COMPRESSED-AIR ENGINE. One mode. of employing air as a motive-power is described in caloric engine (q.v.). Another obvious way is to compress it, and then apply it in the manner of high-pressure steam. Although compressed air has been used for working small engines in confined situations, such as tunnels (q.v.), it is not at all likely that it will ever come into extensive use, owing to the great waste of power attending it. This waste arises from two causes—first, the friction due to forcing the compressed air along a great length of pipe; and secondly, the loss from the dissipation of the great heat which results from its compression. If, say, 100 cubic feet of air is compressed into 1 cubic foot, it will become very hot, and although it is very easy to keep in the air, it is impossible to keep in the heat. In spite of every precaution, the heat will find its way through the vessel in which the air is confined, and through the pipes in which it is being transmitted, and this is equivalent to a portion of the airitself leaking out, because when the air is permitted to expand in working the engine, it will not attain the bulk it originally had of 100 cubic feet. The greater the original com pression of the air, the higher its temperature will rise; and as this caloric, which can not be kept from escaping, is practically a part of the bulk of the air, it follows that the loss of power from this cause will increase with the pressure or tension of the air. Even were it possible to prevent the escape of the heat, by covering the vessels and pipes with some non-conducting substance, it would not be practicable to use the licit air in the same way as steam is used, because the lubricating material necessary to keep the piston and slide-valves from " tearing" would be decomposed by the high tempera ture. In steam-engines, there is always a small quantity of water in the cylinders and slide-valves, arising from the condensation of a portion of the steam, and this suffices to lubricate the piston and valves. It is well known that when steam is superheated so highly as to prevent a slight condensation in the cylinder and slide-vales, they are very rapidly destroyed. Air rises in temperature when very much compressed, and we can not use it until its temperature falls; and as this involves a great waste of power, it follows that where economy is of any consequence, air cannot be used as a mode of trans mitting mechanical power. Indeed, no fluid can be economically used for transmitting
power for any great distance. We have just seen that compressed air is very unsuitable; steam is even more wasteful, because' it condenses into water in long pipes. Water itself loses much of its force from friction in passing through long pipes, unless they are of very large size; and in applying it to hydraulic cranes, where the weight to be raised varies, great waste of power arises from the fact, that the cylinder, in which thd ram works, has to be filled every time the crane is worked with water at the full pressure of COO or 700 lbs. to the square inch, even when a pressure one tenth of that amount would suffice to raise the weight. In short, the power actually used in working a hydraulic crane is always the maximum, even when the weight to be raised is a minimum. It uses as much power to lift a hundred-weight as it does to lift a ton. The extreme handi ness and other practical advantages possessed by the hydraulic cranes leave a large bal ance in their favor, notwithstanding their waste of power.
In boring the Mont Cenis tunnel (see TUNNET), air was compressed at the mouth of the tunnel by the abundant water-power easily obtainable there, and forced along to the working face through small iron pipes, for working the boring-machines. The tunnel through the Hoosac mountain in Massachusetts has also been bored by compressed air working the rock-drills., A plan has been proposed for using ordinary steam-boilers and engines close to the working face In the tunnel, and drawing out the stroke and vitiated air through a wooden trunk. Another fairly successful application of compressed-air engines has been in the working of coal-cutting machines. Of these machines, Firth's in England, Gladhill's in Scotland, and Brown's in America, have been in practical operation for several years; but they can as yet only be economically worked under -exceptionally favorable circumstances. Experiments have been recently tried to propel tramway cars with compressed air. Engines for the compression of air are used in making ice. See REFRIGERATING MACHINES.