All About Engines

52 All About Engines teeth locked together, so that the second was bound to rotate. The most curious thing about the contriv- ance, at any rate to those who have not studied mechanism, is that the flywheel makes two complete revolutions for each “ up and down ” movement of the suspended rod—that is, two revolutions for every two strokes of the engine. The rotation of the flywheel is, therefore, twice as fast, and, in the case of a slow movement of the piston, much more regular than if a crank had been used. Now, though a slow, irregular motion is no dis- advantage in pumping, it is a serious defect in an engine for driving machinery. Watt saw that in obtaining rotary motion from his single-acting engines, in which the steam was only effective during alter- nate strokes, he would have to depend on a heavy flywheel, and a heavy flywheel required power to drive it. Consequently he began to think of means whereby the steam could be made to act upon the piston at every stroke. The idea of a double-acting engine was in his mind—he even had a drawing of it__in 1774, but it was not until he began to develop the rotary engine after 1780 that he commenced to work out his ideas in practice. And his patent of 1782, in addition to laying extra stress on expansive work- ing, covered also the double-acting engine and the “ sun and planet ” motion. The double-acting cylinder involved alterations to the other parts of the engine. The old single- acting cylinder had only pulled the end of the beam, not pushed it; and the piston rod exerted that pull