114 THE DEVELOPMENT OF THE MODERN STEAM-ENGINE.
Boulton & Watt gave them, as compensation for their en- gines, a fraction-usually one-third-of the value of the fuel saved by the use of the Watt engine in place of the engine of Newcomen, the amount due being paid annually or semiannually, with an option of redemption on the part of the purchaser at ten years' purchase. This form of agreement compelled a careful determination, often, of the work done and fuel consumed by both the engine taken out and that put in its place. It was impossible to rely upon any determination by personal observation of the number of strokes made by the engine. Watt therefore made a "counter," like that now familiar to every one as used on gas-meters. It consists of a train of wheels moving point- ers on several dials, the first dial showing tens, the second hundreds, the third thousands, etc., strokes or revolutions. Motion was communicated to the train by means of a pen⭑ dulum, the whole being mounted on the beam of the engine, where every vibration produced a swing of the pendulum. Eight dials were sometimes used, the counter being set and locked, and only opened once a year, when the time arrived for determining the work done during the preceding twelve- month.
The application of his engine to purposes for which careful adjustment of speed was requisite, or where the load was subject to considerable variation, led to the use of a controlling-valve in the steam-pipe, called the "throttle- valve," which was adjustable by hand, and permitted the supply of steam to the engine to be adjusted at any instant and altered to any desired extent: It is now given many forms, but it still is most usually made just as originally designed by Watt. It consists of a circular disk, which just closes up the steam-pipe when set directly across it, or of an elliptical disk, which closes the pipe when standing at an angle of somewhat, less than 90° with the line of the pipe. This disk is carried on a spindle extending through the pipe at one side, and carrying on its outer end
