[44] The illustration shows the several parts of Newcomen’s atmospheric engine. _a_ is the boiler; _b_, the piston moving up and down; _c_, the cylinder; _d_, a pipe proceeding from the top of the boiler, and inserted into the bottom of the cylinder, having a cock, _e_, to interrupt the flow of steam at pleasure; _f_, cold-water cistern, from which the cold water is conveyed by the pipe _g_, called the injection-pipe, and thrown in a jet into the cylinder, _b_, on turning the injection-cock, _h_; the snifting-valve, _i_, enables the air to escape from the cylinder, while the siphon-pipe, _j_, enables the condensed steam to flow from the same cavity in the form of water; _k_, the main lever beam; _l_, the counterpoise or weight hung on the balance-beam, or on _m_, the pump-rod which works the pump, _n_. Thus, step by step, Newcomen’s engine grew in power and efficiency, and became more and more complete as a self-acting machine. It will be observed that, like all other inventions, it was not the product of any one man’s ingenuity, but of many. One contributed one improvement, and another another. The essential features of the atmospheric engine were not new. The piston and cylinder had been known as long ago as the time of Hero. The expansive force of steam and the creation of a vacuum by its condensation had been known to the Marquis of Worcester, Savery, Papin, and many more. Newcomen merely combined in his machine the result of their varied experience, and, assisted by the persons who worked with him, down to the engine-boy Potter, he advanced the invention several important stages; so that the steam-engine was no longer a toy or a scientific curiosity, but had become a powerful machine capable of doing useful work.