of sixteen inches if it is to run eight days. The length of time that the clock will run depends upon three things: 1. The length of the pendulum; 2. The space through which the weight falls; 3. The number of wheels in the train, and the number of teeth in each wheel. We have already seen how the length of the pendulum can be regulated. If the weight has a small space allowed for its fall, the clock may be made to run longer by increasing both the weight and the number of teeth. The number of teeth may be increased by increasing the number of wheels, or by putting in new wheels.
The wheel D, Fig. 3, is called the "center wheel," because it turns once in an hour. It has thirty-six teeth. In former times the wheel A turned once in twelve hours; and the axle, or "arbor," a, went through a hole in the face of the clock. A hand on the end of the arbor passed over certain figures on the face which marked the hours from one to twelve. This hand was called the hour-hand; but, as it could not mark the minutes, the center wheel, D, was so made that it would turn once in an hour, and thus, by carrying a hand over the face outside, marked the minutes. After this change was made no one cared whether the wheel A turned in one hour or in three hours, or whether the wheel C turned in one half minute or in two minutes, if only the wheel D turned in exactly one hour. At d is a "cannon" pinion that sticks to the arbor by friction. The minute-hand, which is placed upon the pinion, may thus be moved without turning the wheel D or any of the other wheels.
We must now provide an hour-hand. The cannon-pinion a (Fig. 4), with twelve leaves, runs on the arbor of the center wheel;
Fig. 4. but it could not be drawn in Fig. 3, because it is behind the center wheel, D. These twelve leaves, A (Fig. 4), run into thirty-six teeth in the wheel B. You will notice that the teeth and the leaves are not drawn in the picture. On the farther side of B is the pinion b, with twelve leaves which run into the forty-eight teeth of the wheel C. The wheel C and the pinion b are marked with dotted lines, because they are behind the pinion a and the wheel B. If a turns once in an hour, B will turn once in three hours, and C once in twelve hours. If what is called a "barrel" is placed over the cannon-pinion of the center wheel, and one end of it is fastened to the wheel C, the other end that comes through the face of the clock will carry the hour-hand. These wheels, in Fig. 4, are independent of the wheels in Fig. 3, except that a, in Fig. 4, fits upon the arbor d, of D, in Fig. 3 so loosely that you may turn the hour-and the minute-hand whenever you choose, and yet tightly enough to turn about with the wheel D if they are not dis-
