188 THE POPULAR SCIENCE MONTHLY. 



turbed. 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 di- 

 rection 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 " main- 

 taining-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 



