An obvious improvement was the introduction of the piston. This was Papin’s idea, and he used it first in 1690. Six years later an engine was constructed by Savery, Newcomen (a Devonshire man), and Cawley, in which the “beam” was introduced, and also the ideas of a distinct boiler separate from a cylinder in which worked a piston. This machine was in operation for about seventy years. The beam worked on an axle in its centre—something like a child’s “see-saw,” and one end being attached to the piston moving in the cylinder, it was worked up and down, the other end of the beam being fastened to the pump-rod, which was thus alternately raised and depressed. The upward movement of the piston having been effected by a rush of steam from the boiler upon its head, the steam was cut off and cold water run in upon it from a cistern. The steam was thus condensed by the water and a vacuum caused, and the piston was pressed down by the weight of the atmosphere—of course dragging down its end of the beam, and raising the pump-rod. The steam was then turned on again and pushed up the piston, and consequently the end of the beam also. Thus the engine continued to work, the turning of the cocks to admit steam and water being performed by an attendant. The engine was, however, made self-acting in this respect, and Smeaton improved this form of engine greatly. The beam is still used in engines for pumping. Nevertheless, improved though it became, it was still clumsy and almost impracticable. It was the genius of James Watt which changed it from a slow, awkward, cumbrous affair into a most powerful, practicable, and useful machine. His great improvements briefly were these: he condensed the steam in a separate vessel from the cylinder, and thus avoided cooling it and the consequent loss of steam power; secondly, he used the steam to push back the piston as well as to push it forward (this is called the “double-acting engine,” and is now always used); thirdly, he introduced the principle of using the steam expansively, causing economy in working; and fourthly, he enabled a change to be made of the up and down motion of the piston into a circular motion by the introduction of the crank. [Illustration: JAMES WATT.] The use of the steam expansively is to stop its rush to the cylinder when the piston has only partially accomplished its stroke, leaving the remainder of the stroke to be driven by the expansion of the steam. In early engines the steam was admitted by conical valves, worked by a rod from the beam. Murdock, we may add in parenthesis, is believed to have invented the slide-valve which came into use as locomotives were introduced, and of which there are now numerous forms. The valve is usually worked by an “eccentric” rod on the shaft of the engine.