A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These programs enable computers to perform a wide range of tasks. A computer system is a nominally complete computer that includes the hardware, operating system (main software), and peripheral equipment needed and used for full operation. This term may also refer to a group of computers that are linked and function together, such as a computer network or computer cluster. A broad range of industrial and consumer products use computers as control systems. Simple special-purpose devices like microwave ovens and remote controls are included, as are factory devices like industrial robots and computer-aided design, as well as general-purpose devices like personal computers and mobile devices like smartphones. Computers power the Internet, which links billions of other computers and users. Early computers were meant to be used only for calculations. Simple manual instruments like the abacus have aided...

A computer worm is a standalone malware computer program that replicates itself in order to spread to other computers. It often uses a computer network to spread itself, relying on security failures on the target computer to access it. It will use this machine as a host to scan and infect other computers. When these new worm-invaded computers are controlled, the worm will continue to scan and infect other computers using these computers as hosts, and this behaviour will continue. Computer worms use recursive methods to copy themselves without host programs and distribute themselves based on the law of exponential growth, thus controlling and infecting more and more computers in a short time. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer. Many worms are designed only to spread, and do not attempt to change the systems they pass through. However, as the Morris worm and Mydoom showed, even these "payload-free" worms can cause major disruption by increasing network...

A computer worm is a standalone malware computer program that replicates itself in order to spread to other computers. It often uses a computer network to spread itself, relying on security failures on the target computer to access it. It will use this machine as a host to scan and infect other computers. When these new worm-invaded computers are controlled, the worm will continue to scan and infect other computers using these computers as hosts, and this behaviour will continue. Computer worms use recursive methods to copy themselves without host programs and distribute themselves based on the law of exponential growth, thus controlling and infecting more and more computers in a short time. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer. Many worms are designed only to spread, and do not attempt to change the systems they pass through. However, as the Morris worm and Mydoom showed, even these "payload-free" worms can cause major disruption by increasing network...

A cylinder seal is a small round cylinder, typically about one inch (2 to 3 cm) in length, engraved with written characters or figurative scenes or both, used in ancient times to roll an impression onto a two-dimensional surface, generally wet clay. According to some sources, cylinder seals were invented around 3500 BC in the Near East, at the contemporary sites of Uruk in southern Mesopotamia and slightly later at Susa in south-western Iran during the Proto-Elamite period, and they follow the development of stamp seals in the Halaf culture or slightly earlier. They are linked to the invention of the latter's cuneiform writing on clay tablets. Other sources, however, date the earliest cylinder seals to a much earlier time, to the Late Neolithic period (7600-6000 BC), hundreds of years before the invention of writing.Cylinder seals are a form of impression seal, a category which includes the stamp seal and finger ring seal. They survive in fairly large numbers and are important as art, especially in the Babylonian and earlier Assyrian periods. Impressions into a soft material can be taken without risk of damage to the seal, and they...

A Petri dish (alternatively known as a Petri plate or cell-culture dish) is a shallow transparent lidded dish that biologists use to hold growth medium in which cells can be cultured, originally, cells of bacteria, fungi and small mosses. The container is named after its inventor, German bacteriologist Julius Richard Petri. It is the most common type of culture plate. The Petri dish is one of the most common items in biology laboratories and has entered popular culture. The term is sometimes written in lower case, especially in non-technical literature.What was later called Petri dish was originally developed by German physician Robert Koch in his private laboratory in 1881, as a precursor method. Petri, as assistant to Koch, at Berlin University made the final modifications in 1887 as used today. Penicillin, the first antibiotic, was discovered in 1929 when Alexander Fleming noticed that mold that had contaminated a bacterial culture in a Petri dish had killed the bacteria all around it. Features and variants Petri dishes are usually cylindrical, mostly with diameters ranging from 30 to...

.

Walden (; first published in 1854 as Walden; or, Life in the Woods) is a book by American transcendentalist writer Henry David Thoreau. The text is a reflection upon the author's simple living in natural surroundings. The work is part personal declaration of independence, social experiment, voyage of spiritual discovery, satire, and—to some degree—a manual for self-reliance.Walden details Thoreau's experiences over the course of two years, two months, and two days in a cabin he built near Walden Pond amidst woodland owned by his friend and mentor Ralph Waldo Emerson, near Concord, Massachusetts. Thoreau makes precise scientific observations of nature as well as metaphorical and poetic uses of natural phenomena. He identifies many plants and animals by both their popular and scientific names, records in detail the color and clarity of different bodies of water, precisely dates and describes the freezing and thawing of the pond, and recounts his experiments to measure the depth and shape of the bottom of the supposedly "bottomless" Walden Pond. Background...

Piazza del Campo is the main public space of the historic center of Siena, Tuscany, Italy and is regarded as one of Europe's greatest medieval squares. It is renowned worldwide for its beauty and architectural integrity. The Palazzo Pubblico and its Torre del Mangia, as well as various palazzi signorili surround the shell-shaped piazza. At the northwest edge is the Fonte Gaia. The twice-a-year horse-race, Palio di Siena, is held around the edges of the piazza. The piazza is also the finish of the annual road cycling race Strade Bianche. History The open site was a marketplace established before the thirteenth century on a sloping site near the meeting point of the three hillside communities that coalesced to form Siena: the Castellare, the San Martino and the Camollia. Siena may have had earlier Etruscan settlements, but it was not a considerable Roman settlement, and the campo does not lie on the site of a Roman forum, as is sometimes suggested. It was paved in 1349 in fishbone-patterned red brick with 8 lines of travertine, which divide the piazza into 9 sections, radiating from the mouth of the gavinone...