Chap. 7.] Syjlem tf Pullies. 65 



dius c b, Becaufe to keep an equilibrium, it is necef- 

 fary that the power fhould be to the refiftance, as the 

 radius is to the diameter* 



But if the direction of the powers is oblique, as for 

 inftance, if one end of the cord is attached to the fixed 

 point , while the other is fuftained by the power P, it 

 ftiil reprefents a lever of the fecond order m /, of which 

 the fulcrum will be at m, and which will be divided 

 into two equal parts m /, i /, by the direction c I of the 

 refiftance. Then the power P will be to the refift- 

 ance R as the radius c b is to the fpace I m of the 

 arch embraced by the cord. 



If inftead of drawing the cord upwards it is necef- 

 fary to draw it downwards, a fixed pulley (fig. 4.) is 

 placed above the moveable pulley m, which makes no 

 change in the effect of the power. And when the 

 power is not fufficient to elevate the burden, a fecond 

 moveable pulley is added, and another fixed one 

 (fig. 5.) or even a greater number, by means of which 

 the power has much greater effect. This fyftem of 

 pullies, fome moveable and fome fixed, and all em- 

 braced by the fame cord, is called by fome a tackle. 

 The fixed pullies i and 4 are fupported in the fame 

 neck or cafe, and the moveable pullies i and 3 by 

 another neck. The lower part M of the neck, which 

 fupports the fixed pullies, ferves as a fixed point for 

 one end of the cord, and it is the lower part R of the 

 neck which fupports the moveable pullies to which 

 the burden is hung. 



- By means of this union of pullies, a very great 

 burden may be raifed by a fmall force; for ic is de- 

 monftrable that the force neceffary to fuftain a weight 

 by means of feveral pullies, is to the weight itfelf as 

 unity is to double the number of moveable pullies. 

 When the directions of the cords are parallel to each 

 VOL. I. F other, 



