324 ON THE CONSEEVATION OF FORCE. 



done, it has regained its former capacity, and can again 

 set the clock in motion. 



We learn from this that a raised weight possesses a 

 TYioving force^ but that it must necessarily sink if this 

 force is to act ; that by sinking, this moving force is 

 exhausted, but by using another extraneous moving force 

 -^that of the arm — its activity can be restored. 



The work which the weight has to perform in driving 

 the clock is not indeed great. It has continually to 

 overcome the small resistances which the friction of the 

 axles and teeth, as well as the resistance of the air, oppose 

 to the motion of the wheels, and it has to furnish the 

 force for the small impulses and sounds which the 

 pendulum produces at each oscillation. If the weight is 

 detached from the clock, the pendulum swings for a 

 while before coming to rest, but its motion becomes each 

 moment feebler, and ultimately ceases entirely, being 

 gradually used up by the small hindrances I have men- 

 tioned. Hence, to keep the clock going, there must be a 

 moving force, which, though small, must be continually 

 at work. Such a one is the weight. 



We get, moreover, from this example, a measure for the 

 amount of work. Let us assume that a clock is driven 

 by a weight of a pound, which falls five feet in twenty- 

 four hours. If we fix ten such clocks, each with a weiglit 

 of one pound, then ten clocks will be driven twenty- four 

 hours ; hence, as each has to overcome the same resistances 

 in the same time as the others, ten times as much work 

 is performed for ten pounds fall through five feet. Hence, 

 we conclude that the height of the fall being the same, 

 the work increases directly as the weiglit. 



Now, if we increase the length of the string so that 

 the weight runs down ten feet, the clock will go two 

 days instead of one ; and, with double the height of fall, 

 the weight will overcome on the second day the same 



