MECHANIC POWERS. 29 



Let A C B (fig. 10.) represent a lever of this 

 sort, bended at C, which is its prop, or centre of 

 motion. P is a power acting upon the longer arm 

 A C, at A, by the means of the cord D A going 

 over the pulley D ; and W is a weight or resist- 

 ance acting upon the end B of the shorter arm 

 C B. If the power be to the weight as C B is to 

 C A, they are in equilibrio : thus, suppose W to 

 be five pounds, acting at the distance of one foot 

 from the centre of motion C, and P to be one 

 pound, acting at A, five feet from the centre C, 

 the power and weight will just balance each other. 



Thus we see, that in every species of lever there 

 will be an equilibrium, when the power is to the 

 weight as the distance of the weight from the ful- 

 crum is to the distance of the power from the 

 fulcrum. 



In making experiments with models of the 

 mechanic powers, some difficulties arise from the 

 weight of the materials ; but as it is impossible 

 to find any that are without weight, care must be 

 taken that they are perfectly balanced, before the 

 weights and powers are applied. Thus the bar, 

 used in making experiments on levers, has the 

 short end so much thicker than the long arm, as 

 will be sufficient to balance it on the prop. 



If the weight to be raised be of considerable 

 bulk, and if it be fixed either above or below the 

 end of the lever, it will vary in its intensity, ac- 

 cording to the position of the lever. Let A B 

 (fig. 11.) represent a lever having a weight fixed 

 above it, as A, of which the centre of gravity is a, 

 and the line of direction a b ; then is b the point 

 in the lever on which the weights acts : but if the 

 lever be moved into the position C D, the line of 

 direction of the weight will fall nearer to the ful- 



