ON THE CONSERVATION OF FORCE 187 



We learn from this that a raised weight possesses a 

 moving 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 pro- 

 duces at each oscillation. If the weight is detached from 

 the clock, the pendulum swings for a while before coming 

 to a 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 the 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 increases as the height of fall. Hence, we may take 

 the product of the weight into the height of fall as a meas- 

 ure of work, at any rate, in the present case. The applica- 

 tion of this measure is, in fact, not limited to the individual 

 case, but the universal standard adopted in manufactured 



