OX THE CONSERVATION OF FORCE. 323 



inorganic moving forces, when their capacity for work is 

 spent, there is a possibility of reproduction, although in 

 general other means must be used to this end than in the 

 case of the human arm. 



From the feeling of exertion and fatigue in our muscles, 

 we can form a general idea of what we understand by 

 amount of work ; but we must endeavour, instead of the 

 indefinite estimate afforded by this comparison, to form a 

 clear and precise idea of the standard by which we have 

 to measure the amount of work. This we can do better 

 by the simplest inorganic moving forces than by the 

 actions of our muscles, which are a very complicated 

 apparatus, acting in an extremely intricate manner. 



Let us now consider that moving force which we know 

 best, and which is simplest gravity. It acts, for ex- 

 ample as such, in those clocks which are driven by a 

 weight. This weight fastened to a string, which is wound 

 round a pulley connected with the first toothed wheel of 

 the clock, cannot obey the pull of gravity without setting 

 the whole clockwork in motion. Now I must beg you to 

 pay special attention to the following points : the weight 

 cannot put the clock in motion without itself sinking ; 

 did the weight not move, it could not move the clock, 

 and its motion can only be such a one as obeys the action 

 of gravity. Hence, if the clock is to go, the weight must 

 continually sink lower and lower, and must at length sink 

 so far that the string which supports it is run out. The 

 clock then stops. The useful effect of its weight is for the 

 present exhausted. Its gravity is not lost or diminished ; 

 it is attracted by the earth as before, but the capacity of 

 this gravity to produce the motion of the clockwork is lost. 

 It can only keep the weight at rest in the lowest point of 

 its path, it cannot farther put it in motion. 



But we can wind up the clock by the power of the arm, 

 by which the weight is again raised. When this has been 

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