$4 ?be movable Pulley. [Book I, 



refiftance I, in order to keep the equilibrium. Hence 

 it follows, that the fimplt pulley neither aids nor hin- 

 ders the power; it only ferves, as has already been 

 obfervcd, to ke^p the power in its mod advantage - 

 ',:.;' ion, to change the direction of the motion, 



li r it conftant. 



'I ;,< y may aifo'Jbe confidered as a lever of 



c , for it has all the properties of that 

 machin- when the refiftance R (fig. 3.) is attached to 

 the neck c i, and one of the ends of the cord which 

 paries under the pulley is attached to the fixed point 

 a> v;hile the cthf r is drawn or fuftained by the power 

 d: The pulley then becomes what is called a mc-je- 

 able pu^ky, and is elevated with the burden ; it con- 

 fequently is analagous to a lever of the fecond order 

 I e, of wnich the fulcrum or prop is at b, and is di- 

 vided into two equal parts b c, c e } by the direction 

 c I of the refiftance R ; it is on this account only 

 neceffary that the power d fhonld pcfTcfs half the force 

 of the refiftance ,R to keep it in equilibrium ; and 

 if the burden is elevated, the power d acts through 

 nvice the fpacc of that of^the refiftance R, and confe- 

 quently with double the velocity : For fuppofe the 

 center c of the pulley is carried to the point />, then 

 there only remains under the line d a the portion of 

 the cord which pafTes under the pulley ; the tv/o por- 

 tions b a and e d have then pafird^ above ; but b a 

 and e d> which mark the fpacc run through by the 

 power, are, taken together, double to c h, the fpace 

 run through by the pulley ; then the power has a 

 velocity double to that of the refiftance. In this cafe 

 the cord embraces half the circumference of the pul- 

 ley, and the directions of the two powers are parallel. 

 The arm of the lever, of power is then the diameter 

 e b of the pulley, that of the refiftance is only the ra- 



