MECHANIC POWERS. 43 



top, as in fig. 9, a different proportion between the 

 power and the weight will take place. Here it is 

 evident, that each pulley doubles the power ; thus, 

 if there are two pulleys, the power will sustain four 

 times the weight ; if three pulleys, eight times 

 the weight; if four pulleys, sixteen times; and 

 so on. 



When pulleys in blocks are placed perpendicu- 

 larly under each other, on separate pins, they 

 occupy considerable space, and would not in general 

 answer; it is, therefore, common to place all the 

 pulleys in each block on the same pin, by the side 

 of each other, as in fig. 10; but the advantage 

 and rule for the power are the same here as in 

 %. 7 and 8. 



A pair of blocks with the rope fastened round 

 it, is commonly called a tackle. 



THE INCLINED PLANE. 



This mechanic pow r er is of very great use in 

 rolling up heavy bodies, such as casks, wheelbar- 

 rows, &c. It is formed by placing boards, earth, 

 or other materials in a sloping direction. 



The force wherewith a body descends upon an 

 inclined plane is to the force of its absolute gravity, 

 by which it would descend perpendicularly in free 

 space, as the height of the plane is to its length. 

 For suppose the plane A B (Plate 8. %. 1.) to 

 be parallel to the horizon, the cylinder C will keep 

 at rest on any part of the plane where it is laid. 

 If the plane be placed perpendicularly, as A B, 

 fig. 2, the cylinder C will descend with its whole 

 force of gravity, because the plane contributes 

 nothing to its support or hindrance ; and therefore 

 it would require a power ecrual to its whole weight 

 to keep it from descending. 



