40 Experimental Philosophy. [Lecture 1(5. 



other as exactly as the arms of a balance or scale 

 beam, which are of equal lengths. Thus it 

 gives a man no advantage, except that he can 

 apply his weight as well as his strength in rais- 

 ing a body from the earth, and then he can lift 

 more than his own weight. 



With a combination of pulleys, however, the 

 case is different. For if a weight W hangs at 

 the lower end of the moveable pulley D, and 

 the cord GF goes under the pulley, and is fixed 

 at the top of the hook H on one side, and nailed 

 to the block C on the other ; it is evident that 

 H and C between them support the whole weight 

 W ; H supports one half, and C the other half. 

 Now suppose I take the support of one of their 

 halves upon myself, but merely change the direc- 

 tion of my power, and instead of holding up the 

 cord at C, throw it over the immoveable pulley 

 fixed there, and exert my strength below at P; 

 it will be evident that I support one half the 

 weight W, and the hook H supports the other. 

 If therefore I draw the cord at P, the weight W 

 will continue to rise, but wherever it rises, I con- 

 tinue to support but half its weight while H sup- 

 ports the other. Thus, one single moveable 

 pulley diminishes one half of the weight to be 

 raised ; if we should add another, it would di- 

 mmish the half of that which remained, and so 

 on. For instance, if a weight of eight hun- 

 dred pounds is to be raised, I use one moveable 

 pulley, and that will lessen the weight one hah , 



