THE WORKS OF A WATCH. 375 



the lesser weight, and seeing how much of the 

 rope is uncoiled in one revolution of the wheel, 

 and comparing that with the length of rope coiled 

 up round the axle in the revolution, and conse- 

 quently in the corresponding time. 



A watch is an instrument where a set of wheels 

 are made to revolve with a uniform velocity, and 

 at a certain rate, so as to become the measure of 

 time. When this motion is obtained, an index, 

 or hand, is put on the axis of one of these wheels, 

 and thus made to revolve on a dial, where the 

 fractions of the revolution are marked. Two or 

 more hands may be applied to different wheels, 

 and these will indicate the subdivision of time in 

 which the several wheels revolve in a minute, an 

 hour, or in twelve hours. 



In this figure, a is the wheel, and the notches 

 or rather the projecting points on it, the teeth* 

 B is the pinion, and the teeth on it are technically 

 called the leaves. This is the wheel and pinion, 



* Tiie resemblancG in these teeth to the jagged edi;c of a spur 

 explains why certain wheels are called the " spiir-gear." The 

 engraver has here Hubstituted, for my sketch, a crown wheel, that 

 is, one where the teeth stand on the edge of a circlet, and parallel 

 to the axis. 



