At the very beginning of the 19th century John Dalton in England, 1801-1807, and Gay-Lussac in France began their investigations of gases and vapours. Dalton was not only the author of the atomic theory, but the discoverer of the leading ideas in the “Constitution of Mixed Gases.” These features were the diffusion of gases, the action of gases on each other in vacuum--the influence of different temperatures upon them, their chemical constituents and their relative specific gravity. Gay-Lussac, continuing his investigations as to expansion of air and gases under increased temperatures, in 1807-10, established the law that when free from moisture they all dilate uniformly and to equal amounts for all equal increments of temperature. He also showed that the gases combine, as to volume, in simple proportions, and that several of them on being compounded contracted always in such simple proportions as one-half, one-third, or one-quarter, of their joint bulk. By these laws all forms of engines which were made to work through the agency of heat are classed as heat engines--so that under this head are included steam engines, air engines, gas engines, vapour engines and solar engines. The tie that binds these engines into one great family is temperature. It is the heat that does the work. Whether it is a cannon, the power of which is manifested in a flash, or the slower moving steam engine, whose throbbing heart beats not until water is turned to steam, or the sun, the parent of them all, whose rays are grasped and used direct, the question in all cases is, what is the amount of heat produced and how can it be controlled? It, then, can make no difference what the agent is that is employed, whether air, or gas, or steam, or the sun, or gunpowder explosion, but what is the temperature to be attained in the cylinder or vessel in which they work. Power is the measure of work done in a given time. Horse power is the unit of such measurement, and it consists of the amount of power that is required to raise one pound through a vertical distance of one foot. This power is pressure and the pressure is heat. The unit of heat is the amount of heat required to raise the temperature of a pound of distilled water one degree--from 39 degrees to 40 degrees F. Its amount or measurement is determined in any instance by a dynamometer. These were the discoveries with which Philosophy opened the nineteenth century so brilliantly in the field of Pneumatics. Before that time it seemed impossible that explosive gases would ever be harnessed as steam had been and made to do continual successful work in a cylinder and behind a piston. As yet means were to be found to make the engine efficient as a double-acting one--to start the untamed steed at the proper moment and to stop him at the moment he had done his work.