ON THE CONSERVATION OF FORCE. 



329 



Fio. 39. 



pulls the rope throiigli four feet, the load only rises one 



foot, for the length through which he pulls the rope, at a, is 



uniformly distributed in the block over four ropes, so that 



each of these is only shortened 



by a foot. To raise the load, 



therefore, to the same height, 



the one man must necessarily 



work four times as long as the 



four together did. But the total 



expenditure of work is the 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 exer- 

 tion of the arm, may be set in 

 motion. The weight of 100 

 pounds sinks, that of 400 rises. 

 Without any measurable expen- 

 diture 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 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 



