186 HELMHOLTZ 



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 continually sink lower and lower, 

 and must at length sink so far that the string which sup- 

 ports it is run out. The clock then stops. The usual 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 has regained its former capacity, and can again 

 set the clock in motion. 



