Compressed air, as a means of transmitting power, rivals such mechanical devices as gearing, belting, and rope drives, when it is applied near the compressor; or it may be conducted for many miles in pipes, thus competing with the electric current; or, finally, it may be transported in tanks to the place where it is to be used, a process analogous to the use of the electric storage battery. Compressed air has, moreover, certain advantages over other methods of transmitting power for a number of special purposes. Thus for use in coal mines it is safer than electricity because it is free from the danger of sparks. There are a great many cases in which the air itself is used in the process to which the power is applied, as in different kinds of air blast, from the simple bellows to the blowers of blast furnaces; also in aerating apparatus, oil and fuel burners, spraying, cleansing, etc. A familiar form of air compressor is the hand pump used for inflating bicycle tires. This simple device illustrates two important facts; first, that a considerable amount of energy must be used to overcome the expansive force of the air, and, second, that part of the energy applied to the pump produces heat. That the heat thus produced and dissipated in the surrounding air represents a loss of energy is apparent; but energy is wasted in another way that is, perhaps, not so evident. When a gas is heated its expansive force is increased. Hence, on account of the heating of the air in the tire, the pump has to do more work to accomplish a given amount of compression than it would need to do if the air remained cool. In order to avoid this loss, the air compressors used for industrial purposes are provided with some sort of device for keeping the air cool during compression. This is accomplished by a spray of water inside the compressor cylinder, or, more commonly, by inclosing the cylinder in a water jacket. In producing high pressures, the air is compressed by degrees in two or more cylinders, and cooled between the successive stages. Lastly, before compressed air is applied to driving tools or machinery, it is often reheated to increase its pressure. For most industrial purposes the pressure of compressed air does not exceed 75 pounds to the square inch (5 “atmospheres”). For charging the tanks of compressed-air locomotives, for liquefying gases, and a few other purposes, much higher pressures are used. In laboratory experiments air has been compressed to the enormous pressure of 60,000 pounds to the square inch, or 4,000 atmospheres. At a pressure of 14,000 pounds to the square inch compressed air has been successfully used for blasting in mines in place of ordinary explosives.