Atmospheric Engine. Under this name was formerly understood an engine operated by the simultaneous pressure of cold air on a small piston and hot air on a large piston, the air being heated and expanded during its passage from the small cylinder into the large one. Since, however, engines have been built to work by the pressure of the air alone, without the addition of heat, engines operated by the latter force have been called caloric engines. (See Caloric Engine.) The use of ordinary atmospheric pressure as a primary source of power has long been a delusion of persons of the class who still seek for perpetual motion. All that has been accomplished in this way has been by making use of the continual changes in the atmospheric pressure, as for instance to move the mercurial column in a syphon barometer of which the two vertical tubes were very far apart, and the whole balanced on a central pivot. An increase in atmospheric pressure would drive more mercury into the long closed end, 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 mean's, 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 cylinder, 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 comparison with several scores of small independent 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 successfully employed by Sommeiller in the construction of the Mont Cenis tunnel; the hydraulic 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 extensively 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 expansion 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 provision 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 drills 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 engines 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).