to shock and fatigue, (3) acrylics, a versatile adhesive family that bonds to oily parts, cures quickly, and has good overall properties, (4) anaerobics, or surface-activated acrylics, which are good for bonding threaded metal parts and cylindrical shapes, (5) cyanoacrylates, which bond quickly to plastic and rubber but have limited temperature and moisture resistance, and (6) silicones, which are flexible, weather well out-of-doors, and provide good sealing properties. Each of these families can be modified to provide adhesives that have a range of physical and mechanical properties, cure systems, and application techniques.

Polyesters, polyvinyls, and phenolic resins are also used in industrial applications but have processing or performance limitations. High-temperature adhesives, such as polyimides, have a limited market.

Hot-melt adhesives

Hot-melt adhesives are employed in many nonstructural applications. Based on thermoplastic resins, which melt at elevated temperatures without degrading, these adhesives are applied as hot liquids to the adherend. Commonly used polymers include polyamides, polyesters, ethylene-vinyl acetate, polyurethanes, and a variety of block copolymers and elastomers such as butyl rubber, ethylene-propylene copolymer, and styrene-butadiene rubber.

Hot-melts find wide application in the automotive and home-appliance fields. Their utility, however, is limited by their lack of high-temperature strength, the upper use temperature for most hot-melts being in the range of 40–65 °C (approximately 100–150 °F). In order to improve performance at higher temperatures, so-called structural hot-melts—thermoplastics modified with reactive urethanes, moisture-curable urethanes, or silane-modified polyethylene—have been developed. Such modifications can lead to enhanced peel adhesion, higher heat capability (in the range of 70–95 °C [160–200 °F]), and improved resistance to ultraviolet radiation.

Pressure-sensitive adhesives

Pressure-sensitive adhesives, or PSAs, represent a large industrial and commercial market in the form of adhesive tapes and films directed toward packaging, mounting and fastening, masking, and electrical and surgical applications. PSAs are capable of holding adherends together when the surfaces are mated under briefly applied pressure at room temperature. (The difference between these adhesives and contact cements is that the latter require no pressure to bond.)

Materials used to formulate PSA systems include natural and synthetic rubbers, thermoplastic elastomers, polyacrylates, polyvinylalkyl ethers, and silicones. These polymers, in both solvent-based and hot-melt formulations, are applied as a coating onto a substrate of paper, cellophane, plastic film, fabric, or metal foil. As solvent-based adhesive formulations are phased out in response to environmental regulations, water-based PSAs will find greater use.

Ultraviolet-cured adhesives

Ultraviolet-cured adhesives became available in the early 1960s but developed rapidly with advances in chemical and equipment technology during the 1980s. These types of adhesive normally consist of a monomer (which also can serve as the solvent) and a low-molecular-weight prepolymer combined with a photoinitiator. Photoinitiators are compounds that break down into free radicals upon exposure to ultraviolet radiation. The radicals induce polymerization of the monomer and prepolymer, thus completing the chain extension and cross-linking required for the adhesive to form. Because of the low process temperatures and very rapid polymerization (from 2 to 60 seconds), ultraviolet-cured adhesives are making rapid advances in the electronic, automotive, and medical areas. They consist mainly of acrylated formulations of silicones, urethanes, and methacrylates. Combined ultraviolet–heat-curing formulations also exist.

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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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#923 2021-02-01 00:39:27

ganesh
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Registered: 2005-06-28
Posts: 33,015

Re: Miscellany

901) Fasteners

Fasteners, In construction, connectors between structural members. Bolted connections are used when it is necessary to fasten two elements tightly together, especially to resist shear and bending, as in column and beam connections. Threaded metal bolts are always used in conjunction with nuts. Another threaded fastener is the screw, which has countless applications, especially for wood construction. The wood screw carves a mating thread in the wood, ensuring a tight fit. Pins are used to keep two or more elements in alignment; since the pin is not threaded, it allows for rotational movement, as in machinery parts. Riveted connections, which resist shearing forces, were in wide use for steel construction before being replaced by welding. The rivet, visibly prominent on older steel bridges, is a metal pin fastener with one end flattened into a head by hammering it through a metal gusset plate. The common nail, less resistant to shear or pull-out forces, is useful for cabinet and finishing work, where stresses are minimal.

A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or dismantled without damaging the joining components. Welding is an example of creating permanent joints. Steel fasteners are usually made of stainless steel, carbon steel, or alloy steel.

Other alternative methods of joining materials include: crimping, welding, soldering, brazing, taping, gluing, cement, or the use of other adhesives. Force may also be used, such as with magnets, vacuum (like suction cups), or even friction (like sticky pads). Some types of woodworking joints make use of separate internal reinforcements, such as dowels or biscuits, which in a sense can be considered fasteners within the scope of the joint system, although on their own they are not general purpose fasteners.

Furniture supplied in flat-pack form often uses cam dowels locked by cam locks, also known as conformat fasteners. Fasteners can also be used to close a container such as a bag, a box, or an envelope; or they may involve keeping together the sides of an opening of flexible material, attaching a lid to a container, etc. There are also special-purpose closing devices, e.g. a bread clip.

Items like a rope, string, wire, cable, chain, or plastic wrap may be used to mechanically join objects; but are not generally categorized as fasteners because they have additional common uses. Likewise, hinges and springs may join objects together, but are ordinarily not considered fasteners because their primary purpose is to allow articulation rather than rigid affixment.

Industry

In 2005, it was estimated that the United States fastener industry runs 350 manufacturing plants and employs 40,000 workers. The industry is strongly tied to the production of automobiles, aircraft, appliances, agricultural machinery, commercial construction, and infrastructure. More than 200 billion fasteners are used per year in the U.S., 26 billion of these by the automotive industry. The largest distributor of fasteners in North America is the Fastenal Company.

Materials

There are three major steel fasteners used in industries: stainless steel, carbon steel, and alloy steel. The major grade used in stainless steel fasteners: 200 series, 300 series, and 400 series. Titanium, aluminum, and various alloys are also common materials of construction for metal fasteners. In many cases, special coatings or plating may be applied to metal fasteners to improve their performance characteristics by, for example, enhancing corrosion resistance. Common coatings/platings include zinc, chrome, and hot dip galvanizing.

Applications

When selecting a fastener for industrial applications, it is important to consider a variety of factors. The threading, the applied load on the fastener, the stiffness of the fastener, and the number of fasteners needed should all be taken into account.

When choosing a fastener for a given application, it is important to know the specifics of that application to help select the proper material for the intended use. Factors that should be considered include:

Accessibility
Environment, including temperature, water exposure, and potentially corrosive elements
Installation process
Materials to be joined
Reusability
Weight restrictions

Types

A threaded fastener has internal or external screw threads.[6] The most common types are the screw, nut and bolt, possibly involving washers. Other more specialized types of threaded fasteners include captive threaded fasteners, stud, threaded inserts, and threaded rods.

For military hardware

American screws, bolts, and nuts were historically not fully interchangeable with their British counterparts, and therefore would not fit British equipment properly. This, in part, helped lead to the development of numerous United States Military Standards and specifications for the manufacturing of essentially any piece of equipment that is used for military or defense purposes, including fasteners. World War II was a significant factor in this change.

A key component of most military standards is traceability. Put simply, hardware manufacturers must be able to trace their materials to their source, and provide traceability for their parts going into the supply chain, usually via bar codes or similar methods. This traceability is intended to help ensure that the right parts are used and that quality standards are met in each step of the manufacturing process; additionally, substandard parts can traced back to their source.

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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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#924 2021-02-02 00:19:27

ganesh
Administrator
Registered: 2005-06-28
Posts: 33,015

Re: Miscellany

902) Internet service provider

Internet service provider (ISP), company that provides Internet connections and services to individuals and organizations. In addition to providing access to the Internet, ISPs may also provide software packages (such as browsers), e-mail accounts, and a personal Web site or home page. ISPs can host Web sites for businesses and can also build the Web sites themselves. ISPs are all connected to each other through network access points, public network facilities on the Internet backbone.

The rise of commercial Internet services and applications helped fuel a rapid commercialization of the Internet. This phenomenon was the result of several other factors as well. One important factor was the introduction of the personal computer (PC) and the workstation in the early 1980s—a development that in turn was fueled by unprecedented progress in integrated circuit technology and an attendant rapid decline in computer prices. Another factor, which took on increasing importance, was the emergence of Ethernet and other “local area networks” (LANs) to link personal computers. But other forces were at work too. Following the restructuring of AT&T Corporation in 1984, the U.S. National Science Foundation took advantage of various new options for its national-level digital backbone service, known as NSFNET. In 1988 the U.S. Corporation for National Research Initiatives received approval to conduct an experiment linking a commercial e-mail service (MCI Mail) to the Internet. This application was the first Internet connection to a commercial provider that was not also part of the research community. Approval quickly followed to allow other e-mail providers access, and the Internet began its first explosion in traffic.

In 1993 federal legislation allowed NSF to open the NSFNET backbone to commercial users. Prior to that time, use of the backbone was subject to an “acceptable use” policy, established and administered by NSF, under which commercial use was limited to those applications that served the research community. NSF recognized that commercially supplied network services, now that they were available, would ultimately be far less expensive than continued funding of special-purpose network services.

Also in 1993 the University of Illinois made widely available Mosaic, a new type of computer program, known as a browser, that ran on most types of computers and, through its “point-and-click” interface, simplified access, retrieval, and display of files through the Internet. Mosaic incorporated a set of access protocols and display standards originally developed at the European Organization for Nuclear Research (CERN) by Tim Berners-Lee for a new Internet application called the World Wide Web (WWW). In 1994 Netscape Communications Corp. (originally called Mosaic Communications Corporation) was formed to develop a Web browser, Navigator, and server software for commercial use. Shortly thereafter the software giant Microsoft Corporation became interested in supporting Internet applications on personal computers and developed its Internet Explorer Web browser (based initially on Mosaic) and other programs. These new commercial capabilities accelerated the growth of the Internet, which as early as 1988 had already been growing at the rate of 100 percent per year.

By the late 1990s there were approximately 10,000 ISPs around the world, more than half located in the United States. However, most of these ISPs provided only local service and relied on access to regional and national ISPs for wider connectivity. Consolidation began at the end of the decade, with many small to medium-sized providers merging or being acquired by larger ISPs. Among these larger providers were groups such as America Online, Inc. (AOL), which had started as a dial-up information service with no Internet connectivity but made a transition in the late 1990s to become the leading provider of Internet services in the world—with more than 25 million subscribers by 2000 and with branches in Australia, Europe, South America, and Asia. Meanwhile, many new state-owned ISPs entered the business in large national markets, such as China, India, and Indonesia, and quickly eclipsed the subscriber base of any traditional commercial ISP.

Dial-up Internet customers continued to shift to broadband service for faster Internet connections. The entry-level broadband service offered by telephone and cable television companies cost as little as dial-up services in some parts of the United States. As a result of the shift, dial-up Internet provider AOL watched its base of dial-up service subscribers decline from nearly 27 million in 2002 to 17.7 million by 2006 and to 2.1 million in 2015. In an effort to reposition itself, AOL no longer sought to be the premier provider of dial-up service and instead tried to become a free advertising-supported Internet portal like Yahoo and Google. AOL offered its customers two approaches: they could still pay for dial-up Internet access from AOL, or they could pay for Internet access from another company and still access many AOL features for free.

With the proliferation of Internet sites such as Netflix that broadcast video and other large files, ISPs have pushed for the right to offer differently priced tiers of service to online content or software providers on the basis of their Internet use. Proponents of net neutrality believe, among other things, that network providers should be required to treat all broadband consumers equally instead of charging some consumers higher prices for using more bandwidth (data-carrying capacity). Opponents of net neutrality question whether cable and telephone companies could afford to invest in advanced security or transmission services if they could not charge a premium for them. In general, big Internet providers of content and software support net neutrality, while the ISPs are against it. Legislation will be required to settle the dispute.

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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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#925 2021-02-03 00:33:45

ganesh
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Registered: 2005-06-28
Posts: 33,015

Re: Miscellany

903) Kremlin

Introduction
   
Several cities in Russia were built around fortresses called kremlins. Russians built kremlins for defense during the Middle Ages. A kremlin was often located along a river. A wall, a moat, and towers usually separated it from the surrounding parts of the city. Kremlins contained cathedrals (churches) and palaces for princes and bishops. They also held government offices and weapons of war.
The most famous kremlin is in central Moscow. It is often called just the Kremlin. Its east side faces the famous plaza called Red Square. The Kremlin has long been a symbol of Russia’s power. The United Nations Educational, Scientific and Cultural Organization (UNESCO) declared the Kremlin and Red Square a World Heritage site in 1990.

Design

The Kremlin in Moscow is shaped like a triangle. Its east side faces Red Square, and it has four gates. A back gate has a secret passage to the Moscow River.
The Kremlin’s red brick walls and 20 towers were built at the end of the 1400s. The Saviour’s Tower, one of the most important towers, was built in 1491. Another important tower is the Saint Nicholas Tower, which faces Red Square. It was first built in 1491 and was then rebuilt in 1806. The two other main gate towers are on the western wall. They are the Trinity Tower and the Borovitskaya Tower.

Three churches are grouped around the central Cathedral Square. They are some of the finest Russian churches. The oldest is the Cathedral of the Assumption, built from 1475 to 1479 and made of white stone. Five golden domes sit on top of the church. Across the square is the Cathedral of the Annunciation, built from 1484 to 1489. It was damaged by fire in 1547 and was rebuilt from 1562 to 1564. Golden roofs and domes top its many small churches.

The third cathedral is the Archangel, built from 1505 to 1508. It is where several princes of Moscow and Russian czars, or rulers, are buried. After the Russian Revolution in 1917, the churches in the Kremlin stopped being places where people worshipped. They are now mostly museums. Since the early 1990s, however, religious services have been offered in some of the Kremlin cathedrals.

To the west of Cathedral Square is a group of palaces that were built at different times. The Palace of Facets was given its name because of its white stone squares on the outside of the building. It was built from 1487 to 1491. Several of the palaces are now museums.

History

The Moscow Kremlin dates back to AD 1156 and was first built of wood. It was rebuilt in brick in the 1300s. Later, it was repaired and changed a number of times.
Its design shows its long history, as there are many styles of building. It lost its importance as a fort in the 1620s. It was the center of Russian government until 1712 and again after 1918. The Kremlin became the base of the Soviet government and the symbol of Communist rule. After the Soviet Union came to an end in 1991, the Kremlin became the home of Russia’s executive branch of government.

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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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