66 Syftem of [Book I. 



other, the powers are then (as has been before ob- 

 ferved) in an inverfe ratio to the velocities. 



Hence it follows, that the number of pullies and 

 the power being given, the weight which the fyftem 

 of pullies is capable of fuftaining is eafily found, by 

 multiplying the power by double the .number of 

 moveable pullies. For example, fuppofe that the 

 power is equal to 60 pounds, and that the number of 

 moveable pulies is 3, 60 multipled by 6 (double the 

 number of 3) will be equal to 360, which is the 

 weight that this fyftem of pullies is .able to fuftain. 



In the fame manner the number of moveable pul- 

 lies being given, as well as the weight which the 

 tackle is able to fuftain, the power is eafily found by 

 dividing the weight by double the number of move- 

 able pullies. Si?ppofe the weight equal to 800 Ib. 

 and the number of moveable pullies to be 4 j 800 

 divided by 8 (that is, by double the number of pul- 

 lies) gives the quotient loolbs. which is the force 

 neceffary to fuftain 800 Ibs. with fuch an union of 

 pullies *. 



To 



* ' It may be obferved, that in all contrivances by which 



* power is gained, a proportional lofs is fuffered in time. If one 



* man, by means of a tackle, can raiie as much weight as ten 

 ' men could by their unafliited ftrength, he will be ten times as 



* long about it. 



' It is convenience alone, and not any aftnal increafe of fore?, 

 ' which we obtain from mechanics. This may be illaftrated by 

 the following example : 



' Suppofe a man at the top of a houfe draws up ten weights, 

 ' one at a time, by a fingle rope, in ten minutes. Let him have a 



* tackle of five lower puilies, and he will draw up the whole ten 

 at once with the fame eafe as he before raifed up one j but in 

 ' ten times the time, that is, in ten minutes. Thus we fee the 



* fame work is performed in the fame time, whether the tackle be 



* ufcd or not; but the convenience is, that if 'the whole ten 



weights, 



