JAMES WATT AND HIS INVENTIONS. 115
an arm by means of which it may be turned into any posi- tion. When placed with its face in line with the pipe, it offers very little resistance to the flow of steam to the en- gine. When set in the other position, it shuts off steam entirely and stops the engine. It is placed in such position at any time, that the speed of the engine is just that re- quired at the time. In the engraving of the double-acting engine with fly-wheel (Fig. 31), it is shown at T, as con- trolled by the governor.
FIG. 29.-The Governor.
The governor, or "fly-ball governor," as it is often distinctively called, was another of Watt's minor but very essential inventions. Two heavy iron or brass balls, B B', were suspended from pins, CC, in a little cross-piece car- ried on the head of a vertical spindle, A A', driven by the engine. The speed of the engine varying, that of the spindle changed correspondingly, and the faster the balls were swung the farther they separated. When the engine's speed de- creased, the period of revolution of the balls was increased, and they fell back toward the spindle. Whenever the veloc- ity of the engine was uniform, the balls preserved their dis- tance from the spindle and remained at the same height, their
