ATMOSPHERIC ENGINE ATOMIC THEORY 85 pivot. An increase in atmospheric pressure would drive more mercury into thejong closed vnd, and cause this to descend; a Decrease in atmospheric pressure would cause the mercury to return to the short open end, and cause this in its turn to descend ; while wheelwork was so arranged as to produce motion by a descent either way. Such a contrivance, however, or any other based on the same principle of the changes in atmospheric pressure, even when constructed on the largest practicable scale, can only produce a weak power. It is evident that in order to produce an available motive power , by the application of atmospheric pressure, this pressure ought to be made as strong as steam pressure; for which purpose the air must be | compressed by mechanical means, or at least a vacuum created. In this way, however, the air can only be employed for the transmission of power, and this is actually the case in all atmospheric engines. None of them are prime : movers, but the air which drives them is | compressed by another power either steam, falling water, or animal force. There are ! several ways of using this compressed air. One is to fill with it a large strong cylin- der, the equivalent of a locomotive boiler, and use this compressed air to work the piston, in the same way as steam is used. This is only applicable upon cars traversing short distances, so that the engine can periodically receive new supplies. It is argued that a very large steam engine, creating the power for a great number of small engines, by compressing air in large reservoirs, to supply all the engines of a city line of railroad cars, is very economical in com- parison with several scores of small indepen- dent motors, each with its furnace and boiler. Another method of supplying atmospheric pressure from one prime motor to different | small engines, is to conduct the air in tubes ' from the former to the latter. This was sue- j cessfully employed by Soimneiller in the con- struction of the Mont Cenis tunnel ; the hy- draulic power of a cataract near the entrance of the tunnel being used as a prime motor to compress the air in reservoirs, whence it was conducted by flexible tubes to the rock- ! boring machines. This method is now exten- sively in use in the United States, the prime motor being ordinarily steam power. One of the chief advantages of atmospheric engines of this class is that, in place of heat and steam escaping, as is the case with steam engines, pure atmospheric air escapes, which by its expan- sion becomes cold, and thus supplies the end of the mining shaft with pure and cool air, securing a most perfect ventilation ; while the use of steam in such a locality, even if a pro- vision were made to carry off the escaping steam, would raise the temperature to such a degree as to make further work impossible. It is now acknowledged that the boring of such tunnels as the Mont Cenis, the St. Gothard, and the Hoosac would be impracticable but for drill* worked by atmospheric engines. When the boring is performed by percussion of steel drills, the atmospheric pressure moves a piston connected with them. When the boring is performed by rotation, as is the case with the diamond drill, the atmospheric engine may be either a rotary or a reciprocating one. In fact' the- arrangement of all atmospheric engines is nearly identical with that of non-condensing steam engines. As atmospheric pressure may be easily transmitted through tubes in any direction, and therefore also the power of a prime motor, it is expected that in the course of time the power of large cataracts will be utilized in this way to drive atmospheric en- gines for several miles around. A piston may also be propelled through a very long tube by atmospheric pressure or by a vacuum ; this has been applied to transmitting small packages, and also to the propulsion of railroad trains. (See PNEUMATIC DESPATCH, and PNEUMATIC RAILWAY.) ATNAHS, or Atenis, an Indian tribe of British America, called also Shoushwap or Chin In- dians. They are a Selish tribe on Frazer and Salmon rivers, an energetic, industrious people, manufacturing blankets of good quality from the wool of a native goat or sheep. Another tribe called Atnas is mentioned in the early accounts of the northwest as living on Copper river, Alaska, and seems to be now included in the Koloshians. ATOLL, the Malay name of a peculiar form of coralline island common in Polynesia and the Indian ocean, which consists of a circular reef, seldom more than a few hundred yards wide, enclosing a sheet of water connected with the ocean by an open passage. These la- goons are sometimes 30 m. in diameter and from 100 to 400 feet deep, and afford safe har- bors, the opening never being on the windward side. The reefs generally support vegetation, and are sometimes inhabited. ATOMIC THEORY, the doctrine that matter consists of ultimate particles or atoms incapable of division. This idea was first maintained speculatively in opposition to the notion that matter is capable of being divided to infinity. Modern science has adopted this idea, not merely as a speculation which cannot be veri- fied, but as a proposition which interprets and harmonizes a wide range of experimental facts. Inasmuch as it offers an explanation of the facts and principles of chemistry, these require to be noticed before we can understand the use and necessity of the theory. Modern chemistry took its rise with the abandonment of the old notion of phlogiston, and the eluci- dation of the principles of combustion by La- voisier. He introduced the balance as a fun- damental instrument of chemical inquiry, and thus placed the science upon a firm quantitative basis. As weighing became general and ac- curate, it was soon discovered that chemical combination is definite, and chemical compo- sition constant. A certain weight of alkali, for example, combines with a given weight of acid
