60 -7r,v;-. [Book I. 



prop L , is in the direction L M, which tends to the 

 point of contact M of the two forces. 



It would be the fame if the powers/ and g (fig. 12.) 

 were in equilibrio by the inequality of their diftances 

 from the prop H, that is, in cafe their malTes were in 

 an inverfc ratio to their diltances / H and g H from 

 the prop. The charge upon the prop can never 

 be greater than the fum of the two forces, or in 

 other words the fum of the oppofed maiTes ; but it 

 would be equal to this fum if the powers were in a di- 

 rection parallel to each other, and it would be iefs than 

 this fum if the lines of direction e c , e e were inclined 

 towards each other, then the force upon the prop H 

 would be exerted in the line H I, which would tend 

 to the point I, where the two mafies would meet ac- 

 cording to th,e direction in which they act. 



In levers of the fecond and third orders the prop . 

 fupports only a part of the 'effort of the two forces. 

 In other words, it acts in conjunction with the power 

 in levers of the fecond order, and in conjunction with 

 the refiftance in levers of the third order ; as when two 

 men carry a burden with a ftaff upon their fhoulders ; 

 : two men, one of whom may be regarded as the 

 power, and the other as the prop, only carry each a 

 part of the burden ; and he who is the nearell the 

 burden carries the greater (hurc of it, and that in pro- 

 portion to his nearnelsi to i:. 



II. The PULLEY is a fniall wheel moveable upon its 

 axis, with the circumference hollowed to receive the 

 cord, which is attached on the one hand to the moving 

 power, and on the otlvr to the refitting force. The 

 wheel or pulley is commonly fixed in a block, or cafe, 

 which admits the rope or cord to pafs freely over the 

 circumference of the wheel, and the gorge of the pul- 

 ley, 



