As to gas and vapor engines generally, it may now be said, in comparison with steam, that although the steam engine is now regarded as almost perfect in operation, and that it can be started and stopped and otherwise controlled quietly, smoothly, instantaneously, and in the most uniform and satisfactory manner, yet there is the comparatively long delay in generating the steam in the boiler, and the loss of heat and power as it is conducted in pipes to the working cylinder, resulting in the utilisation of only ten per cent of the actual power generated, whereas gas and vapour engines utilise twenty-five per cent of the power generated, and the flame and explosions are now as easily and noiselessly controlled as the flow of oil or water. The world is coming to agree with Prof. Fleeming Jenkins that “Gas engines will ultimately supplant the steam.” The smoke and cinder nuisance with them has been solved. The sister invention of the gas engine is the air engine. There can be no doubt about the success of this busy body, as it is now a swift and successful motor in a thousand different fields. Machines in which air, either hot or cold, is used in place of steam as the moving power to drive a piston, or to be driven by a piston, are known generally as air, caloric, or hot-air engines, air compressors, or compressed air engines, and are also classed as pneumatic machines, air brakes, or pumps. They are now specifically known by the name of the purpose to which they are applied, as air ship, ventilator, air brake, fan blower, air pistol, air spring, etc. The attention of inventors was directed towards compressed and heated air as a motor as soon as steam became a known and efficient servant; but the most important and the only successful air machine existing prior to this century was the air pump, invented by Guericke in 1650, and subsequently perfected by Robert Boyle and others. The original pump and the Magdeburg hemispheres are still preserved. It is recorded that Amontons of France, in 1699, had an atmospheric fire wheel or air engine in which a heated column of air was made to drive a wheel. It has already been noted what Papin (1680-1690) proposed and did in steam. His last published work was a Latin essay upon a new system for raising water by the action of fire, published in 1707. The action of confined and compressed steam and gases, and air, is so nearly the same in the machines in which they constitute the motive power that the history, development, construction, and operation of the machines of one class are closely interwoven with those of the others.