A ship is a large vehicle used to travel on water. It is bigger than a boat. Most are cargo ships, which carry most of the world's international trade. There are also many warships, passenger ships and other kinds for different purposes.

When people talk about a ship, they often use the pronoun "she". For example: She was launched on 29 December 1878.

History

Early ships
The first ships used oars or the wind (or both) to make them move.

From about 4000 BC the Ancient Egyptians were making wooden sail boats. By 1200 BC the Phoenicians and Greeks had begun to make bigger sailing ships which were about 30 metres (100 feet) long and could carry 90180 tonnes of cargo. The Romans made even bigger ships which could carry up to 1,000 people and 1,000 tonnes of cargo. The 8th century saw the rise of the Vikings, who were famous for their "longships" and which were mainly used for raiding other countries, but also for trading. The longships had flat bottoms so they could move in shallow (not deep) rivers.

Age of sail
Sailing ships were used for thousands of years, but they were very important from the Age of Discovery to the 19th century. The Chinese admiral Zheng He commanded a fleet of large 'treasure ships' on seven voyages all over Asia up to East-Africa in the early fifteenth century. The most successful and largest fleet in the 17th century was the Dutch fleet (see the Netherlands). For trade and transport the Dutch often used a particular kind of trading ship, called a flute (fluyt in Dutch). Transport of people and cargo on sailing ships became rare in the early 20th century.

Some famous ships from this era include:
 Niña, Pinta, Santa Maria - Christopher Columbus' ships
 The Mayflower - The ship that carried the Pilgrims to Massachusetts
 Queen Anne's Revenge - The pirate Blackbeard's ship
 HMS Bounty - Captain Bligh's ship. Site of the most famous mutiny of all time
 USS Constitution - The most powerful ship in the early United States Navy
 HMS Victory - Admiral Horatio Nelson's flagship at the Battle of Trafalgar

The age of steam
In the 19th century, steamboats became commonplace.

At one time, the steamships Titanic, Olympic, and Britannic were the largest ships in the world, Titanic sank on her maiden voyage after hitting an iceberg, becoming one of the most famous shipwrecks of all time, the Olympic was Titanic 's nearly identical twin, and actually set sail before Titanic and was scrapped in the 1930s after a very successful career including her being a passenger liner and a warship in World War I. The Britannic was the largest of these three sister ships, and was supposed to be more grand and elegant than the Titanic, but before she set sail on her maiden voyage, WWI broke out and she was stripped of her elegant furniture and elaborate paneling and became a hospital ship. During her term as a hospital ship, she was sunk by either a mine or torpedo, no one knows for sure. The Titanic lies at the bottom of the North Atlantic Ocean, off the coast of Nova Scotia, and the Britannic lies in the Aegean Sea, off the coast of the Island of Kea.

Modern ships
After World War II ships with diesel engines became commonplace. Passenger airliners replaced passenger ships for long trips in the late 20th century. Cargo ships became much bigger. The main kinds are container ships for mostly manufactured goods, and bulk carriers including oil tankers.

Largest ship
The world's largest ship is the Prelude, owned by Shell. It is being built on Geoje Island, South Korea. It is 488m long and has the internal capacity greater than the total volume of four large aircraft carriers. What it will do is collect natural gas off the coast of Australia, and liquify it. When liquified, the hydrocarbon takes up 600 times less space than its gas. Smaller tankers will take the liquid gas to its buyers. The ship will do liquifying and temporary storage, which is usually done on land. Shell believes this justifies the cost of the ship.

The shipping yard builds all kinds of structures for the oil industry. It employs 30,000 workers.

Some names for parts of a ship

 Amidships - near the middle of the ship.
 Bow - the front of the ship.
 Stern - the back of the ship.
 Aft - in the direction of the stern.
 Astern - behind the ship.
 Starboard - the right side of the ship.
 Port - the left side of the ship.
 Bridge - the room in which the ship is controlled.
 Cabin - a room where a crew member lives.
 Decks - the floors.
 Galley - the kitchen.
 Hold - an area inside the ship used to carry goods.
 Hull - the main body of the ship.
 Keel - a beam running from stern to bow.
 Mast - a central pole on which sails are hung.
 Brig - Prison cells in the ship.

Some types of ships

 Bulk carrier - very large ship used for carrying very heavy cargo.
 Catamaran - a ship with two hulls.
 Cruise ship - a large passenger ship that takes people on holiday or vacations.
 Ferry - a passenger ship which often carries vehicles as well as people.
 Supertanker - a very large ship usually used for carrying oil.
 Warship
 Aircraft carrier - a warship which carries aircraft
 Battleship - a large warship
 PT boat- a small warship
 Submarine - an underwater boat

References

Basic English 850 words

Navigation

Ship


A file extension or filename extension is a suffix at the end of a filename. It is used to show the type of a computer file. This suggests what program can understand it. For example, MYWORDS.TXT is a plain text file and may be opened by a text editor. Filename extensions come after the name of the file, and many of them are three letters long.

Some examples of common file extensions are:
 HASH are hash program files
 TXT files are plain text files
 BMP are bitmap picture files
 JPG are picture files in the JPEG format
 MP3 are music files in the MP3 format
 AAC are Advanced Audio Coding music files
 FLAC are Free Lossless Audio Codec
 ALAC are Apple Lossess Audio Codec
 WMA are Windows Media Audio sound files
 MPEG are motion picture encoded video files
 MKV are Mastroska Video Codec
 WMV are Windows Media Video files
 HTM or *HTML are Hyper Text Markup Language files such as web pages
 PHP are web server scripts which create web pages
 ODT are Open Document text files
 DOC are text documents in Microsoft Word format
 XLS are Microsoft Excel spreadsheet documents
 PPT are Microsoft Power Point presentation files
 EXE are Microsoft Windows executable program files
 DLL are Dynamic Link Libraries, a type of executable file, in MS Windows
 ZIP are Compressed files (Lempel Ziff algorithm in an archive)
 Z are Unix / Linux compressed files
 bz2 are bzip2 block compressed files (very good compression)
 jar are Java archive files
 JSP are Java server pages files
 java are Java source code files
 class are Java compiled source code files
 oc are Java run-time library files
 tar are Unix / Linux tape archive files
 sh are Unix / Linux shell script files
 awk are Unix C like pattern processing language source code files
 sed are Unix stream editor command files
 lex are Unix / Linux lexical analyzer C code generator specification files
 c are C programming language source files
 o are C programming language compiled (object) files
 cs are C# programming language files
 rb are Ruby programming language files
 pl are Perl or Prolog programming language files

There are many other commonly used file extensions.

You can change the program defaults so that the computer knows which program to open each type of file extension with.

Software
Computer file formats

File extension


Robert Brown   (21 December 1773 – 10 June 1858) was a Scottish botanist and paleobotanist who made important contributions to botany largely through his pioneering use of the microscope. His contributions include one of the earliest detailed descriptions of the cell nucleus and cytoplasmic streaming; the observation of Brownian motion; early work on plant pollination and fertilisation, including being the first to recognise the fundamental difference between gymnosperms and angiosperms; and some of the earliest studies in palynology. He also made numerous contributions to plant taxonomy, notably erecting a number of plant families that are still accepted today; and numerous Australian plant genera and species, the fruit of his exploration of that continent with Matthew Flinders.

Early life

Robert Brown
was born in Montrose on 21 December 1773, in a house that existed on the site where Montrose Library currently stands. He was the son of James Brown, a minister in the Scottish Episcopal Church with Jacobite convictions so strong that in 1788 he defied his church's decision to give allegiance to George III. His mother was Helen Brown née Taylor, the daughter of a Presbyterian minister. As a child Brown attended the local Grammar School (now called Montrose Academy), then Marischal College at Aberdeen, but withdrew in his fourth year when the family moved to Edinburgh in 1790. His father died late the following year. He had a friend namely Ishaan lanker who accompanied him with his various researchs.

Brown enrolled to study medicine at the University of Edinburgh, but developed an interest in botany and ended up spending more of his time on the latter than the former. He attended the lectures of John Walker; made botanical expeditions into the Scottish Highlands, alone or with nurserymen such as George Don; and wrote out meticulous botanical descriptions of the plants he collected. He also began corresponding with and collecting for William Withering, one of the foremost British botanists of his day. Highlights for Brown during this period include his discovery of a new species of grass, Alopecurus alpinus, and his first botanical paper, "The botanical history of Angus", read to the Edinburgh Natural History Society in January 1792 but not published in print in Brown's lifetime.

Brown dropped out of his medical course in 1793. Late in 1794 he enlisted in the Fifeshire Fencibles, and his regiment was posted to Ireland shortly after. In June 1795 he was appointed Surgeon's Mate. His regiment saw very little action and he had a good deal of leisure time, almost all of which he spent on botany. He was frustrated by his itinerant lifestyle, which prevented him from building his personal library and specimen collection as he would have liked, and cut him off from the most important herbaria and libraries.

During this period Brown was especially interested in cryptogams, and these would be the subject of Brown's first, albeit unattributed, publication. Brown began a correspondence with James Dickson, and by 1796 was sending him specimens and descriptions of mosses. Dickson incorporated Brown's descriptions into his Fasciculi plantarum cryptogamicarum britanniae, with Brown's permission but without any attribution.

By 1800 Brown was firmly established amongst Irish botanists and was corresponding with a number of British and foreign botanists, including Withering, Dickson, James Edward Smith and José Correia da Serra. He had been nominated to the Linnean Society of London; had contributed to Dickson's Fasciculi; was acknowledged in a number of other works; and had had a species of algae, Conferva brownii (now Aegagropila linnaei) named after him by Lewis Weston Dillwyn. He had also begun experimenting with microscopy. However, as an army surgeon stationed in Ireland there seemed little prospect of him attracting the notice of those who could offer him a career in botany.

To Australia on the Investigator
In 1798 Brown heard that Mungo Park had withdrawn from a proposed expedition into the interior of New Holland (now Australia), leaving a vacancy for a naturalist. At Brown's request, Correia wrote to Sir Joseph Banks, suggesting Brown as a suitable replacement:  He was not selected, and the expedition did not end up going ahead as originally proposed, though George Caley was sent to New South Wales as a botanical collector for Banks. In 1800, however, Matthew Flinders put to Banks a proposal for an expedition that would answer the question whether New Holland was one island or several. Banks approved Flinders' proposal, and in December 1800 wrote to Brown offering him the position of naturalist to the expedition. Brown accepted immediately.

Preparations
Brown was told to expect to sail at the end of 1800, only a few weeks after being offered the position. A succession of delays meant the voyage did not get under way until July 1801. Brown spent much of the meantime preparing for the voyage by studying Banks' Australian plant specimens and copying out notes and descriptions for use on the voyage.

Though Brown's brief was to collect scientific specimens of all sorts, he was told to give priority to plants, insects, and birds, and to treat other fields, such as geology, as secondary pursuits. In addition to Brown, the scientific staff comprised the renowned botanical illustrator Ferdinand Bauer; the gardener Peter Good, whose task was to collect live plants and viable seed for the use of Kew Gardens; the miner John Allen, appointed as mineralogist; the landscape artist William Westall; and the astronomer John Crosley, who would fall ill on the voyage out and leave the ship at the Cape of Good Hope, being belatedly replaced at Sydney by James Inman. Brown was given authority over Bauer and Good, both of whom were instructed to give any specimens they might collect to Brown, rather than forming separate collections. Both men would provide enthusiastic and hard-working companions for Brown, and thus Brown's specimen collections contain material collected by all three men.

Desertas, Madeira and the Cape of Good Hope
Investigator sailed from London on 18 July. They made brief landfalls at Bugio Island (Desertas Islands) and Madeira, but Brown was disappointed to collect almost nothing of note from either site. They arrived at the Cape of Good Hope on 16 October, staying a little over two weeks, during which time Brown made extensive botanical expeditions and climbed Table Mountain at least twice. Many years later he would write to William Henry Harvey, who was considering emigrating there, that "some of the pleasantest botanizing he ever had was on Devil's Mountain, near Cape Town, and he thought I could not pitch on a more delightful field of study." Amongst the plants collected at the Cape were two new species of Serruria (Proteaceae), S. foeniculacea and S. flagellaris.

Australia
 arrived in King George Sound in what is now Western Australia in December 1801. For three and a half years Brown did intensive botanic research in Australia, collecting about 3400 species, of which about 2000 were previously unknown.  A large part of this collection was lost when  was wrecked en route to England.

Brown remained in Australia until May 1805. He then returned to Britain where he spent the next five years working on the material he had gathered.  He published numerous species descriptions; in Western Australia alone he is the author of nearly 1200 species. The list of major Australian genera that he named includes: Livistona, Triodia, Eriachne, Caladenia, Isolepis, Prasophyllum, Pterostylis, Patersonia, Conostylis, Thysanotus, Pityrodia, Hemigenia, Lechenaultia, Eremophila, Logania, Dryandra, Isopogon, Grevillea, Petrophile, Telopea, Leptomeria, Jacksonia, Leucopogon, Stenopetalum, Ptilotus, Sclerolaena and Rhagodia.

Subsequent career

In early 1809 he read his paper called On the natural order of plants called Proteaceae to the Linnean Society of London. This was subsequently published in March 1810 as On the Proteaceae of Jussieu. It is significant for its contribution to the systematics of Proteaceae, and to the floristics of Australia, and also for its application of palynology to systematics. This work was extensively plagiarised by Richard Anthony Salisbury, who had memorised much of the Linnean reading and then inserted it in Joseph Knight's 1809 publication On the cultivation of the plants belonging to the natural order of Proteeae.

In 1810, he published the results of his collecting in his famous Prodromus Florae Novae Hollandiae et Insulae Van Diemen, the first systematic account of the Australian flora. Over half of New Zealand's orchid genera were first described in the work.  That year, he succeeded Jonas C. Dryander as Sir Joseph Banks' librarian, and on Banks' death in 1820 Brown inherited his library and herbarium. This was transferred to the British Museum in 1827, and Brown was appointed Keeper of the Banksian Botanical Collection.

In 1818 he published Observations, systematical and geographical, on the herbarium collected by Professor Christian Smith, in the vicinity of the Congo. In 1822, he was elected a Fellow of the Linnean Society and a foreign member of the Royal Swedish Academy of Sciences. In 1827 he became correspondent of the Royal Institute of the Netherlands, three years later he became associated member. When the institute became the Royal Netherlands Academy of Arts and Sciences in 1851 Brown joined as foreign member. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1849.

In a paper read to the Linnean society in 1831 and published in 1833, Brown named the cell nucleus. The nucleus had been observed before, perhaps as early as 1682 by the Dutch microscopist Leeuwenhoek, and Franz Bauer had noted and drawn it as a regular feature of plant cells in 1802, but it was Brown who gave it the name it bears to this day (while giving credit to Bauer's drawings). Neither Bauer nor Brown thought the nucleus to be universal, and Brown thought it to be primarily confined to Monocotyledons.

After the division of the Natural History Department of the British Museum into three sections in 1837, Robert Brown became the first Keeper of the Botanical Department, remaining so until his death. He was succeeded by John Joseph Bennett.

He served as president of the Linnean Society from 1849 to 1853.

Brown died at 17 Dean Street, Soho Square in London, on 10 June 1858. He was buried in Kensal Green Cemetery in London.

Legacy
Brown's name is commemorated in the Australian herb genus Brunonia as well as numerous Australian species such as Eucalyptus brownii, Banksia brownii and the moss Brown's Tetrodontium Moss (Tetrodontium brownianum), a species which he discovered growing at Roslin near Edinburgh whilst still a student. The plant can still be found at the site of its discovery. Brown's River, south of Hobart, upon the banks of which he collected botanical samples, was named in his honour. In South Australia, Mount Brown and Point Brown (near Smoky Bay) were named for him by Flinders during the Investigator expedition. Mount Brown  in British Columbia, Canada was named for him by David Douglas.

In 1938 the London County Council commemorated Brown, as well as botanists Joseph Banks and David Don, and meetings of the Linnean Society, with a rectangular stone plaque at 32 Soho Square.

A small New Zealand tree Pisonia brunoniana was named in recognition of him, and Cape Brown (Greenland) was named by William Scoresby (1789–1857) in 1822 in his honour.

Brownian motion
In 1827, while examining grains of pollen of the plant Clarkia pulchella suspended in water under a microscope, Brown observed minute particles, now known to be amyloplasts (starch organelles) and spherosomes (lipid organelles), ejected from the pollen grains, executing a continuous jittery motion.  He then observed the same motion in particles of inorganic matter, enabling him to rule out the hypothesis that the effect was life-related.  Although Brown did not provide a theory to explain the motion, the phenomenon is now known as Brownian motion.

In recent years controversy arose over whether Brown's microscopes were sufficient to reveal phenomena of this order. Brown's discoveries were denied in a brief paper in 1991. Shortly thereafter, in an illustrated presentation, British microscopist Brian J. Ford presented to Inter Micro 1991 in Chicago a reprise of the demonstration using Brown's original microscope. His video sequences substantiated Brown's observations, suggesting Brown's microscope was sufficient to allow him to see motion.  Physicist Phil Pearle and colleagues presented a detailed discussion of Brown's original observations of particles from pollen of Clarkia pulchella undergoing Brownian motion, including the relevant history, botany, microscopy, and physics.

Publications
For a list of Brown's publications, see Wikisource.

Further reading

Joel Schwartz. Robert Brown and Mungo Park: Travels and Explorations in Natural History for the Royal Society. Cham Switzerland: Springer Nature, 2022.

See also
 Prodromus Florae Novae Hollandiae et Insulae Van Diemen
 Brown's taxonomic arrangement of Banksia
 List of Australian plant species authored by Robert Brown
 European and American voyages of scientific exploration
 Character and description of Kingia
 Taxa named by Robert Brown

Notes

Further reading

 Mabberley, David (2002), 'Brown, Robert', in R. Aitken and M. Looker (eds), Oxford Companion to Australian Gardens, South Melbourne, Oxford University Press, pp. 108–10.
 Moore, D. T. and Groves, E.W . A catalogue of plants written by Robert Brown (1773–1858) in New South Wales: first impressions of the flora of the Sydney region. Archives of Natural History 24 (2): 281–293 (June 1997).
 Munster, P., (2002), 'Robert Brown at Swan Bay', Australian Garden History, 14 (3), p. 10.

External links

 Classic papers by Robert Brown PDFs of several original papers by Robert Brown are available from this webpage.
 Robert Brown's Australian Botanical Specimens, 1801–1805 at the British Museum (BM) A comprehensive database.
 Robert Brown's work on orchids.
 Robert Brown on Ask.com

Botanical collectors active in Australia
Botanists active in Australia
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British pteridologists
British taxonomists
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Botany in Western Australia
Paleobotanists
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Honorary Fellows of the Royal Society of Edinburgh
Members of the Royal Netherlands Academy of Arts and Sciences
Members of the Royal Swedish Academy of Sciences
Corresponding members of the Saint Petersburg Academy of Sciences
Honorary members of the Saint Petersburg Academy of Sciences
Presidents of the Linnean Society of London
Recipients of the Copley Medal
Recipients of the Pour le Mérite (civil class)
Alumni of the University of Aberdeen
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Burials at Kensal Green Cemetery

Robert Brown (botanist, born 1773)


Transport Layer Security (TLS) Protocol and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide security and data integrity for communications over TCP/IP networks such as the Internet. Several versions of the protocols are common in applications such as web browsing, electronic mail, Internet faxing, instant messaging and voice-over-IP (VoIP).

Description
The TLS protocol allows applications to communicate across a network in a way designed to prevent eavesdropping, tampering, and message forgery. TLS provides endpoint authentication and communications confidentiality over the Internet using cryptography. Most of the time, only the server is authenticated (i.e., its identity is ensured) while the client remains unauthenticated; this means that the end user (whether an individual or an application, such as a Web browser) can be sure with whom it is communicating. The next level of security is known as mutual authentication. Mutual authentication requires public key infrastructure (PKI) deployment to clients unless TLS-PSK or the Secure Remote Password (SRP) protocol are used, which provide strong mutual authentication without needing to deploy a PKI.

Usage
SSL and TLS have been widely used in several open source software projects. Programmers may use the OpenSSL, NSS, or GnuTLS libraries for SSL/TLS functionality. Microsoft Windows includes a usage of SSL and TLS as part of its Secure Channel package. Delphi programmers may use a library called Indy.

Standards
The current approved version is 1.2, which is specified in:
 RFC 5246: "The Transport Layer Security (TLS) Protocol Version 1.2".

The current standard obsoletes these former versions:
 RFC 2246: "The TLS Protocol Version 1.0".
 RFC 4346: "The Transport Layer Security (TLS) Protocol Version 1.1".

Other RFCs subsequently extended TLS, including:
 RFC 2595: "Using TLS with IMAP, POP3 and ACAP". Specifies an extension to the IMAP, POP3 and ACAP services that allow the server and client to use transport-layer security to provide private, authenticated communication over the Internet.
 RFC 2712: "Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)". The 40-bit ciphersuites defined in this memo appear only for the purpose of documenting the fact that those ciphersuite codes have already been assigned.
 RFC 2817: "Upgrading to TLS Within HTTP/1.1", explains how to use the Upgrade mechanism in HTTP/1.1 to initiate Transport Layer Security (TLS) over an existing TCP connection. This allows unsecured and secured HTTP traffic to share the same well known port (in this case, http: at 80 rather than https: at 443).
 RFC 2818: "HTTP Over TLS", distinguishes secured traffic from insecure traffic by the use of a different 'server port'.
 RFC 3207: "SMTP Service Extension for Secure SMTP over Transport Layer Security". Specifies an extension to the SMTP service that allows an SMTP server and client to use transport-layer security to provide private, authenticated communication over the Internet.
 RFC 3268: "AES Ciphersuites for TLS". Adds Advanced Encryption Standard (AES) ciphersuites to the previously existing symmetric ciphers.
 RFC 3546: "Transport Layer Security (TLS) Extensions", adds a mechanism for negotiating protocol extensions during session initialisation and defines some extensions. Made obsolete by RFC 4366.
 RFC 3749: "Transport Layer Security Protocol Compression Methods", specifies the framework for compression methods and the DEFLATE compression method.
 RFC 3943: "Transport Layer Security (TLS) Protocol Compression Using Lempel-Ziv-Stac (LZS)".
 RFC 4132: "Addition of Camellia Cipher Suites to Transport Layer Security (TLS)".
 RFC 4162: "Addition of SEED Cipher Suites to Transport Layer Security (TLS)".
 RFC 4217: "Securing FTP with TLS".
 RFC 4279: "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", adds three sets of new ciphersuites for the TLS protocol to support authentication based on pre-shared keys.
 RFC 4347: "Datagram Transport Layer Security" specifies a TLS variant that works over datagram protocols (such as UDP).
 RFC 4366: "Transport Layer Security (TLS) Extensions" describes both a set of specific extensions, and a generic extension mechanism.
 RFC 4492: "Elliptic Curve Cryptography  (ECC) Cipher Suites for Transport Layer Security (TLS)".
 RFC 4507: "Transport Layer Security (TLS) Session Resumption without Server-Side State".
 RFC 4680: "TLS Handshake Message for Supplemental Data".
 RFC 4681: "TLS User Mapping Extension".
 RFC 4785: "Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for Transport Layer Security (TLS)".

Related pages
 Certificate authority
 Public key certificate
 Extended Validation Certificate
 SSL acceleration
 Datagram Transport Layer Security
 Multiplexed Transport Layer Security
 X.509
 Virtual private network
 SEED
 Server gated cryptography

Software
 OpenSSL: a free (and very popular) implementation (BSD license with some banter extensions)
 GnuTLS: a free LGPL licensed implementation
 JSSE: a Java implementation included in the Java Runtime Environment
 Network Security Services (NSS): FIPS 140 validated open source library

References

Further reading

Cryptography
Internet standards
Internet protocols

Transport Layer Security


In electricity generation, a generator is a device that converts motion-based power (potential and kinetic energy) or fuel-based power (chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines, wind turbines and even hand cranks. The first electromagnetic generator, the Faraday disk, was invented in 1831 by British scientist Michael Faraday. Generators provide nearly all the power for electrical grids.

In addition to electricity- and motion-based designs, photovoltaic and fuel cell powered generators use solar power and hydrogen-based fuels, respectively, to generate electrical output.

The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators are very similar. Many motors can generate electricity from mechanical energy.

Terminology

Electromagnetic generators fall into one of two broad categories, dynamos and alternators.

 Dynamos generate pulsing direct current through the use of a commutator.
 Alternators generate alternating current.

Mechanically, a generator consists of a rotating part and a stationary part which together form a magnetic circuit:

Rotor: The rotating part of an electrical machine.
Stator: The stationary part of an electrical machine, which surrounds the rotor.

One of these parts generates a magnetic field, the other has a wire winding in which the changing field induces an electric current:
Field winding or field (permanent) magnets: The magnetic field-producing component of an electrical machine. The magnetic field of the dynamo or alternator can be provided by either wire windings called field coils or permanent magnets. Electrically-excited generators include an excitation system to produce the field flux. A generator using permanent magnets (PMs) is sometimes called a magneto, or a permanent magnet synchronous generator (PMSG).
Armature: The power-producing component of an electrical machine.  In a generator, alternator, or dynamo, the armature windings generate the electric current, which provides power to an external circuit.
The armature can be on either the rotor or the stator, depending on the design, with the field coil or magnet on the other part.

History
Before the connection between magnetism and electricity was discovered, electrostatic generators were invented. They operated on electrostatic principles, by using moving electrically charged belts, plates and disks that carried charge to a high potential electrode. The charge was generated using either of two mechanisms: electrostatic induction or the triboelectric effect. Such generators generated very high voltage and low current. Because of their inefficiency and the difficulty of insulating machines that produced very high voltages, electrostatic generators had low power ratings, and were never used for generation of commercially significant quantities of electric power. Their only practical applications were to power early X-ray tubes, and later in some atomic particle accelerators.

Faraday disk generator

The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by Michael Faraday. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.

He also built the first electromagnetic generator, called the Faraday disk; a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage.

This design was inefficient, due to self-cancelling counterflows of current in regions of the disk that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counterflow limited the power output to the pickup wires and induced waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.

Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher, more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.

Jedlik and the self-excitation phenomenon
Independently of Faraday, Ányos Jedlik started experimenting in 1827 with the electromagnetic rotating devices which he called electromagnetic self-rotors. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. It was also the discovery of the principle of dynamo self-excitation, which replaced permanent magnet designs. He also may have formulated the concept of the dynamo in 1861 (before Siemens and Wheatstone) but did not patent it as he thought he was not the first to realize this.

Direct current generators

A coil of wire rotating in a magnetic field produces a current which changes direction with each 180° rotation, an alternating current (AC). However many early uses of electricity required direct current (DC). In the first practical electric generators, called dynamos, the AC was converted into DC with a commutator, a set of rotating switch contacts on the armature shaft. The commutator reversed the connection of the armature winding to the circuit every 180° rotation of the shaft, creating a pulsing DC current. One of the first dynamos was built by Hippolyte Pixii in 1832.

The dynamo was the first electrical generator capable of delivering power for industry.
The Woolrich Electrical Generator of 1844, now in Thinktank, Birmingham Science Museum, is the earliest electrical generator used in an industrial process. It was used by the firm of Elkingtons for commercial electroplating.

The modern dynamo, fit for use in industrial applications, was invented independently by Sir Charles Wheatstone, Werner von Siemens and Samuel Alfred Varley. Varley took out a patent on 24 December 1866, while Siemens and Wheatstone both announced their discoveries on 17 January 1867, the latter delivering a paper on his discovery to the Royal Society.

The "dynamo-electric machine" employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field. Wheatstone's design was similar to Siemens', with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in Wheatstone's design they were in parallel. The use of electromagnets rather than permanent magnets greatly increased the power output of a dynamo and enabled high power generation for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s Siemens used electromagnetic dynamos to power electric arc furnaces for the production of metals and other materials.

The dynamo machine that was developed consisted of a stationary structure, which provides the magnetic field, and a set of rotating windings which turn within that field. On larger machines the constant magnetic field is provided by one or more electromagnets, which are usually called field coils.

Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution. Before the adoption of AC, very large direct-current dynamos were the only means of power generation and distribution. AC has come to dominate due to the ability of AC to be easily transformed to and from very high voltages to permit low losses over large distances.

Synchronous generators (alternating current generators)

Through a series of discoveries, the dynamo was succeeded by many later inventions, especially the AC alternator, which was capable of generating alternating current. It is commonly known to be the Synchronous Generators (SGs). The synchronous machines are directly connected to the grid and need to be properly synchronized during startup. Moreover, they are excited with special control to enhance the stability of the power system.

Alternating current generating systems were known in simple forms from Michael Faraday's original discovery of the magnetic induction of electric current. Faraday himself built an early alternator. His machine was a "rotating rectangle", whose operation was heteropolar - each active conductor passed successively through regions where the magnetic field was in opposite directions.

Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. The first public demonstration of an "alternator system" was given by William Stanley, Jr., an employee of Westinghouse Electric in 1886.

Sebastian Ziani de Ferranti established Ferranti, Thompson and Ince in 1882, to market his Ferranti-Thompson Alternator, invented with the help of renowned physicist Lord Kelvin. His early alternators produced frequencies between 100 and 300 Hz. Ferranti went on to design the Deptford Power Station for the London Electric Supply Corporation in 1887 using an alternating current system. On its completion in 1891, it was the first truly modern power station, supplying high-voltage AC power that was then "stepped down" for consumer use on each street. This basic system remains in use today around the world.

After 1891, polyphase alternators were introduced to supply currents of multiple differing phases. Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.

Self-excitation

As the requirements for larger scale power generation increased, a new limitation rose: the magnetic fields available from permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic field coil allowed the generator to produce substantially more power. This concept was dubbed self-excitation.

The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of remanent magnetism present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This "bootstrap" process continues until the magnetic field in the core levels off due to saturation and the generator reaches a steady state power output.

Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.

Specialised types of generator

Direct current (DC)
A dynamo uses commutators to produce direct current. It is self-excited, i.e. its field electromagnets are powered by the machine's own output. Other types of DC generators use a separate source of direct current to energise their field magnets.

Homopolar generator

A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polarity depending on the direction of rotation and the orientation of the field.

It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. The voltage is typically low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage. They are unusual in that they can produce tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.

Magnetohydrodynamic (MHD) generator

A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25 MW demonstration plant in 1987. In the Soviet Union from 1972 until the late 1980s, the MHD plant U 25 was in regular utility operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time. MHD generators operated as a topping cycle are currently (2007) less efficient than combined cycle gas turbines.

Alternating current (AC)

Induction generator

Induction AC motors may be used as generators, turning mechanical energy into electric current. Induction generators operate by mechanically turning their rotor faster than the simultaneous speed, giving negative slip. A regular AC non-simultaneous motor usually can be used as a generator, without any changes to its parts. Induction generators are useful in applications like minihydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls. They do not require another circuit to start working because the turning magnetic field is provided by induction from the one they have. They also do not require speed governor equipment as they inherently operate at the connected grid frequency.

An induction generator must be powered with a leading voltage; this is usually done by connection to an electrical grid, or by powering themselves with phase correcting capacitors.

Linear electric generator

In the simplest form of linear electric generator, a sliding magnet moves back and forth through a solenoid - a copper wire or a coil. An alternating current is induced in the wire, or loops of wire, by Faraday's law of induction each time the magnet slides through. This type of generator is used in the Faraday flashlight. Larger linear electricity generators are used in wave power schemes.

Variable-speed constant-frequency generators
Grid-connected generators deliver power at a constant frequency. For generators of the synchronous or induction type, the primer mover speed turning the generator shaft must be at a particular speed (or narrow range of speed) to deliver power at the required utility frequency. Mechanical speed-regulating devices may waste a significant fraction of the input energy to maintain a required fixed frequency.

Where it is impractical or undesired to tightly regulate the speed of the prime mover, doubly fed electric machines may be used as generators. With the assistance of power electronic devices, these can regulate the output frequency to a desired value over a wider range of generator shaft speeds. Alternatively, a standard generator can be used with no attempt to regulate frequency, and the resulting power converted to the desired output frequency with a rectifier and converter combination. Allowing a wider range of prime mover speeds can improve the overall energy production of an installation, at the cost of more complex generators and controls. For example, where a wind turbine operating at fixed frequency might be required to spill energy at high wind speeds, a variable speed system can allow recovery of energy contained during periods of high wind speed.

Common use cases

Power station

A power station, also known as a power plant or powerhouse and sometimes generating station or generating plant, is an industrial facility that generates electricity. Most power stations contain one or more generators, or spinning machines converting mechanical power into three-phase electrical power. The relative motion between a magnetic field and a conductor creates an electric current. The energy source harnessed to turn the generator varies widely. Most power stations in the world burn fossil fuels such as coal, oil, and natural gas to generate electricity. Cleaner sources include nuclear power, and increasingly used renewables such as the sun, wind, waves and running water.

Vehicular generators

Roadway vehicles

Motor vehicles require electrical energy to power their instrumentation, keep the engine itself operating, and recharge their batteries. Until about the 1960s motor vehicles tended to use DC generators (dynamos) with electromechanical regulators. Following the historical trend above and for many of the same reasons, these have now been replaced by alternators with built-in rectifier circuits.

Bicycles
Bicycles require energy to power running lights and other equipment. There are two common kinds of generator in use on bicycles: bottle dynamos which engage the bicycle's tire on an as-needed basis, and hub dynamos which are directly attached to the bicycle's drive train. The name is conventional as these they are small permanent-magnet alternators, not self-excited DC machines as are dynamos. Some electric bicycles are capable of regenerative braking, where the drive motor is used as a generator to recover some energy during braking.

Sailboats
Sailing boats may use a water- or wind-powered generator to trickle-charge the batteries. A small propeller, wind turbine or turbine is connected to a low-power generator to supply currents at typical wind or cruising speeds.

Recreational vehicles
Recreational vehicles need an extra power supply to power their onboard accessories, including air conditioning units, and refrigerators. An RV power plug is connected to the electric generator to obtain a stable power supply.

Electric scooters
Electric scooters with regenerative braking have become popular all over the world. Engineers use kinetic energy recovery systems on the scooter to reduce energy consumption and increase its range up to 40-60% by simply recovering energy using the magnetic brake, which generates electric energy for further use. Modern vehicles reach speed up to 25–30 km/h and can run up to 35–40 km.

Genset

An engine-generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used, and there are even hybrid diesel-gas units, called dual-fuel units. Many different versions of engine-generators are available - ranging from very small portable petrol powered sets to large turbine installations. The primary advantage of engine-generators is the ability to independently supply electricity, allowing the units to serve as backup power sources.

Human powered electrical generators

A generator can also be driven by human muscle power (for instance, in field radio station equipment).

Human powered electric generators are commercially available, and have been the project of some DIY enthusiasts. Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries, and in some cases are designed with an integral inverter. An average "healthy human" can produce a steady 75 watts (0.1 horsepower) for a full eight hour period, while a "first class athlete" can produce approximately 298 watts (0.4 horsepower) for a similar period, at the end of which an undetermined period of rest and recovery will be required. At 298 watts, the average "healthy human" becomes exhausted within 10 minutes. The net electrical power that can be produced will be less, due to the efficiency of the generator. Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio. During the mid 20th century, pedal powered radios were used throughout the Australian outback, to provide schooling (School of the Air), medical and other needs in remote stations and towns.

Mechanical measurement
A tachogenerator is an electromechanical device which produces an output voltage proportional to its shaft speed. It may be used for a speed indicator or in a feedback speed control system. Tachogenerators are frequently used to power tachometers to measure the speeds of electric motors, engines, and the equipment they power. Generators generate voltage roughly proportional to shaft speed. With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds.

Equivalent circuit

An equivalent circuit of a generator and load is shown in the adjacent diagram. The generator is represented by an abstract generator consisting of an ideal voltage source and an internal impedance. The generator's  and  parameters can be determined by measuring the winding resistance (corrected to operating temperature), and measuring the open-circuit and loaded voltage for a defined current load.

This is the simplest model of a generator, further elements may need to be added for an accurate representation. In particular, inductance can be added to allow for the machine's windings and magnetic leakage flux, but a full representation can become much more complex than this.

See also

 Diesel generator
 Electricity generation
 Electric motor
 Engine-generator
 Faraday's law of induction
 Gas turbine
 Generation expansion planning
 Goodness factor
 Hydropower
 Steam generator (boiler)
 Steam generator (railroad)
 Steam turbine
 Superconducting electric machine
 Thermogenerator
 Thermal power station
 Tidal stream generator

References

English inventions
19th-century inventions

Electric generator


A proton is part of an atom. They are found in the nucleus of an atom along with neutrons. The periodic table groups atoms according to how many protons they have. A single atom of hydrogen (the lightest kind of atom) is made up of an electron moving around a proton. Most of the mass of this atom is in the proton, which is almost 2000 times heavier than the electron. Protons and neutrons make up the nucleus of every other kind of atom. In any one element, the number of protons is always the same. An atom's atomic number is equal to the number of protons in that atom.

Protons are made of quarks. A proton is believed to be made up of 3 quarks, two up quarks and one down quark. One down quark has a charge of -1/3, and two up quarks have a charge of +2/3 each. This adds to a charge of +1. A proton has a very small mass. The mass of the proton is about one atomic mass unit. The mass of the neutron is also about one atomic mass unit. The size of a proton is determined by the vibration of the quarks that are in it, and these quarks effectively form a cloud. This means that a proton is not so much a hard ball as an area that contains quarks.

Related pages
 Proton decay
 Neutron
 Electron
 Quarks

References

Subatomic particles

Proton


Leggings are several types of leg attire that have varied through the years. Modern usage from the 1960s onwards has come to refer to elastic close-fitting high-rise garments worn over the legs typically by women, such as leg warmers or tights. Usage from the 18th century refers to men's wear usually made of cloth or leather that is wrapped around the leg down to the ankle. In the 19th century, leggings usually referred to infants' leg clothing that were matched with a jacket, as well as leg-wrappings made of leather or wool and worn by soldiers and trappers. Leggings prominently returned to women's fashion in the 1960s, drawing from the form-fitting clothing of dancers. With the widespread adoption of the synthetic fibre Lycra and the rise in popularity of aerobics, leggings came to further prominence in the 1970s and 1980s, and eventually made their way into streetwear. Leggings are a part of the late 2010s into the 2020s athleisure fashion trend of wearing activewear outside sporting activities and in casual settings.

History

Leggings in various forms and under various names have been worn for warmth and protection by both men and women throughout the centuries. The separate hose worn by men in Europe from the 13th to 16th centuries (the Renaissance period) were a form of leggings, as are the trews of the Scottish Highlands. Separate leggings of buckskin leather were worn by some Native Americans. These were adopted by some Long Hunters, French fur trappers, and later by mountain men. They are the leatherstockings of James Fenimore Cooper's Leatherstocking Tales. The Buckskins were mostly a dull grey brain-tan, not the bright, glossy vegetable tanned leather commonly used today.

Cowboys wore leggings of buckskin to protect from chapping caused by riding, wear and tear to their pants, and bites from animals, such as snakes or insects in the scrub, such as ticks.

In many places, especially in colder countries such as Russia and Korea, men and women continued to wear wool leggings into modern times, often as an additional outer layer for warmth.

The linen pantalettes worn by girls and women under crinolines in the mid-19th century were also a form of leggings, and were originally two separate garments. Leggings became a part of fashion in the 1960s, as trousers similar to capri pants but tighter.

Military use

Since the late 19th century, soldiers of various nations, especially infantry, often wore leggings to protect their lower leg, to keep dirt, sand, and mud from entering their shoes, and to provide a measure of ankle support. At first, these were usually puttees—strips of thick woolen cloth resembling a large bandage—which were wrapped around the leg to support the ankle. They were usually held in place by a strap attached to the cloth. Later, puttees were replaced by some armies with canvas leggings fastened with buckles or buttons, usually secured at the bottom with an adjustable stirrup that passed under the sole of the shoe, just in front of the heel. The soldier placed the leggings around his calf with the buttoned side facing out and adjusted them and the strap to achieve a proper fit. Leggings typically extended to mid-calf and had a garter strap to hold them up and were secured with a tie just below the knee. Military leggings extended to the bottom of the knee and buttoned to the bottom button on the knee-breeches. They are sometimes confused with gaiters, which extend to the high ankle and are worn with full leg trousers.

During World War II, United States Army foot soldiers were referred to as legs by paratroopers and other U.S. forces that did not wear the standard Army leggings issued with the field service shoe. Late in World War II, after experiments with general issue of high-top combat boots and jump boots for soldiers, leggings began to disappear from military service. In 1943, the United States Army modified their field service shoe by adding a taller leather upper that reached to the lower calf; secured by a combination of laces and buckles, the new design was designated the Type III Field Boot. However, the United States Marine Corps retained canvas leggings throughout the war, and used them in combat as late as the Korean War; Marines were referred to as Yellow Leg troops by North Korean and Chinese Communist forces.

By the 1960s, the old style of field shoe had given way to combat boots in most military forces, and leggings of any kind were obsolete. Leggings, usually bright white and often made of patent leather or buff are now worn primarily for ceremonial purposes.

Modern fashion

Leggings in the form of skin-tight trousers, a tighter version of the capris ending at mid-calf or near ankle length, made their way into women's fashion in the 1960s, and were worn with a large belt or waistband and slip-on high heels or ballet flat–styled shoes.

Leggings made from a nylon-lycra blend (usually 90% nylon, 10% lycra) have long been worn during exercise. Nylon lycra leggings are often referred to as bicycle or running tights, and are shinier in appearance than those made from cotton. Some have racing stripes or reflective patterns to further distinguish them as athletic wear and provide extra safety. However, since the 1980s exercise-style leggings have also been worn for fashion and as street wear.

Leggings made from cotton-lycra, or a cotton-polyester-lycra combination, are more typically worn for fashion, but are also worn as exercise wear. Cotton-lycra leggings are available in many colors, prints and designs; but black, navy and various shades of gray remain the most commonly worn.

Wearing black leggings under long, often diaphanous, skirts was part of a general fashion trend of wearing gym or dance clothes as street wear that evolved along with the fitness craze and under the influence of the movie Flashdance and the long-running Broadway show A Chorus Line. A more recent trend has been the wearing of black leggings with miniskirts.

By the early-1990s, leggings were actually outselling jeans in many parts of the United States. It was very common to see leggings worn with long oversized tee-shirts, oversized sweatshirts or oversized sweaters, slouch socks and Keds by girls from toddler to over college age. Fashion turned against leggings for a short time in the late 1990s and early 2000s.

In 2005, leggings made a "comeback" into fashion, particularly in indie culture, with capri-length leggings being worn with miniskirts and dresses with ballet flats or short length flat boots. Consequently, leggings are also now popular to wear with oversized, long sweaters, sweatshirts and tops denim miniskirts, plaid skirts, short dresses, short shorts and with socks worn over them. Leggings are also worn under athletic shorts i.e. Nike Tempo shorts especially in colder weather. Leggings also come in capri length and bike short length. The bike short length is popular under sports uniform shorts and under skirts and dresses as a fashionable item and to keep from showing too much.

For men
At the Marni Men's show during the Fall 2007 Fashion Week, outfits with leggings designed solely for men were introduced.

Men's leggings, dubbed "meggings" (a portmanteau of the words "men" and "leggings") were presented as the latest fashion trend for men at spring/summer 2011 fashion runways, supposed to be styled and layered beneath shorts and preferably with large, baggy, loose or long tops such as T-shirts.

K-pop idols have been wearing male leggings under shorts as a fashion item since the 2010s.

Shiny leggings

Shiny leggings, sometimes called leather-look leggings, have a shiny, metallic (lamé), or wet-like appearance. They emerged as a popular fashion trend in the late-2000s (decade), particularly in 2008 as reported by Stylesignal and other trend forecasters. These leggings are most often a blend of nylon and spandex and come in a variety of colors, although most commonly in black, silver, or gold. These types of leggings are notable for their leather, or even latex appearance and are most often worn as evening or clubwear.

Shiny leggings were featured on fashion runways by many leading designers and were further popularized by celebrities such as Lauren Conrad, Kelly Clarkson and Mary-Kate and Ashley Olsen. Popular makers of shiny leggings include Members Only and American Apparel.

Jeggings

"Jeggings", a registered brand name of Sanko Group, are leggings made to look like skin-tight denim jeans.  Jeggings were brought on by the resurgence in style of skinny jeans in the late 2000s, when a higher demand for an even tighter style of pants came about. In 2011 "Jeggings" was entered into the twelfth edition of Concise Oxford English Dictionary.

In sports

Leggings are sometimes worn during sports and other vigorous activities. Runners, dancers, and exercisers may wear them alone or with athletic shorts over them—particularly in cold weather under a sports uniform, e.g. in soccer, with soccer shorts, shin guards and soccer socks over the leggings, under a cheerleading uniform with socks over the leggings, or field hockey players under their skirts with socks over the leggings. Leggings have even been worn by a hurling full back. Boys and men can also be seen wearing them under athletic shorts with socks over them when exercising or running track or cross country etc. Also, boys and men dancers wear leggings. Leggings and bike shorts are now accepted and allowed to be worn as part of a tennis outfit on various leagues and the professional WTA tour.

In school uniforms

Leggings have become a part of many school uniforms. Many schools that have uniform requirements allow leggings as an option in cooler and colder weather as long as the required skirt or dress and required socks are worn over them.

Use as outerwear

There has been societal debate about whether leggings are clothing on their own, which can be worn without covering, or are an accessory only to be worn with other items covering them, such as skirts, dresses or shorts.  In a 2016 poll of its readers Glamour magazine said that 61% of its readers thought that leggings should only be worn as an accessory, whereas an article that same year from Good Housekeeping concluded that "...Leggings do, in fact, count as pants—provided they are opaque enough that they don't show your underwear."

There have been a number of instances of people wearing leggings as pants who have been restricted or criticized for their actions. In 2013, schools in Sonoma County, California, banned students from wearing them as outerwear, as did a Massachusetts school in 2015. Schools in Oklahoma, Illinois, and North Carolina have enforced or suggested similar dress codes. A state legislator in Montana introduced a bill in 2015 intended to ban leggings and yoga pants.

In March 2017, three children flying on a company pass were barred from boarding a United Airlines flight by a gate agent who decided that their leggings were inappropriate.  United Airlines defended its position, while rival airline Delta stated via Twitter that leggings were welcome on its flights; United said in a statement that it does not bar regular female passengers from boarding if they are wearing leggings. Although some public figures, including actress Patricia Arquette and model and actress Chrissy Teigen, were critical of United's decision, a survey encompassing 1,800 travelers carried out by Airfarewatchdog found that 80% of their respondents backed the airline's decision to ban "inappropriate clothing", although the term was not defined in the poll.

Restrictions on wearing leggings is sometimes linked to slut shaming or body shaming, with critics noting that "...not being able to wear leggings because it's 'too distracting for boys' is giving us the impression we should be guilty for what guys do."

Gallery

See also
 Compression garment
 Stockings
 Tibialia

References

External links

1980s fashion
1990s fashion
2000s fashion
2010s fashion
2020s fashion
American fashion
Canadian fashion
Dancewear
Hosiery
Hurling equipment
Sportswear
United States Army uniforms
World War II military equipment of the United States

Leggings


A steam engine is an engine that uses steam from boiling water to make it move. The steam pushes on the engine parts to make them move. Steam engines can power many kinds of machines including vehicles and electric generators.

Steam engines were used in mine pumps starting in the early 1700s century and were much improved by James Watt in the 1770s. They were very important during the industrial revolution where they replaced horses, windmills and watermills to work machines.

The first steam engines were piston engines. The steam pressure pushed on a piston which made it move along a cylinder and so they had a reciprocal (back-and-forth) motion. This could move a pump directly or work a crank to turn a wheel and work a machine. They operated at low pressure and had to be very big to make a lot of power.

Steam engines were used in factories to work machines and in mines to move pumps. Later smaller engines were built that could move railway locomotives and steam boats.

The steam to power a steam engine is made in a boiler that heats water to make steam. In most places fire heats the boiler. Fuel for the fire may be wood, coal, or petroleum. Nuclear energy or solar energy may be used instead of fire. The steam coming out of the boiler applies the force on a piston. A valve sends the steam to one end of the piston, then the other, to make it move backwards and forwards. The moving piston pushes and pulls the piston rod, crosshead and connecting rod, to turn wheels or drive other machinery. The heavy spinning flywheel smooths out the power from the piston. The governor controls the speed of the engine.

Today many steam engines are still at work. During the 20th century the pistons were replaced by turbines which spin like a windmill pushed by jets of steam. These turn faster with more energy efficiency than the original kinds of piston steam engines. They are used in power plants to operate generators which make electricity. Some ships are also powered by steam turbines. The boilers of steam turbines can be heated by many different types of fuel, even a nuclear reactor in some power stations and warships.

Related pages
Aeolipile
Steam turbine

Motors
Engineering

Steam engine


Spiders (class Arachnida, order Araneae) are air-breathing arthropods. They have eight legs, and mouthparts (chelicerae) with fangs that inject venom. Most make silk. The spiders are seventh in number of species of all animal orders. About 48,000 spider species, and 120 families have been recorded by taxonomists. Over twenty different classifications have been proposed since 1900.p3

Spiders live on every continent except for Antarctica, and in nearly every habitat with the exceptions of air and sea.
Almost all spiders are predators, and most eat insects. They catch their prey in several ways. Some build a spider web, and some use a thread of silk that they throw at the insect. Some kinds of spiders hide in holes in the ground, then run out and grab an insect that walks by. Others will make web 'nets' to throw at passing insects. Or they go out and simply attack their prey. Some can jump quite well and hunt by sneaking close to an insect and then jumping on it.

Anatomy

Spiders have a two-part body, the front part (cephalothorax) and the abdomen. Unlike insects, spiders do not have antennae. The more advanced spiders have a centralized nervous system, with their ganglia fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs, and extend them by hydraulic pressure.

Size range
The smallest of full-grown spiders can be less than 4 mm. (0.1 inch). The largest of spiders can have a body length of 10 cm. (4 inches) or more. The largest can weigh 150 grams (5.3 oz). The largest of spiders are the tarantulas and the huntsman spiders. Some huntsman spiders in South East Asia can have a leg span of around 250–300 mm (9.8–11.8 in).

Spider eyes and other senses

Most spiders have four pairs of eyes on the top-front area of the body, arranged in patterns that vary from one family to another. The main eyes at the front are capable of forming images. Jumping spiders have visual acuity which is ten times better than that of dragonflies, which have by far the best vision among insects. The spiders do this by a telephoto-like series of lenses, a four-layered retina. They can swivel their eyes and put together images from different stages in the scan. The downside is that the scanning and integrating processes are relatively slow.

Spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various sensors, mostly bristles, respond to touch, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell. Spiders also have in the joints of their limbs sensors that detect forces and vibrations. In web-building spiders all these mechanical and chemical sensors are more important than the eyes. The eyes are more important to spiders that hunt actively. Like most arthropods, spiders lack balance sensors, and rely on their eyes to tell them which way is up.

Fangs
The fangs are hollow, like the needles used to give injections. Spiders use their fangs to inject toxins that kill the insects they will eat. Some kinds of spider venom attack the nervous systems of their prey, and other kinds of venom attack body tissues.

Behavior

What most spiders eat

Spiders are predators, and eat insects and other arthropods (including other spiders). Most use venom from their fangs to kill their prey. It is rare for spiders to capture prey that are much larger than they are. It is also difficult for most spiders to capture prey that are very much smaller than they are. There are some spiders that form colonies. Spiders eat not only spiders of other species, but spiders of their own species.

The black widow spider got its name because the females sometimes eat the males that mate with them. This may also happen in other species. Each species of spider has its own way of communicating with other spiders they meet. Because of the danger of being eaten, in some species the males have special hooks on their front legs with which they hold the female while they mate. Others bring the female something to eat. There are a few species in which the male spiders construct their own little webs that are connected to the webs of the females. In these species, the male is so much smaller than the female that it would be difficult for her to actually capture the male.

Most spiders have such poor vision that they will not even notice a dead insect. Jumping spiders are one exception to this rule. They have such good vision that they can find recently dead flies or other insects to eat.

Some spiders are not predators
Spiders are basically predators, but some have other food sources. The jumping spider Bagheera kiplingi gets over 90% of its food from fairly solid plant material produced by acacias as part of a beneficial relationship with a species of ant.

Young spiders of several families feed on plant nectar. Studies have shown that they do this for long periods. They also clean themselves regularly while feeding. These spiders also prefer sugar solutions to plain water, which shows that they are looking for nutrients. Many spiders are nocturnal, they are most active during the night. The extent of nectar consumption by spiders may therefore have been under-estimated. Nectar contains amino acids, lipids, vitamins and minerals in addition to sugars. Studies have shown that other spider species live longer when nectar is available. Feeding on nectar also avoids the risks of struggles with prey, and the costs of producing venom and digestive enzymes. Since nectar is a flowering plant production, its consumption by spiders came quite late in their evolution.

Various species are known to feed on dead arthropods (scavenging), web silk, and their own shed exoskeletons. Pollen caught in webs may also be eaten, and studies have shown that young spiders have a better chance of survival if they have the opportunity to eat pollen. In captivity, several spider species are also known to feed on bananas, marmalade, milk, egg yolk and sausages.

Methods of catching prey

Webs
The best-known method spiders use to capture prey is a sticky web. The placing of the web allows different spiders to trap different insects in the same area. Flat, horizontal webs allow them to trap insects that fly up from vegetation underneath, for example. Flat vertical nets allow them to trap insects in flight. The spiders that build webs usually do not see very well, but they are very sensitive to vibrations.

Underwater
Females of the water spider Argyroneta aquatica build underwater "diving bell" webs which they fill with air and use while eating their prey, molting, mating, and raising offspring. They live almost entirely within the bells, darting out to catch prey animals that touch the bell or the threads that anchor it. A few spiders use the surfaces of lakes and ponds as "webs", detecting trapped insects by the vibrations that these cause while struggling.

Bola casting
Net-casting spiders weave only small webs but then manipulate them to trap prey. They stretch their webs and then release them when prey strike them. Those of the family Deinopidae weave even smaller webs, hold them outstretched between their first two pairs of legs, and lunge and push the webs as much as twice their own body length to trap prey, and this move may increase the webs' area by a factor of up to ten. Experiments have shown that Deinopis spinosus has two different techniques for trapping prey: backwards strikes to catch flying insects, whose vibrations it detects; and forward strikes to catch ground-walking prey that it sees. These two techniques have also been observed in other deinopids. Walking insects form most of the prey of most deinopids, but one population of Deinopis subrufus appears to live mainly on tipulid flies that they catch with the backwards strike.

Mature female bolas spiders of the genus Mastophora build "webs" that consist of only a single "trapeze line", which they patrol. They also construct a bolas made of a single thread, tipped with a large ball of very wet sticky silk. They emit chemicals that resemble the pheromones of moths, and then swing the bolas at the moths. They catch about the same weight of insects per night as web-weaving spiders of similar size. The spiders eat the bolas if they have not made a kill in about 30 minutes, rest for a while, and then make new bolas. Juveniles and adult males are much smaller and do not make bolas. Instead they release different pheromones that attract moth flies, and catch them with their front pairs of legs.

Using trapdoors

The primitive Liphistiidae, the "trapdoor spiders" (family Ctenizidae) and many tarantulas are ambush predators. They lurk in burrows, often closed by trapdoors and surrounded by networks of silk threads that alert these spiders to the presence of prey. Other ambush predators do without such aids, including many crab spiders. A few species that prey on bees, which see ultraviolet, can adjust their ultraviolet reflectance to match the flowers in which they are lurking. Wolf spiders, jumping spiders, fishing spiders and some crab spiders capture prey by chasing it, and rely mainly on vision to get their prey.

Capturing insects without using webs
Not all spiders use silk to net their prey. Instead, these spiders may capture insects by grabbing them and then biting them. Among these kinds of spiders the two best known are the wolf spiders and the jumping spiders.

Wolf spiders

A wolf spider will usually wait until an insect comes near to it, and then rush at the insect, grab it using its front legs, and then bite the insect so that its venom can do its work.

Female wolf spiders lay their eggs on a pad of silk and then draw the edges together to create a round ball that they carry along with them wherever they go. They hold their egg balls to their tail ends by using their silk. When the eggs hatch, the little spiders will crawl onto the mother's back, and she will carry them along with her for days or weeks.

Wolf spiders are very good mothers and will strongly protect both their egg balls and their infants. When the time comes, the little spiders will leave the mother and each will go its own way.

Jumping spiders

Jumping spiders have very good eyes and can see well. They sneak as close to an insect as they can, and then they jump onto the insect and immediately bite it. Since they often hunt in trees, bushes, and on the sides of walls, if the jumping spider misses it may fall off. But they have a way to save themselves from harm. Before they jump they fasten their silk to the place where they have been standing, and as they jump they let out a silk safety line. So if they fall they will catch themselves when they reach the end of their silk safety line. Sometimes a jumping spider will catch an insect and then fall while still holding onto the insect. But the spider is still safe.

Jumping spiders make little silken "tents" for themselves to sleep in. When they lay eggs they keep them inside such a shelter. They do not take their eggs with them when they go out to hunt.

For all male spiders it is dangerous to seek a mate. The female spider may not realize that the male is a spider of her kind, so she may try to eat it. The jumping spiders not only have visual patterns that identify them to each other, but the male jumping spider will do a special dance when it approaches a female of the same species. That way, the female can recognize that it is a male of her species. She will generally forget about eating for long enough to mate with the visiting male spider.

Jumping spiders have such good eyes that they will usually watch any human who tries to watch them. Some species are very shy and will run away if the human gets too close. But some species, such as Phidippus audax (the audacious or brave jumping spider) and Platycryptus undatus, can become calm if the human comes close to them slowly. Sometimes they will jump onto one of your fingers and then jump from finger to finger and from hand to hand. They seem to want to explore.

Hunting other spiders
Some jumping spiders of the genus Portia hunt other spiders in ways that seem intelligent, outflanking their victims or luring them from their webs. Laboratory studies show that Portias instinctive tactics are only starting points for a trial-and-error approach from which these spiders learn very quickly how to overcome new prey species. However, they seem to be relatively slow thinkers, which is not surprising as their brains are vastly smaller than those of mammalian predators.

 Disguising as ants

Ant-mimicking spiders face several challenges: they generally develop slimmer abdomens and false "waists" to mimic the three distinct regions (tagmata) of an ant's body; they wave the first pair of legs in form to their heads to mimic antennae, which spiders lack, and to conceal the fact that they have eight legs rather than six. They have large color patches round one pair of eyes to disguise the fact that they generally have eight simple eyes, while ants have two compound eyes; they cover their bodies with reflective hairs to resemble the shiny bodies of ants. In some spider species males and females mimic different ant species, as female spiders are usually much larger than males.

Ant-mimicking spiders also modify their behavior to resemble that of the target species of ant, for example many adopt a zig-zag pattern of movement, ant-mimicking jumping spiders avoid jumping, and spiders of the genus Synemosyna walk on the outer edges of leaves in the same way as Pseudomyrmex. Ant-mimicry in many spiders and other arthropods may be for protection from predators that hunt by sight, including birds, lizards and spiders. However several ant-mimicking spiders prey either on ants or on the ants "livestock" such as aphids. When at rest the ant-mimicking crab spider Amyciaea does not closely resemble Oecophylla, but while hunting it imitates the behavior of a dying ant to attract worker ants. After a kill some ant-mimicking spiders hold their victims between themselves and large groups of ants to avoid being attacked.

 Reproduction and life cycle

Spiders reproduce sexually and fertilization is internal but indirect. In other words, the sperm is not inserted into the female's body by the male's genitals but by an intermediate stage. Unlike many land-living arthropods, male spiders do not produce ready-made spermatophores (packages of sperm). Instead, they spin small sperm webs on to which they ejaculate and then transfer the sperm to structures on the tips of their pedipalps. When a male detects signs of a female nearby he checks whether she is of the same species and whether she is ready to mate; for example in species that produce webs or 'safety ropes', the male can identify the species and sex of these objects by smell.

Spiders generally use elaborate courtship rituals to prevent the large females from eating the small males before fertilization, except where the male is so much smaller that he is not worth eating. In some species males mate with newly molted females, which are too weak to be dangerous to the males.

In web-weaving species precise patterns of vibrations in the web are a major part of the rituals, while patterns of touches on the female's body are important in many spiders that hunt actively, and may 'hypnotize' the female. Gestures and dances by the male are important for jumping spiders, which have excellent eyesight. If courtship is successful, the male injects his sperm from the pedipalps into the female's genital opening on the underside of her abdomen. Female reproductive tracts vary. Some are simple tubes, but others have chambers where females store sperm, and release it when they are ready.

Males do get eaten in some species. Males of the genus Tidarren cut off one of their palps, and enter adult life with one palp only. The palps are 20% of male's body mass in this species, and detaching one of the two improves mobility. In the Yemeni species Tidarren argo, the remaining palp is then torn off by the female. The separated palp remains attached to the female's opening for about four hours. In the meantime, the female feeds on the palpless male. In over 60% of cases the female of the Australian redback spider kills and eats the male after it inserts its second palp into the female's genital opening; in fact the males co-operate by trying to impale themselves on the females' fangs. Observation shows that most male redbacks never get an opportunity to mate, and the 'lucky' ones increase the likely number of offspring by ensuring that the females are well-fed. However males of most species survive a few matings, limited mainly by their short life spans. Some even live for a while in their mates' webs.p176/212

Females lay up to 3,000 eggs in one or more silk egg sacs, which maintain a fairly constant humidity level. In some species the females die afterwards, but females of other species protect the sacs by attaching them to their webs, hiding them in nests, carrying them in the chelicerae or attaching them to the spinnerets and dragging them along.

 Development of young
Baby spiders pass all their larval stages inside the egg and hatch as spiderlings, very small and sexually immature but similar in shape to adults. Some spiders care for their young, for example a wolf spider's brood cling to rough bristles on the mother's back, and females of some species respond to the "begging" behaviour of their young by giving them their prey, provided it is no longer struggling, or even regurgitate food.

Like other arthropods, spiders have to moult to grow as their cuticle ("skin") cannot stretch. Most spiders live for only one to two years, although some tarantulas can live in captivity for over 20 years.p232

 Spiders and humans
Of the 40,000 spiders, less than 12 are known to be dangerous to humans. Most of the time, being bitten by a spider is painful. Most spiders use venom to paralyse their prey; they kill it through eating, or through a bite. A few spiders have venoms that can be dangerous to weakened people and those allergic to it. Since 1927, 13 people have died, after a bite from a Atrax robustus spider from Australia.

The spider that kills the most people, the black widow and other spiders in the genus Latrodectus are around 1 cm. in body length. The Atrax and Phoneutria spiders, which are also capable of killing people, both average around 2.5 cm. or one inch, and even the Widow spiders are large enough to be easily noticed.

Even relatively small spiders like Phidippus audax can give a painful bite if you hurt them, but spiders are very helpful to human beings because they control insects that eat our crops.

Being afraid of spiders is a very common phobia (fear). The widow spiders (black widows and other members of their genus) never willingly leave their webs, so usually people get bitten when they touch the spider by mistake.

 Gallery Click on a picture to see it larger:'''

 References

 Other websites

 Platnick N.I. 20013. The world spider catalog''. Hosted by the American Museum of Natural History, and edited by Peter Merrett and H. Don Cameron.

Spider


The Natural History Museum (NHM) is a museum in London. It is the largest natural history museum in Britain. It includes the nation's main collections of specimens of life, rocks and minerals. The museum employs scientific staff, conducts research, stores specimens, and manages the public galleries.

The NHM is one of the four large institutions on Exhibition Road in South Kensington, London. The others are the Science Museum, the Victoria and Albert Museum and Imperial College London.

History
The NHM was at first a department of the famous British Museum. When the present NHM building was opened in 1881, it was then called the British Museum (Natural History) or BMNH. Its head reported to the Director of the British Museum. Not until 1963 did the NHM become fully independent, with its own Board of Trustees, and not until 1992 was it permitted to use its present name.

In 1986, the museum absorbed the adjacent Geological Museum of the British Geological Survey, which is to just to the north of the NHM. The museum's galleries were completely rebuilt and relaunched in 1996 as The Earth Galleries, with the exhibitions in the Waterhouse building retitled The Life Galleries. The Natural History Museum's own Mineralogy displays remain largely unchanged as an example of the 19th-century display techniques of the Waterhouse building.

The newly developed Darwin Centre (named after Charles Darwin) is designed as a new home for the museum's collection of tens of millions of preserved specimens, and new work spaces for the museum's scientific staff. Built in two phases, with two new buildings adjacent to the main Waterhouse building, it is the most important new project in the museum's history.

Scientific work
The museum employs 300 scientific staff, and has huge collections of fossils, rocks and minerals, as well as the preserved specimens of present-day plants and animals. There are over 70 million specimens in the collections. Data bases of the collections are being developed.

Other websites
 Natural History Museum official site

References

Museums in London
National museums
Natural history museums

Natural History Museum


Crucifixion is a way to execute (kill) someone, usually because they have committed a crime. It was used as a capital punishment by the government of Rome. A prisoner was beaten with whips, and made to carry their own cross. They were either tied or nailed to the cross. The Romans used different types of crosses: T-shaped, X-shaped, and sometimes just trees. It killed people by making them bleed, go into shock, and making it hard for them to breathe, eventually dying of suffocation. Some of the people would live for several days before dying. The Roman soldiers (which were like the police of that time) would often break the legs of the criminal to speed it up.

Jesus Christ

The most famous person to have been crucified was Jesus Christ, the Son of God according to the Christian religion. He was crucified along with two thieves. The cross later became a symbol of Christianity. Six of the twelve apostles of Jesus were also crucified later on, but most of them asked to be crucified in a different way from Jesus. For example, Andrew (who later became the patron saint of Scotland) was crucified on an X-shaped cross. In the year 337 AD, Emperor Constantine I abolished crucifixion in the Roman Empire.

References

Capital punishment
Christian symbols

Crucifixion


A church is a building that was constructed to allow people to meet to worship together. These people are usually Christians, or influenced by Christianity. Some other non-Christian religious groups also call their religious buildings churches, most notably Scientology.

The following description is about Roman Catholic churches, although some parts are the same in Episcopalian and Lutheran churches. Depending on the number of people that are in a community, the churches come in different sizes. Small churches are called chapels. The churches in a particular geographical area form a group called the diocese.  Each diocese has a cathedral.  In most cases, the cathedral is a very big church. Cathedrals are the seat of bishops.

History of church buildings

In the early days of Christianity people met in private buildings. Church buildings are mentioned for the first time around the year 260 A.D. when the Emperor Galienus ordered an end of a persecution and to return the places of worship. In the third century we hear of large church buildings. We do not know, how these early buildings looked. Only in Dura-Europos (Syria) a building was discovered, which had been a private house modified for Christian services.

After the death of the Roman emperor Constantine in 337 A.D. Christians were allowed to have buildings to worship in. These first churches were built on a similar plan to Roman basilicas. This plan was later used for the fine Gothic cathedrals and churches that were built at the end of the Middle Ages.

The parts of a church

There are several parts in the architecture of a church. Not all churches will have all these parts:

 The nave is the main part of the church where the congregation (the people who come to worship) sit.
 The aisles are the sides of the church which may run along the side of the nave.
 The transept, if there is one, is an area which crosses the nave near the top of the church. This makes the church shaped like a cross, which is a symbol of Jesus's death on a cross.
 The chancel leads up to the altar at the top of the church. The altar is in the sanctuary. The word "sanctuary" means "sacred place". People were not allowed to be arrested in the sanctuary, so they were safe. The altar is usually at the east end of the church. People in the church sit facing the altar. We say that the church "faces east".
 Churches will also have a tower or steeple, usually at the west end. If the church has a transept the tower may be above the centre of the transept.

In Roman Catholic churches there is always a stoup (bowl) of holy water near the entrance of the church. This tradition comes from the fact that Roman basilicas had a fountain for washing in front of the entrance. The font is a bowl where people (often babies) are baptized. This is also near the entrance of the church. This is a symbol of the fact that it is welcoming the people into the Christian church.

Traditionally the nave has long benches for the congregation to sit on. These are called pews. Some churches may now have replaced their pews with chairs so that they can be moved about for different occasions. At the front of the nave is the pulpit where the priest preaches (these talks are called "sermons"). There is also a lectern (like a large music stand) from where the lessons (the Bible readings) are read.

If there are aisles along the side of the nave there will be pillars which hold up the roof. In large churches or cathedrals there may be a row of little arches along the top of these pillars. This is called the triforium. Over the triforium is the clerestory which is a row of windows high up in the church wall.

The chancel is the most holy part of the church, and this is why it is often separated from the nave by a screen which can be made of wood or stone, or occasionally iron. The congregation can see through the screen. On the top of the screen there may be a cross. This is called a rood (pronounce like "rude") screen. Priests used to climb up a staircase to the top of the rood screen to read the epistle and the gospel. Sometimes people sang from there.

Inside the chancel are the benches where the choir sit. These are called choir stalls. They are on both sides. The two sides of the choir sit facing one another. The choir members who sit on the left (north side) are called "cantoris" (the side where the "cantor" sits) and those on the right (south side) are called "decani" (the side where the deacon sits). In some large churches or cathedrals the seats for the priests tip up. The top of these seats, when they are tipped up, are called misericords (from the Latin word for "mercy"). This is because the priests or monks were able to lean against them when they got tired if they had to stand up for a long time.

Sometimes there are holes in the walls of the screen so that the congregation can see through. These are called squints. If there is a recess in the wall it is called an aumbry. It is a cupboard for communion wine and bread that have been consecrated by a priest.

The altar may be right at the east end of the church, but in larger churches or cathedrals it is often much farther forward. In that case the very east end is called an apse. Sometimes it is a separate chapel called the "Lady Chapel".

Churches through the ages

The design of churches changed a lot during the course of history. Often churches were made bigger. When this happened there may be a mixture of architectural styles. These styles vary a lot in different countries.

English churches

In English churches there were several different periods of architecture:

 The Saxon period (700–1050) was a time when churches were very simple. The end of the church (end of the sanctuary) was often rounded. Hardly any are left now because they were mostly made of wood.
 The Norman period (1050–1190) came from the style called Romanesque which was popular in Europe. The arches had ornaments which were called "mouldings". The tops of the pillars looked like cushions, so they were called "cushion capitals". The windows were narrow and rounded at the top.
 Early English or Gothic architecture (1190–1280) was not as solid and heavy as Norman architecture. Towers were elegant and tall, like the tower of Salisbury Cathedral.
 The Decorated style of architecture (1280–1360)was popular at a time when the plague (Black Death) was raging and a third of the people in England died. For that reason, not so much building was done then. There were lots of stone carvings were made in churches at that time.
 The Perpendicular style (1360–1540) was very grand. It had lots of straight upward lines and fan vaulting. This can be seen in Westminster Abbey and King's College Chapel, Cambridge. Many churches that can be seen in England were built in this period.

In the 1600s, churches were built in a variety of styles. Often they copied some of the older styles. After the Great Fire of London many new churches were built by the architect Sir Christopher Wren. They were built in the classical style. Churches continued to be built in later centuries like this, but also the Gothic style continued to be used.

Modern churches often do not have the traditional cross-shape. It is difficult for the congregation to see and hear what is happening in the chancel. Modern churches bring the congregation, choir and priests in closer touch. An example is the round design for the Church of Christ the Cornerstone in Milton Keynes. Modern churches are often simpler but with a warmer character than the Gothic churches. Many have beautiful mosaic glass windows. Coventry Cathedral is a famous example of a modern church building.

Related pages
Cathedral
Chapel
Choir (music)

References

 Encyclopædia Britannica, 1973

Other websites

 Virtual Church

Church (building)


The Transmission Control Protocol (TCP) is one of the main protocols of the Internet Protocol Suite. TCP is part of the popular "TCP/IP" combination used by the Internet. The Internet Protocol, or IP, makes sure data on the internet gets to the right place. Then TCP makes sure the data is put in the right order, and none of it is missing. TCP also helps to control traffic on the internet so it does not get overloaded. These protocols, which are kind of like languages that computers use, are designed so that any computer, and any program (such as a Web browser or e-mail client), can use them.

Importance of TCP
TCP makes it easier for computer programs to communicate with each other, typically over a computer network. When a program wants to send a lot of data, TCP is in charge of dividing the data, sending it through the network, and putting it back together correctly on a different computer. In the process, IP chooses which wires and "intersections" get used to send all the data pieces in the fastest way. It does this using packets.

If there are a lot of people using the network, it can get overloaded. Also sometimes weather, power outages, and other problems can make communication hard. Routers on the Internet use something called load balancing to try to fix some of these problems. But still, information can be delivered out of order, get lost, or even duplicated. TCP is designed to notice all of these problems and try to fix them. This can take a while, which is why the Internet sometimes seems slow. Once the TCP receiver has finally reassembled a perfect copy of the data originally transmitted, it passes that data to the computer program that asked for it. In this way, the program does not have to know about the network, and TCP does not have to know about the program's data.

Further reading
 W. Richard Stevens. TCP/IP Illustrated, Volume 1: The Protocols.
 W. Richard Stevens and Gary R. Wright. TCP/IP Illustrated, Volume 2: The Implementation.
 W. Richard Stevens. TCP/IP Illustrated, Volume 3: TCP for Transactions, HTTP, NNTP, and the UNIX Domain Protocols.

Other websites
 Dissertation about TCP improvements in wired and wireless networks  (Dissertation)
 IANA Port Assignments
 John Kristoff's Overview of TCP (Fundamental concepts behind TCP and how it is used to transport data between two endpoints)
 RFC 675 - Specification of Internet Transmission Control Program, December 1974 Version
 RFC 793 - TCP v4
 RFC 1122 - includes some error corrections for TCP
 RFC 1323 - TCP-Extensions
 RFC 1379 - Extending TCP for Transactions—Concepts
 RFC 1948 - Defending Against Sequence Number Attacks
 RFC 2018 - TCP Selective Acknowledgment Options
 RFC 2581 - TCP Congestion Control
 RFC 2988 - Computing TCP's Retransmission Timer
 RFC 4614 - A Roadmap for TCP Specification Documents
 TCP, Transmission Control Protocol
 TCP EFSM diagram - A detailed description of TCP states.
 The basics of Transmission Control Protocol
 The Law of Leaky Abstractions by Joel Spolsky
 Checksum example

Computer protocols

Transmission Control Protocol