Athy´, a town in Ireland, county of Kildare, 37 miles south-west of Dublin, on the Barrow, which is here joined by the Grand Canal. Its chief trade is in corn. Pop. 3535.
Atit´lan, a lake and mountain of Central America in Guatemala. The lake is about 24 miles long and 10 broad; the mountain is an active volcano 12,160 feet high.
Atlan´ta, a city in the United States, capital of Georgia, on an elevated ridge, 7 miles south-east of the Chattahoochee River. It is an important railway centre; carries on a large trade in grain, paper, cotton, flour, and especially tobacco, and possesses flour-mills, paper-mills, ironworks, &c. Here are Atlanta University for negro men and women, a theological college, a medical college, &c. Atlanta suffered severely during the Civil War, and a battle was fought there on 22nd July, 1864. A fire which broke out on 21st May, 1917, caused damage estimated at more than £1,000,000. Pop. (1920), 200,600.
Atlan´tes, orTelamōnes, in architecture, male figures used in place of columns or pilasters for the support of an entablature or cornice. Female figures so employed are termedcaryatides.
Atlantic City, a fashionable watering-place of the United States, on the coast of New Jersey. It is an important air port, and has an aerodrome covering about 160 acres. Pop. 50,682.
Atlantic Ocean, the vast expanse of sea lying between the west coasts of Europe and Africa and the east coasts of North and South America, and extending from the Arctic to the Antarctic Ocean; greatest breadth, between the west coast of Northern Africa and the east coast of Florida, 4150 miles; least breadth, between Norway and Greenland, 930 miles. The total area of the North Atlantic (including the inland seas) is 13,262,000 sq. miles; the area of the South Atlantic is 12,627,000 sq. miles. The principal inlets and bays are Baffin's and Hudson's Bays, the Gulf of Mexico, the Caribbean Sea, the North Sea, the Bay of Biscay, and the Gulf of Guinea. The principal islands north of the equator are Iceland, the Faroe and British Islands, the Azores, Canaries, and Cape Verde Islands, Newfoundland, Cape Breton, and the West India Islands; and south of the equator, Ascension, St. Helena, and Tristan da Cunha.
The great currents of the Atlantic are the Equatorial Current (divisible into the Main, Northern, and Southern Equatorial Currents), the Gulf Stream, the North African and Guinea Current, the Southern Connecting Current, the Southern Atlantic Current, the Cape Horn Current, Rennel's Current, and the Arctic Current. The current system is primarily set in motion by the trade-winds which drive the water of the intertropical region from Africa towards the American coasts. The Main Equatorial Current, passing across the Atlantic, is turned by the S. American coast, along which it runs at a rate of 30 to 50 miles a day, till, having received part of the North Equatorial Current, it enters the Gulf of Mexico. Issuing thence between Florida and Cuba under the name of the Gulf Stream, it flows with a gradually-expanding channel nearly parallel to the coast of the United States. It then turns north-eastward into the mid-Atlantic, the larger proportion of it passing southward to the east of the Azores to swell the North African and Guinea Current created by the northerly winds off the Portuguese coast. The Guinea Current, which takes a southerly course, is divided into two on arriving at the region of the north-east trades, part of it flowing east to the Bight of Biafra and joining the South African feeder of the Main Equatorial, but the larger portion being carried westward into the North Equatorial drift. Rennel's Current, which is possibly a continuation of the Gulf Stream, enters the Bay of Biscay from the west, curves round its coast, and then turns north-west towards Cape Clear. The Arctic Current runs along the east coast of Greenland (being here called the Greenland Current), doubles Cape Farewell, and flows up towards Davis' Strait; it then turns to the south along the coasts of Labrador and the United States, from which it separates the Gulf Stream by a cold band of water. Immense masses of ice are borne south by this current from the Polar seas. In the interior of the North Atlantic there is a large area comparatively free from currents, called the Sargasso Sea, from the large quantity of sea-weed (of the genus Sargassum) which drifts into it. A similar area exists in the South Atlantic. In the South Atlantic the portion of the Equatorial Current which strikes the American coast below Cape St. Roque flows southward at the rate of from 12 to 20 miles a day along the Brazil coast under the name of the Brazil Current. It then turns eastward and forms the South Connecting Current, which, on reaching the South African coast, turns northward into the Main and Southern Equatorial Currents. Besides the surface currents, an under current of cold water flows from the poles to the equator, and an upper current of warm water from the equator towards the poles.
The greatest depth as yet discovered is north of Porto Rico, in the West Indies, namely 27,360 feet. Cross-sections of the North Atlantic between Europe and America show that its bed consists of two great valleys lying in a north-and-south direction, and separated by a ridge, on which there is an average depth of 1800 fathoms.The mean depth of the North Atlantic is 2047 fathoms, that of the South Atlantic 2067 fathoms. A ridge, called theWyville-Thomson Ridge, with a depth of little more than 200 fathoms above it, runs from near the Butt of Lewis to Iceland, cutting off the colder water of the Arctic Ocean from the warmer water of the Atlantic. The South Atlantic, of which the greatest depth yet found is over 3000 fathoms, resembles the North Atlantic in having an elevated plateau or ridge in the centre with a deep trough on either side. The saltness and specific gravity of the Atlantic gradually diminish from the tropics to the poles, and also from within a short distance of the tropics to the equator. In the neighbourhood of the British Isles the salt has been stated at one thirty-eighth of the weight of the water. The North Atlantic is the greatest highway of ocean traffic in the world. It is also a great area of submarine communication, by means of the telegraphic cables that are laid across its bed. SeeOceanography.
Atlantic Telegraph.SeeTelegraph.
Atlan´tides(-dēz), a name given to the Pleiades, which were fabled to be the seven daughters of Atlas or of his brother Hesperus.
Atlan´tis, an island which, according to Plato, existed in the Atlantic over against the Pillars of Hercules (Straits of Gibraltar), was the home of a great nation, and was finally swallowed up by the sea. The legend has been accepted by some as fundamentally true; but others have regarded it as the outgrowth of some early discovery of the New World.
Atlan´tosaurus, a gigantic fossil reptile, ord. Dinosauria, obtained in the upper Jurassic strata of the Rocky Mountains, attaining a length of 110 feet or more.
Atlas, an extensive mountain system in North Africa, starting near Cape Nun on the Atlantic Ocean, traversing Morocco, Algiers, and Tunis, and terminating on the coast of the Mediterranean; divided generally into two parallel ranges, runningW.toE., the Greater Atlas lying towards the Sahara and the Lesser Atlas towards the Mediterranean. The principal chain is about 1500 miles long, and the principal peaks rise above or approach the line of perpetual congelation, Miltsin in Morocco being 11,400 feet high, and Tizi Likumpt being 13,150. The highest elevation is perhaps Tizi Tamyurt, estimated at fully 15,000 feet. Silver, antimony, lead, copper, iron, &c., are among the minerals. The vegetation is chiefly European in character, except on the low grounds and next the desert.
Atlas, in Greek mythology, the name of a Titan whom Zeus condemned to bear the vault of heaven.—The same name is given to a collection of maps and charts, and was first used by Gerard Mercator in the sixteenth century, the figure of Atlas bearing the globe being given on the title-pages of such works.
Atlas, in anatomy, is the name of the first vertebra of the neck, which supports the head. It is connected with the occipital bone in such a way as to permit of the nodding movement of the head, and rests on the second vertebra oraxis, their union allowing the head to turn from side to side.
At´las, a kind of silk or silk-satin fabric of Eastern manufacture.
Atmidom´eter, an instrument for measuring the evaporation from water, ice, or snow. It somewhat resembles Nicholson's hydrometer, being constructed so as to float in water and having an upright graduated stem, on the top of which is a metal pan. Water, ice, or snow is put into the pan, so as to sink the zero of the stem to a level with the cover of the vessel, and as evaporation goes on the stem rises, showing the amount of evaporation in grains.
Atmom´eter, an instrument for measuring the amount of evaporation from a moist surface in a given time. It is often a thin hollow ball of porous earthenware in which is inserted a graduated glass tube. The cavity of the ball and tube being filled with water and the top of the tube closed, the instrument is exposed to the free action of the air; the relative rapidity with which the water transuding through the porous substance is evaporated is marked by the scale on the tube as the water sinks.
At´mosphere, primarily the gaseous envelope which surrounds the earth; but the term is applied to that of any orb. Twilight effects show that the atmosphere is sufficiently dense up to a height of 40 miles to scatter or reflect to an appreciable degree the sun's rays, while the phenomena of meteors, which are rendered luminous through friction, show that it extends, though in extremely attenuated form, to 100 or even 200 or more miles. It exerts on every part of the earth's surface a pressure of about 15 (14.73) lb. per sq. inch. The existence of this atmospheric pressure was first proved by Torricelli, who thus accounted for the rush of a liquid to fill a vacuum, and who, working out the idea, produced the first barometer. The average height of the mercurial column counterbalancing the atmospheric weight at the sea-level is a little less than 30 inches; but the pressure varies from hour to hour, and, roughly speaking, diminishes in geometrical progression with arithmetical increase in altitude. Of periodic variations there are two maxima of daily pressure, occurring when the temperature is about the mean of the day, and two minima, when it is at its highest and lowest respectively; but the problems of diurnal and seasonal oscillations have yet to be fully solved. The pressureupon the human body of average size is no less than 14 tons, but as it is exerted equally in all directions no inconvenience is caused by it. It is sometimes convenient to take the atmospheric pressure as a standard for measuring other fluid pressures; thus the steam pressure of 30 lb. per sq. inch on a boiler is spoken of as a pressure of two atmospheres.
The atmosphere, first subjected to analysis by Priestley and Scheele in the latter part of the eighteenth century, consists practically of oxygen and nitrogen in the almost constant proportion of 20.81 volumes of oxygen to 79.19 volumes of nitrogen, or, by weight, 23.01 parts of oxygen to 76.99 of nitrogen. The gases are associated together, not as a chemical compound, but as a mechanical mixture. Upon the oxygen present depends the power of the atmosphere to support combustion and respiration, the nitrogen acting as a diluent to prevent its too energetic action. It had long been known that atmospheric nitrogen appeared to have a very slightly greater density than nitrogen obtained from other sources. Lord Rayleigh and Sir William Ramsay found that the fact was due to a still more inert gas which forms nearly 1 per cent of the air, and which had not previously been separated from nitrogen. This has been namedargon. Besides these gases, the atmosphere also contains aqueous vapour in variable quantity, ozone, carbonic acid gas, traces of ammonia, nitric acid, and, in towns, sulphuretted hydrogen and sulphurous acid gas. In addition to its gaseous constituents the atmosphere is charged with dust, bacteria, &c. For other gases which are present in traces, seeNeon. SeeClimate;Meteorology.—Bibliography: C. Flammarion,L'Atmosphère; Sir Napier Shaw,The Weather Map.
Atmospheric Engine, name given by early inventors to engines in which the piston is restored to the bottom of its stroke by atmospheric pressure.
Atmospheric Railway, so called in consequence of the motive power being derived from the pressure of the atmosphere, or from compressed air. The idea of thus obtaining motion was first suggested by the French engineer Papin, about 200 years ago. In 1810, and again in 1827, Medhurst published a scheme for 'propelling carriages through a close-fitting air-tight tunnel by forcing in air behind them'; and in 1825 a similar project was patented by Vallance of Brighton. About 1835 H. Pinkus, an American residing in England, patented a pneumatic railway. The carriages were to travel on an open line of rails, along which a cast-iron tube of between 3 and 4 feet diameter was to be laid, having a longitudinal slit from 1 to 2 inches wide and closed by a flexible valve along its upper side, through which a connection could be formed between the leading carriage and a piston working within the tube. This method was improved by Messrs. Clegg & Samuda, who in 1840 tried some experiments on a portion of the West London Railway with sufficient success to induce the Government to advance a loan to the Dublin and Kingstown Railway Company, for the construction of a pneumatic line from Kingstown to Dalkey. It was opened for passenger traffic at the end of 1843, and was worked for many months. The London and Croydon Company subsequently obtained powers for laying down an atmospheric railway by the side of their other line from London to Croydon, and in experimental trips in 1845 a speed of 30 miles an hour was obtained with sixteen carriages, and of 70 miles with six carriages. But during the intense heat of the summer of 1846 the iron tube frequently became so hot as to melt the composition which sealed the valve, and the line had to be worked by locomotives. The mechanical difficulty of commanding a sufficient amount of rarefaction led to the abandonment of the system for railway purposes. It has been revived, however, for the conveyance of letters and parcels in towns by means of tubes of moderate diameter laid beneath the streets. SeePneumatic Dispatch.
AtollAtoll
Atoll´, the Polynesian name for coral islands of the ringed type enclosing a lagoon in the centre. They are found chiefly in the Pacific in archipelagos, and occasionally are of large size. Suadiva Atoll is 44 miles by 34; Rimsky 54 by 20. SeeCoral.
Atomic Theory, a theory as to the existence and properties of atoms (seeAtoms); especially, in chemistry, the theory accounting for the fact that in compound bodies the elements combine in certain constant proportions, by assuming that all bodies are composed of ultimate atoms, the weight of which is different in different kinds of matter. It is associated with the name of Dalton, who systematized and extended the imperfect results of his predecessors. On its practical side the atomic theory asserts threeLaws of Combining Proportions: (1) The Law of Constant or Definite Proportions, teaching that in every chemical compound the nature and relative weights of the constituent elements are definite and invariable; thus water invariably consists of 8 parts by weight of oxygen to 1 part by weight of hydrogen; (2) The Law of Multiple Proportions, according to which the several proportions in which one element unites with a given weight of another invariably bear towards each other a simple relation; thus 1 part by weight of hydrogen unites with 8 parts by weight of oxygen to form water, and with 16 (i.e. 8×2) parts of oxygen to form peroxide of hydrogen; (3) The Law of Combinationin Reciprocal Proportions, that the proportions in which two elements combine with a third also represent the proportions in which, or in some simple multiple of which, they will themselves combine; thus in olefiant gas hydrogen is present with carbon in the proportion of 1 to 6, and in carbonic oxide, oxygen is present with carbon in the proportion of 8 to 6, 1 to 8 being also the proportions in which hydrogen and oxygen combine with each other. The theory that theseproportional numbersare, in fact, nothing else but the relative weights of atoms so far accounts for the phenomena that the existence of these laws might have been predicted by the aid of the atomic hypothesis long before they were actually discovered by analysis. In themselves, however, the laws do not prove the theory of the existence of ultimate particles of matter of a certain relative weight; and although many chemists, even without expressly adopting the atomic theory itself, have followed Dalton in the use of the termsatomandatomic weight, in preference toproportion,combining weight,equivalent, and the like, yet in using the wordatomit should be held in mind that it merely denotes the combining weights of the elements. These will remain the same whether the atomic hypothesis which suggested the employment of the term be true or false. Dalton supposed that the atoms are spherical, and invented certain symbols to represent the mode in which he conceived they might combine. The latest atomic hypothesis is one which assigns an electrical structure to the atom. SeeChemistry;Electricity;Matter. Cf. H. E. Roscoe and A. Harden,New View of Dalton's Atomic Theory; Sir J. J. Thomson,Atomic Theory.
Atomic Weights.SeeChemistry;Molecular Weights.
Atomists.SeeAtoms.
Atoms, for many years regarded as the ultimate indivisible particles of the chemical elements. The idea originated with some of the ancient philosophers (the atomists), more especially Democritus (450B.C.), Epicurus, and Lucretius (99-55B.C.), and was developed into a definite theory by Dalton (1804). According to Dalton the atoms of any one element are alike in all their properties, but differ from the atoms of other elements, and when chemical combination occurs it takes place between the atoms of the combining elements (seeChemistry). Various views have been held with regard to the nature of atoms. Newton regarded them as hard, ponderable particles, perfectly unalterable, and concluded that the difference between substances was due to different kinds of atoms. Lord Kelvin propounded the view that the properties of atoms might be explained by those of vortices or vortex rings in a homogeneous frictionless fluid. As a result of the researches of British and French physicists on radium, the latest view is that matter and electricity are closely connected, that atoms are not indivisible, but complex aggregates containing positive and negative electrons, the differences between the atoms depending mainly on the numbers of these electrons and their velocity. SeeRadium.
Atonement, in Christian theology, the special work of Christ effected by His life, sufferings, and death. The first explicit exposition of the evangelical doctrine of the atonement is ascribed to Anselm, Archbishop of Canterbury, in 1093.
Atrato(a˙-trä´tō), a river of S. America, in the north-west of Colombia, emptying itself by nine mouths into the Gulf of Darien; it is navigable by steamers of some size for 250 miles, and has long been the subject of schemes for establishing water-communication between the Atlantic and Pacific.
Atrauli, a town of India, United Provinces, Aligarh district, clean, well built, and with a good trade. Pop. 16,560.