The use of pneumatic tools began in the sixties of the last century, when pneumatic drills were employed with conspicuous success in the construction of the Mont Cenis and Hoosac tunnels. Such tools are now indispensable adjuncts not only of tunneling and mining, but also of nearly every department of metal-working and wood-working, and have contributed incalculably to the welfare of mankind. Imagine a workman with an ordinary hammer driving such a tool as a chisel, punch, or calking iron, and estimate the amount of work accomplished in the course of a day spent in this wearisome labor. Then consider how such operations are performed with the help of that versatile substance, air. The pneumatic hammer consists of a piston working in a cylinder, to which compressed air is conveyed from a compressor by means of a flexible hose. The hammer is so designed that the air causes the piston to work back and forth with great rapidity. A chisel, rammer, or other percussion tool is loosely fitted in the nose of the hammer, so that the piston will strike it a blow at each forward motion. The workman has nothing to do but hold the tools in place. With a common hammer or mallet a workman will strike from twenty to a hundred blows a minute, according to the nature of the work. The speed of the pneumatic hammer ranges from 1,000 to 20,000 blows per minute, so that its sound is a continuous buzz. Such hammers are used for calking, chipping, riveting, and a great number of other purposes. In another large class of pneumatic tools work is done by rotation instead of percussion. The piston is replaced by a motor, which turns an auger, drill, or other tool for such operations as boring, screwing, reaming, etc. The use of pneumatic tubes for transporting letters, parcels, and the like, although suggested as early as 1667, has been in practical operation only since 1854, when a tube 220 yards long was built in London to convey telegraphic dispatches. The articles to be transported are placed in a carrier fitting closely inside the tube and propelled either by introducing air under pressure behind it or by exhausting the air in front of it. Scores of miles of such tubes laid underground are now in operation in London, Paris, Berlin, New York, and other large cities for carrying mail matter. In the United States the pneumatic cash carrier, used in stores, is the commonest application of “pneumatic dispatch,” as this system of transportation is called. The use of compressed air instead of a brush for applying paint, varnish, and whitewash is a further illustration of the versatile possibilities of air as a means of transmitting power.