PRIMITIVE CLOCKS. 187 



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 in- 

 creased 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 ; 

 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 FlG 4 



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- 



