282 ON THE CONSERVATION OF FORCE. 



inorganic nature ; indeed, that this capacity of the human arm 

 of being tired is only one of the consequences of the law with 

 which we are now concerned. When fatigue sets in, recovery 

 is needed, and this can only be effected by rest and nourishment. 

 We shall find that also in the inorganic moving forces, when 

 their capacity for work is spent, there is a possibility of repro- 

 duction, 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 example, 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 con- 

 tinually 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 done, it 



