PRIMITIVE CLOCKS. 185 



line ef. Great care is taken in preparing the spring at a, so that the 

 bob will have no other motion than that from e tof. Should it move 

 sidewise, or twist about, the clock will be spoiled. The bob was for- 

 merly flat, like a small plate, or round, like a ball. It was then a diffi- 

 cult matter to run the pendulum-wire through the exact center, and 

 therefore the best bobs are now made in the form of a cylinder. A 

 nut at the end of the wire keeps the bob from slipping off. If the nut 

 is turned to the right, the pendulum is shortened, and the clock goes 

 faster. If it is turned to the left, the clock goes slower. Sometimes 

 it is necessary to regulate the pendulum without stopping it. This is 

 done by placing small weights on the parts of it that project. In 

 order to keep them of the same length, both in summer and in winter, 

 pendulums were often made of wood ; but it has been found that if 

 the bob is made of bars of iron and zinc, or brass and steel, in the 

 form of a gridiron, the different expansions of the two metals keep the 

 pendulum at the right length. The pendulum-rod sometimes ends in 

 a cup of mercury at the bob. When the heat expands the rod, the 

 mercury is forced upward in the cup and nearer the fixed end of the 

 pendulum. The object of both the gridiron pendulum and the mercu- 

 rial is to bring the " center of oscillation " as near as possible to the 

 "center of gravity." Another kind of a pendulum is called the 

 "rotary," because the bob moves in a circle instead of going from 

 side to side, but this is not thought to be at all reliable. 



From what has been said already, you will see that the weight h 

 (Fig. 1) would soon run away with the scape- wheel unless the pallets 

 defg dodged in and out among the teeth and stopped it from going 

 so fast. The pendulum, too, instead of moving back and forth be- 

 tween b and c, would stop half-way between them in a vertical or up- 

 and-down line, like the plummets that the bricklayers use. A clock 

 with simply the scape-wheel and the pendulum will soon run down ; 

 you must therefore have more wheels and a heavier weight to move 

 them, or else your wheels will not move evenly enough to carry the 

 minute- and hour-hands over the " face " that is outside. In Fig. 3 

 you will see that we have added other wheels ; but you will recognize 

 the scape-wheel in c, and the weight hanging to the wheel a. As it 

 descends, the weight pulls the wheel a in the direction of the arrow. 

 The wheel A turns with the wheel a, and it has seventy-eight "teeth," 

 as the cogs are called. At b is a small wheel called a " pinion," with 

 six "leaves," as the cogs are called. The large wheel, B, has also 

 seventy-eight teeth ; and the pinion c has also six leaves. While A 

 is turning round once, B and b turn thirteen times, because b has one 

 thirteenth as many teeth as A. In the same way C and c turn thirteen 

 times as fast as B and b. I have a clock before me in which the wheel 

 A turns once in one hundred and thirty minutes, or two hours and ten 

 minutes. The wheel B turns in ten minutes, and the wheel C in ten 

 thirteenths of a minute. You will see that the scape- wheel C does 



