ON THE CONSERVATION OF FORCE. 283 



has regained its former capacity, and can again set the clock in 

 motion. 



We learn from this that a raised weight possesses a moving 

 fores, 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 mentioned. 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 weight 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 weight. 



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 ihe second day the same resistances as on the first, 

 and will therefore do twice as much work as when it can only 

 run down five feet. The weight being the same, the work in- 

 creases as the height of fall. Hence, we may take the product 

 of the weight into the height of fall as a measure of work, at 



