Selasa, 10 Februari 2009



The word is from the Latin virus referring to poison and other noxious substances, first used in English in 1392.[4] Virulent, from Latin virulentus (poisonous) dates to 1400.[5] A meaning of "agent that causes infectious disease" is first recorded in 1728,[4] before the discovery of viruses by Dmitry Ivanovsky in 1892. The adjective viral dates to 1948.[6] The term virion is also used to refer to a single infective viral particle. The plural of virus is "viruses".

[edit] History
Martinus Beijerinck in his laboratory in 1921

In 1884, the French microbiologist Charles Chamberland invented a filter, (known today as the Chamberland filter or Chamberland-Pasteur filter), with pores smaller than bacteria. Thus, he could pass a solution containing bacteria through the filter and completely remove them from the solution.[7] In 1892 the Russian biologist Dimitri Ivanovski used this filter to study what is now known to be tobacco mosaic virus. His experiments showed that the crushed leaf extracts from infected tobacco plants are still infectious after filtration. Ivanovski suggested the infection might be caused by a toxin produced by bacteria, but did not pursue the idea.[8] At the time it was thought that all infectious agents could be retained by filters and grown on a nutrient medium—this was part of the germ theory of disease.[9] In 1899 the Dutch microbiologist Martinus Beijerinck repeated the experiments and became convinced that this was a new form of infectious agent.[10] He went on to observe that the agent multiplied only in dividing cells, but as his experiments did not show that it was made of particles, he called it a contagium vivum fluidum (soluble living germ) and re-introduced the word virus.[8] Beijerinck maintained that viruses were liquid in nature, a theory later discredited by Wendell Stanley, who proved they were particulate.[8] In the same year, 1899, Friedrich Loeffler and Frosch passed the agent of foot and mouth disease (aphthovirus) through a similar filter and ruled out the possibility of a toxin because of the high dilution; they concluded that the agent could replicate.[8]

In the early 20th century, the English bacteriologist Frederick Twort discovered the viruses that infect bacteria, which are now called bacteriophages,[11] and the French-Canadian microbiologist Félix d'Herelle described viruses that, when added to bacteria growing on agar, would produce areas of dead bacteria. He accurately diluted a suspension of these viruses and discovered that the highest dilutions, rather than killing all the bacteria, formed discrete areas of dead organisms. Counting these areas and multiplying by the dilution factor allowed him to calculate the number of viruses in the suspension.[12]

By the end of the nineteenth century, viruses were defined in terms of their infectivity, filterability, and their requirement for living hosts. Viruses had only been grown in plants and animals. In 1906, Harrison invented a method for growing tissue in lymph, and, in 1913, E. Steinhardt, C. Israeli and R. A. Lambert used this method to grow vaccinia virus in fragments of guinea pig corneal tissue.[13] In 1928, H. B. Maitland and M. C. Maitland grew vaccinia virus in suspensions of minced hens' kidneys. Their method was not widely adopted until the 1950s, when poliovirus was grown on a large scale for vaccine production.[14]

Another breakthrough came in 1931, when the American pathologist Ernest William Goodpasture grew influenza and several other viruses in fertilised chickens' eggs.[15] In 1949 John F. Enders, Thomas Weller and Frederick Robbins grew polio virus in cultured human embryo cells, the first virus to be grown without using solid animal tissue or eggs. This work enabled Jonas Salk to make an effective polio vaccine.[16]
Rosalind Franklin

With the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll came the first images of viruses.[17] In 1935 American biochemist and virologist Wendell Stanley examined the Tobacco mosaic virus and found it to be mostly made from protein.[18] A short time later, this virus was separated into protein and RNA parts.[19] Tobacco mosaic virus was the first one to be crystallised and whose structure could therefore be elucidated in detail. The first X-ray diffraction pictures of the crystallised virus were obtained by Bernal and Fankuchen in 1941. Based on her pictures, Rosalind Franklin discovered the full structure of the virus in 1955.[20] In the same year, Heinz Fraenkel-Conrat and Robley Williams showed that purified Tobacco mosaic virus RNA and its coat protein can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably how viruses assembled within their host cells.[21]

The second half of the twentieth century was the golden age of virus discovery and most of the 2,000 recognised species of animal, plant and bacterial viruses were discovered during these years.[1][22] In 1957, equine arterivirus and the cause of Bovine virus diarrhea (a pestivirus) were discovered. In 1963, the hepatitis B virus was discovered by Baruch Blumberg,[23] and in 1965, Howard Temin described the first retrovirus. Reverse transcriptase, the key enzyme that retroviruses use to translate their RNA into DNA, was first described in 1970, independently by Howard Temin and David Baltimore.[24] In 1983 Luc Montagnier's team at the Pasteur Institute in France, first isolated the retrovirus now called HIV.[25]



Most lizards retain the typical tetrapod body plan of a short neck, four limbs of roughly equal size ending in five toes each, a moderately long body, and a long tail. Most lizards possess external ears and have movable eyelids. Encompassing forty families, there is tremendous variety in color, appearance and size. Many are also capable of regenerating tails. Almost all lizards are carnivorous, though most are so small that insects are their primary prey. About 120 species, mostly iguanids, are known to be herbivorous. A few species are omnivorous, and others have reached sizes where they can prey on other vertebrates. Many lizards are good climbers or fast sprinters. Some can run bipedally, such as the collared lizard, and some can even run across the surface of water to escape, notably the basilisk. The fastest lizard is the Costa Rican spiny-tailed iguana (Ctenosaura similis), which can sprint at up to 21.5 miles/h (34.6 km/h).[1]

Many lizards can change colour in response to their environments or in times of stress. The most familiar example is the chameleon, but more subtle colour changes occur in other lizard species as well, such as the anole, also known as the "American chameleon", "house chameleon" or "chameleon."

Lizards have relatively low rates of metabolism, as compared with birds or mammals, and are termed ectothermic, meaning that they rely on external sources for heat. Many species bask in the sun to raise their body temperature. By this behavioral thermoregulation mechanism, many species are able to maintain high and quite stable body temperatures while they are active. At night, however, or during cold times of the year, behavioral thermoregulation cannot achieve high body temperatures, so their temperature tends to vary with that of their surroundings, and lizards are also termed poikilothermic. Lizards are most diverse and abundant in areas with consistently high temperatures, including many deserts. Lizards are rarely seen in the upper half of the United States and most European countries.

[edit] Senses and communication

Lizards employ many diverse methods of communication. Like many other animals, they have an acute sense of smell, detecting scents of their prey or pheromones from other lizards. The primary organ of scent in lizards is a vomeronasal organ in the roof of the mouth, and lizards gather scents by flicking out their tongues, then retracting them and delivering the captured odor molecules to this organ. Some large carnivorous lizards, such as tegus and monitor lizards, have forked tongues like snakes, to take advantage of this organ better. As a result, many male lizards possess enlarged pores on the underside of their thighs, which they rub against objects to mark their territory.

While most lizards can hear well, few are capable of vocalizations or otherwise making noise. The exception to this rule is the geckos, which communicate through a wide variety of barks, chirps and whistles, with each species having specific patterns and sounds.

Sight is quite important for most lizards, both for locating prey and for communication, and as such, many lizards have highly acute color vision. Most lizards rely heavily on body language, using specific postures, gestures and movements to define territory, resolve disputes, and entice mates. Some species of lizard also utilize bright colors, such as the iridescent patches on the belly of Sceloporus. These colors would be highly visible to predators, so are often hidden on the underside or between scales and only revealed when necessary.

A particular innovation in this respect is the dewlap, a brightly colored patch of skin on the throat, usually hidden between scales. When a display is needed, the lizards erect the hyoid bone of their throat, resulting in a large vertical flap of brightly colored skin beneath the head which can be then used for communication. Anoles are particularly famous for this display, with each species having specific colors, including patterns only visible under ultraviolet light, as lizards can often see UV.

[edit] Evolution and relationships
Frilly-necked lizard Chlamydosaurus kingii

The retention of the basic tetrapod body form by lizards makes it tempting to assume any similar animal, alive or extinct, is also a lizard. However, this is not the case, and lizards are part of a well-defined group.

The first reptile was superficially lizard-like, but had a solid, box-like skull, with openings only for eyes, nostrils, etc (termed Anapsid). These organisms later gave rise to two new groups with additional holes in the skull to make room for and anchor larger jaw muscles.Those with a single hole, the Synapsids, became modern mammals. The Diapsids, possessing two holes, continued to diversify. The Archosaurs retained the basic Diapsid skull, and gave rise to a bewildering array of animals, most famous being the dinosaurs and their descendants, birds. The Lepidosaurs began to reduce the skull bones, making the skull lighter and more flexible. Modern tuataras retain the basic Lepidosaur skull, distinguishing them from true lizards in spite of superficial similarities. Squamates, including snakes and all true lizards, further lightened the skull by eliminating the lower margin of the lower skull opening.



Dragons are usually shown in modern times with a body like a huge lizard, or a snake with two pairs of lizard-type legs, and able to emit fire from its mouth. The European dragon has bat-type wings growing from its back. A dragon-like creature with no front legs is known as a wyvern. Following discovery of how pterosaurs walked on the ground, some dragons have been drawn without front legs and using the wings as front legs pterosaur-fashion when on the ground, as in the movie Reign of Fire.


Like most mythological creatures, dragons are perceived in different ways by different cultures. Dragons are sometimes said to breathe and spit fire or poison, and ice. They are commonly portrayed as serpentine or reptilian, hatching from eggs and possessing typically feathered or scaly bodies. They are sometimes portrayed as having large yellow or red eyes, a feature that is the origin for the word for dragon in many cultures. They are sometimes portrayed with a row of dorsal spines, keeled scales, or leathery bat-like wings. Winged dragons are usually portrayed only in European dragons while Oriental versions of the dragon resemble large snakes. Dragons can have a variable number of legs: none, two, four, or more when it comes to early European literature. Dragons always hate mirrors.[citation needed] Also, some dragons in Greek literature were known to have millions of legs at a time. Modern depictions of dragons tend to be larger than their original representations, which were often smaller than humans, but grew in the myths and tales of man over the years.

Although dragons occur in many legends around the world, different cultures have varying stories about monsters that have been grouped together under the dragon label.

Dragons are often held to have major spiritual significance in various religions and cultures around the world. In many Asian cultures dragons were, and in some cultures still are, revered as representative of the primal forces of nature, religion and the universe. They are associated with wisdom—often said to be wiser than humans—and longevity. They are commonly said to possess some form of magic or other supernatural power, and are often associated with wells, rain, and rivers. In some cultures, they are also said to be capable of human speech.

The term dragoon, for infantry that moved around on horseback yet still fought as foot soldiers, is derived from their early firearm, the "dragon", a wide-bore musket that spat flame when it fired, and was thus named for the mythical creature.

Greek; etymology

In Ancient Greece the first mention of a "dragon" is derived from the Iliad where Agamemnon is described as having a blue dragon motif on his sword belt and a three-headed dragon emblem on his breast plate.[2]; however, the Greek word used (δράκων drakōn, genitive δράκοντοϛ drakontos) could also mean "snake". δράκων drakōn is a form of the aorist participle active of Greek δέρκομαι derkomai = "I see", and originally likely meant "that which sees", or "that which flashes or gleams" (perhaps referring to reflective scales). This is the origin of the word "dragon". (See also Hesiod's Theogony, 322.)

In 217 A.D., Philostratus discussed dragons (δράκων, drakōn) in India in The Life of Apollonius of Tyana (II,17 and III,6-8). The Loeb Classical Library translation (by F.C. Conybeare) mentions (III,7) that “In most respects the tusks resemble the largest swine’s, but they are slighter in build and twisted, and have a point as unabraded as sharks’ teeth.”


Main article: European dragon

European dragons exist in folklore and mythology among the overlapping cultures of Europe. Despite having wings, the dragon is generally depicted as having an underground lair or cave, making it an ancient creature of the earth element.



Magnetic field

Main article: Magnetic field

The magnetic field (usually denoted B) is called a field because it has a value at every point in space. The magnetic field (at a given point) is specified by two properties: (1) its direction, which is along the orientation of a compass needle; and (2) its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction. Direction and magnitude makes B a vector, so B is a vector field. (B can also depend on time.) In SI units the strength of the magnetic field is given in teslas.

[edit] Magnetic moment

Main article: Magnetic moment

A magnet's magnetic moment (also called magnetic dipole moment, and usually denoted μ) is a vector that characterizes the magnet's overall magnetic properties. For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole[1], and the magnitude relates to how strong and how far apart these poles are. In SI units the magnetic moment is specified in terms of A·m².

A magnet both produces its own magnetic field and it responds to magnetic fields. The strength of the magnetic field it produces is at any given point proportional to the magnitude of its magnetic moment. In addition, when the magnet is put into an "external" magnetic field produced by a different source, it is subject to a torque tending to orient the magnetic moment parallel to the field. The amount of this torque is proportional both to the magnetic moment and the "external" field. A magnet may also be subject to a force driving it in one direction or another, according to the positions and orientations of the magnet and source. If the field is uniform in space the magnet is subject to no net force, although it is subject to a torque.

A wire in the shape of a circle with area A and carrying current I is a magnet, with a magnetic moment of magnitude equal to IA.

[edit] Magnetization

Main article: Magnetization

The magnetization of a magnetized material is the local value of its magnetic moment per unit volume, usually denoted M, with units A/m. It is a vector field, rather than just a vector (like the magnetic moment), because different areas in a magnet can be magnetized with different directions and strengths (for example, due to domains, see below). A good bar magnet may have a magnetic moment of magnitude 0.1 A·m² and a volume of 1 cm³, or 0.000001 m³, and therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million A/m. Such a large value explains why magnets are so effective at producing magnetic fields.

[edit] Two models for magnets: magnetic poles and atomic currents


Binding and cover

Principal types of binding are padding, perfect, spiral, comb, sewn, clasp, disc, and pressure, some of which can be combined. Binding methods can affect whether a notebook can lie flat when open and whether the pages are likely to remain attached. The cover material is usually distinct from the writing surface material, more durable, more decorative, and more firmly attached. It also is stiffer than the leaves, even taken together. Cover materials should not contribute to damage or discomfort.

It is frequently cheaper to purchase notebooks that are spiral-bound, meaning that a spiral of wire is looped through large perforations at the top or side of the page. Other bound notebooks are available that use glue to hold the pages together; this process is commonly referred to as "padding". [1] Today it is common for pages in such notebooks to include a thin line of perforations that make it easier to tear out the page. Spiral-bound pages can be torn out but frequently leave thin scraggly strips from the small amount of paper that is within the spiral, as well as an uneven rip along the top of the torn-out page. Hard-bound notebooks include a sewn spine, and the pages are not easily removable. Some styles of sewn bindings allow pages to open flat, while others cause the pages to drape.

Variations of notebooks that allow pages to be added, removed, and replaced are bound by either rings, rods, or discs. In each of these systems the pages are modified with perforations that facilitate the specific binding mechanism's ability to secure them. Ring-bound and rod-bound notebooks secure their contents by threading perforated pages around straight or curved prongs. In the open position, the pages can be removed and re-arranged. In the closed position, the pages are kept in order. Disc-bound notebooks remove the open or closed operation by modifying the pages themselves. A page perforated for a disc-bound binding system contains a row of teeth along the side edge of the page that grip onto the outside raised perimeter of individual discs. Pages can be added or removed at any time by peeling the perforations away from each disc.

[edit] Preprinting

Notebooks used for drawing and scrapbooking are usually blank. Notebooks for writing usually have some kind of printing on the writing material, if only lines to align writing or facilitate certain kinds of drawing. Inventor's notebooks have page numbers preprinted to support priority claims. Many notebooks have graphic decorations. Personal organizers can have various kinds of preprinted pages.

[edit] Uses
Notes in a notebook

Artists often use large notebooks which include wide spaces of blank paper appropriate for drawing. Lawyers are also known for using rather large notebooks known as legal pads that contain lined paper (often yellow in color) and are appropriate for use on tables and desks. These horizontal lines or "rules" are sometimes classified according to their space apart with "wide rule" the farthest, "college rule" closer, "legal rule" slightly closer and "narrow rule" closest, allowing more lines of text per page. When sewn into a pasteboard backing, these may be called composition books, or in smaller signatures may be called "blue books" or exam books and used for essay exams. In contrast, journalists prefer small, hand-held notebooks for portability (often called reporters' notebooks), and sometimes use shorthand when taking notes. Scientists and other researchers use lab notebooks to document their experiments. The pages in lab notebooks are sometimes graph paper to make it easier to plot data. Police officers are required to write notes on what they observed whilst on duty, to do this they use a Police notebook. The most common notebooks are Mead, Staples, and Five Star.


History of computing

Main article: History of computer hardware

The Jacquard loom was one of the first programmable devices.

It is difficult to identify any one device as the earliest computer, partly because the term "computer" has been subject to varying interpretations over time. Originally, the term "computer" referred to a person who performed numerical calculations (a human computer), often with the aid of a mechanical calculating device.

The history of the modern computer begins with two separate technologies - that of automated calculation and that of programmability.

Examples of early mechanical calculating devices included the abacus, the slide rule and arguably the astrolabe and the Antikythera mechanism (which dates from about 150-100 BC). Hero of Alexandria (c. 10–70 AD) built a mechanical theater which performed a play lasting 10 minutes and was operated by a complex system of ropes and drums that might be considered to be a means of deciding which parts of the mechanism performed which actions and when.[3] This is the essence of programmability.

The "castle clock", an astronomical clock invented by Al-Jazari in 1206, is considered to be the earliest programmable analog computer.[4] It displayed the zodiac, the solar and lunar orbits, a crescent moon-shaped pointer travelling across a gateway causing automatic doors to open every hour,[5][6] and five robotic musicians who play music when struck by levers operated by a camshaft attached to a water wheel. The length of day and night could be re-programmed every day in order to account for the changing lengths of day and night throughout the year.[4]

The end of the Middle Ages saw a re-invigoration of European mathematics and engineering, and Wilhelm Schickard's 1623 device was the first of a number of mechanical calculators constructed by European engineers. However, none of those devices fit the modern definition of a computer because they could not be programmed.

In 1801, Joseph Marie Jacquard made an improvement to the textile loom that used a series of punched paper cards as a template to allow his loom to weave intricate patterns automatically. The resulting Jacquard loom was an important step in the development of computers because the use of punched cards to define woven patterns can be viewed as an early, albeit limited, form of programmability.

It was the fusion of automatic calculation with programmability that produced the first recognizable computers. In 1837, Charles Babbage was the first to conceptualize and design a fully programmable mechanical computer that he called "The Analytical Engine".[7] Due to limited finances, and an inability to resist tinkering with the design, Babbage never actually built his Analytical Engine.

Large-scale automated data processing of punched cards was performed for the U.S. Census in 1890 by tabulating machines designed by Herman Hollerith and manufactured by the Computing Tabulating Recording Corporation, which later became IBM. By the end of the 19th century a number of technologies that would later prove useful in the realization of practical computers had begun to appear: the punched card, Boolean algebra, the vacuum tube (thermionic valve) and the teleprinter.

During the first half of the 20th century, many scientific computing needs were met by increasingly sophisticated analog computers, which used a direct mechanical or electrical model of the problem as a basis for computation. However, these were not programmable and generally lacked the versatility and accuracy of modern digital computers.


Types of trains
Steam locomotive-hauled passenger train
German ICE high speed passenger train (a form of multiple unit)
British Rail Class 153 (single-unit) diesel railcar
An electric locomotive -hauled freight train

There are various types of train designed for particular purposes. A train can consist of a combination of one or more locomotives and attached railroad cars, or a self-propelled multiple unit (or occasionally a single powered coach, called a railcar). Trains can also be hauled by horses, pulled by a cable, or run downhill by gravity.

Special kinds of trains running on corresponding special 'railways' are atmospheric railways, monorails, high-speed railways, maglev, rubber-tired underground, funicular and cog railways.

A passenger train may consist of one or several locomotives, and one or more coaches. Alternatively, a train may consist entirely of passenger carrying coaches, some or all of which are powered as a "multiple unit". In many parts of the world, particularly Japan and Europe, high-speed rail is utilized extensively for passenger travel.

Freight trains comprise wagons or trucks rather than carriages, though some parcel and mail trains (especially Travelling Post Offices) are outwardly more like passenger trains.

Trains can also be 'mixed', comprising both passenger accommodation and freight vehicles. Such mixed trains are most likely to occur where services are infrequent, and running separate passenger and freight trains is not cost-effective, though the differing needs of passengers and freight usually means this is avoided where possible.

Special trains are also used for track maintenance; in some places, this is called maintenance of way.

In the United Kingdom, a train hauled by two locomotives is said to be "double-headed", and in Canada and the United States it is quite common for a long freight train to be headed by three or more locomotives. A train with a locomotive attached at each end is described as 'top and tailed', this practice typically being used when there are no reversing facilities available. Where a second locomotive is attached temporarily to assist a train up steep banks or grades (or down them by providing braking power) it is referred to as 'banking' in the UK, or 'helper service' in North America. Recently, many loaded trains in the US have been made up with one or more locomotives in the middle or at the rear of the train, operated remotely from the lead cab. This is referred to as "DP" or "Distributed Power."

[edit] Official terminology

The railway terminology that is used to describe a 'train' varies between countries.

United Kingdom

In the United Kingdom, the interchangeable terms set and unit are used to refer to a group of permanently or semi-permanently coupled vehicles, such as those of a multiple unit. While when referring to a train made up of a variety of vehicles, or of several sets/units, the term formation is used. (Although the UK public and media often forgo 'formation', for simply 'train'.) The word rake is also used for a group of coaches or wagons.

In the United Kingdom Section 83(1) of the Railways Act 1993 defines "train" as follows:

a) two or more items of rolling stock coupled together, at least one of which is a locomotive; or
b) a locomotive not coupled to any other rolling stock.

United States

In the United States, the term consist is used to describe the group of rail vehicles which make up a train. When referring to motive power, consist refers to the group of locomotives powering the train. Similarly, the term trainset refers to a group of rolling stock that is permanently or semi-permanently coupled together to form a unified set of equipment (the term is most often applied to passenger train configurations).

The Atchison, Topeka and Santa Fe Railway's 1948 operating rules define a train as: "An engine or more than one engine coupled, with or without cars, displaying markers."[2]