OF THE MECHANICAL POWERS. OO 



weight ; the advantage gained by this is as 4 to the u--n 

 working power. Thus, if one end of -^x-\'-^ 



the rope K MO Q be fixed to a hook 

 at /, and the rope passes over the pul- 

 leys N and R, and under the pulleys 

 L and P, and has a weight T, of one 

 pound, hung to its other end at T, this 

 weight will balance and support a 

 weight W of four pounds hanging by 

 a hook at the moveable block [7, allow- 

 ing the said block as a part of the 

 weight. And if as much more power 

 be added, as is sufficient to overcome 

 the friction of the pulleys, the power 

 will descend with four times as much velo- 

 city as the weight rises, and consequent- 

 ly through four times as much space. 

 f' The two pulleys in the fix- 



ed block X, and the two in the 

 \ moveable block Y., are in the 

 same case with the last men- 

 tioned; and those in the lower 

 block give the same advantage to the power. 

 As a system of pulleys has no great weigh;, 

 and lies in a small compass, it is easily car- 

 ried about ; and can be applied in a great 

 many cases, for raising weights, where other 

 engines cannot. But they have a great deal 

 of friction on three accounts : 1. Because 

 the diameters of their axes bear a \rrv 

 considerable proportion to their own dia- 

 r*\L_l/ meters; 2. Because in working they are 

 apt to rub against one another, or against 

 the sides of the block ; 3. Because of inc 

 stiffness of the rope that goes over and un- 

 der them. 



F 



