Chap. 7.] Bullies. 67 



To find the number of moveable pullies which are 

 neceflary to fuftain a given weight with a given power, 

 the weight muft be divided by the power, and then 

 half the quotient will be the number fought. Suppofe, 

 for example, the weight to be 500 Ibs. and the power 

 50 j the apparatus ought to have 5 moveable pullies, 

 for 500 divided by 50 gives 10 for the quotient, the 

 half of which is 5. 



In all thefe cafes it has been prefumed, that the di- 

 rection of the cords is parallel to each other. If it is 

 oblique, then the burden to be fuftained is to the 

 power, as the fum of the fines * of the angles, which 

 the cords of the moveable pullies make with the ho- 

 rizon, is to the whole fine. It follows then, in this 

 cafe, that the power muft be greater than is required 

 in the former cafe j the direction therefore of the cords 

 ought if poffible to be always parallel to each other. 



To prevent the friction of the ropes one againft 

 another, which occafions a confiderable refiftance and 

 wears them much, it has been found necefiary to em- 

 ploy in a fyftem of pullies fome of a fmaller diameter, 

 which is inconvenient on account of the ftiffhefs of the 

 rope. It is therefore better to place the pullies of 



* weights be joined into one, they may be raifed with the tackle, 

 ' chough it would be impoffible to move them by the unaffiUed 

 ' ftrength of one man. 



* Or, fuppofe, inflead of ten weights, a man draws ten buckets 

 ' of water from the hold of a fhip in ten minutes, and that the; 

 ' fhip being leaky, [admits an equal quantity in the fame time. 



* It is propofed, that by means of a tackle, he mall raife a bucket 

 ' ten times as capacious. With this affiltance he performs it, but 

 ' in as long a time as he employed to draw the ten, and therefore 

 ' is as far from gaining on the waLer in the latter cafe as in the 



* former.' Nichol. Phil. vol. i. p. 74.. 



* The fine is the meafure of xn angle, or a right line drawn 

 from the extremity of one leg to that of the other. 



F 2 each 



