288 



OX THE CONSERVATION OF FORCE. 



same, whether four labourers work for a quarter of an hour or 

 one works for an hour. 



If; instead of human labour, we introduce the work of a 

 weight, and hang to the block a load of 400, and at a, where 

 otherwise the labourer works, a weight of 100 pounds, the block 

 is then in equilibrium, and, without any appreciable exertion of 

 the arm, may be set in motion. The weight of 100 pounds 

 sinks, that of 400 rises. Without any measurable expenditure 

 of force, the heavy weight has been raised by the sinking of the 

 smaller one. But observe that the smaller weight will have 

 sunk through four times the distance that the greater one has 



FIG. 40. 



risen. But a fall, of 100 pounds through four feet is just as 

 much 400 foot-pounds as a fall of 400 pounds through one foot. 

 The action of levers in all their various modifications is pre- 

 cisely similar. Let a b, Fig. 40, be a simple lever, supported 

 at c, the arm c b being four times as long as the other arm a c. 

 Let a weight of one pound be hung at b, and a weight of four 

 pounds at a, the lever is then in equilibrium, and the least pres- 

 sure of the finger is sufiicient, without any appreciable exertion 

 of force, to place it in the position a' b ! , in which the heavy 

 weight of four pounds has been raised, while the one-pound 

 weight has sunk. But here, also, you will observe no work has 

 been gained, for while the heavy weight has been raised through 

 one inch, the lighter one has fallen through four inches ; and 



