PRIMITIVE CLOCKS. 



183 



moves a notch, the pallet g has moved to /and catches the wheel be- 

 low. When the pendulum swings from b back to c,/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 es- 

 caped 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 beinor alwavs hori- 

 zontal, or running at risrht an- 

 gles 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/, it will throw /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 ex- 

 plained 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 



