Page:Popular Science Monthly Volume 29.djvu/200

188. You can, therefore, move the two hands of the clock without disturbing any of the wheels in Fig. 3.

We have seen that the weight must keep pulling, or the clock will stop. Sometimes, instead of the weight, a spring is used, especially if the clock is small. The spring simply pushes the wheel A in the direction of the arrow (Fig. 3). When the spring is used the clock may have a pendulum escapement, or it may have a wheel escapement like that of a watch. But if the pressure of the spring is removed, or if the weight (should there be one) is lifted, the clock will stop. When you wind up the clock it is the same thing as taking away the weight, or the spring, while you are winding. How, then, can you wind it and still keep it going? This is done by what is called a "going-barrel," or "maintaining-works." In Fig. 3 you will notice that the wheel A turns in the direction of the arrow when the weight pulls down. When you wind up the clock the force of the weight is taken off. A strong spring is placed on the side of the wheel A that pushes it along in the direction of the arrow for the few seconds that you take in winding. Another wheel, or barrel, a, is placed on the large wheel A, and on this the string that holds the weight is wound. This wheel you turn in the opposite direction to that of the arrow. At the same time the spring pushes A in the direction of the arrow. You will sometimes see an old clock with an endless chain so arranged that, by pulling on a small weight, you may lift a large weight, and thus wind the clock. Others of the old time-pieces have weights that are hung by chains with rings at the upper end. When the weight has run down you can pull on the ring and the weight is lifted. You will find that all the best clocks, and all the watches, have the "maintaining-works."

The striking part of a clock is a very interesting study. It has a train of wheels and a weight entirely separate from the train that tells the hours and minutes by the hands. The large wheel, B, in Fig. 5, really consists of two wheels fastened together. The larger or outer wheel has seventy-eight teeth that run into a pinion, with thirteen leaves. The cord that holds the weight is wound on the axle of, on which A is also fastened. There are thirteen pins on the surface of A. They can not be seen, because they are on the other side of the wheel; but they have been drawn in the picture so that the explanation may be more easily understood. As the wheel A turns, each pin strikes the end of the lever c, which, when it is released, springs back and strikes the bell d. The smaller wheel, B, has notches all about it—first, one notch; then two notches close together; then three notches close together; and so on until you find twelve notches all in one place. This makes seventy-eight notches in all. Behind the wheel B is a pinion that you can not see. It is turned by the wheel A, but it is entirely independent of B, although it turns on the same axis. This independent pinion turns a wheel almost as large as B, which