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 or axis, 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 pressure
upon 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 named argon. 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, see Neon. See Climate; 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. See Pneumatic Dispatch.