Page:Popular Science Monthly Volume 29.djvu/195

 moves a notch, the pallet g has moved to f and catches the wheel below. When the pendulum swings from b back to c, f is moved to g, and the pallet d stops the wheel from going any farther. So that, while the pendulum has gone from c to b and back again, only one tooth of the wheel has escaped instead of two. The arm of the pendulum which acts upon the teeth of the wheel and the wheel itself are called the "escapement," because they let only a little of the power in the weight escape at a time—just as the hour-glass allowed but a little of the sand to escape at once, and as the clepsydra allowed only a little of the water to run out at a time.

The earliest form of an escapement was that of Vick. It was a small wheel that was turned back and forth by a twisted string. Afterward it was turned by a spiral spring, the wheel being always horizontal, or running at right angles to the other wheels, that were vertical. A new "scape-wheel," as it is called, was invented by Dr. Hooke, which moved vertically, or in the same plane with the other wheels. This is the wheel that is shown in Fig. 1. You will see by the figure that, when the bob is at b, and the tooth of the wheel comes on the pallet f, it will throw f over to g and help the bob to move from b to c. This is called the "recoil" escapement, because the force of the wheel gives such a sudden jerk to the pendulum. The cheaper clocks frequently have the recoil escapement. Very much of this jerking motion is saved by the "dead-beat" escapement, invented by Graham, an Englishman. It is so called because the tooth of the wheel falls dead upon the pallet and stays there until the pendulum starts back and releases it. The teeth of the dead-beat scape-wheel are of a different shape from those shown in Figs. 1 and 2. The "gravity" escapement is so called because another weight beside the principal weight gives an impulse, or motion, to the pallet. There are many other kinds of escapements, that are too difficult to be explained here. I have described only the simpler kinds.

In Fig. 1 the pendulum is made very much shorter than it should be, so that it will not take up the whole of the page. At the earth's