II 



OF THE MECHANICAL POWERS. 67 



dicular height D is equal to half its 

 length A B, the cylinder will roll 

 down upon the plane with a force 

 equal to half its weight; for it would 

 require a power (acting in the direction 



of A B) equal to half its weight, to _J 



keep it from rolling. If the plane A B 

 be elevated, so as to be perpendicular to the 

 horizon., the cylinder C will descend with its 

 whole force of gravity, because the plane con- 

 tributes nothing to its support or hindrance ; 

 and therefore, it would require a power equal to 

 its whole weight to keep it from descending. 



Let the cylinder C be made to turn upon 

 slender pivots in the frame D, in which there 

 is a hook e, with a 

 line G tied to it ; let 

 this line go over the 

 fixed pulley H, and 

 have its other end 

 tied to the hook in 

 the weight /. If the 

 weight of the body 

 /, be to the weight 

 of the cylinder C, 



added to that of its frame D, as the perpendicular heigl.t 

 of the plane L Mis to its length A B, the weight v;'l 

 just support the cylinder upon the plane, and a small 

 touch of a finger will either cause it to ascend or de- 

 scend with equal ease : then, if a little addition be made 

 to the weight /, it will descend, and draw the cylinder 

 up the plane. In the time that the cylinder moves from 

 A to B, it will rise through the whole height of the plane 

 M L ; and the weight will descend from Hto K, through 

 a space equal to the whole length of the plane A B. 



F 2 



