SQ MECHANIC POWERS. 



which rests on it, must be raised also ; but one- 

 fifth of the power applied at the point L of the 

 lever C, and pressing it downwards, will as effec- 

 tually raise the point S of the same lever, as if the 

 whole power were applied at S, and pushed up the 

 same ; consequently a power equal to one-fifth of 

 one-twentieth, that is, one-hundredth part of the 

 weight P, being applied to the point L of the 

 lever C, will balance the weight at the point S of 

 the lever A. 



This method of combining levers is frequently 

 used in machines and instruments, and is of great 

 service, either in obtaining a greater power, or in 

 applying it with more convenience. 



The balance, an instrument of very extensive 

 use in comparing the weights of bodies, is a lever 

 of the first kind, whose arms are of equal length. 

 The points from which the weights are suspended, 

 being equally distant from the centre of motion, 

 will move with equal velocity ; consequently, if 

 equal weights be applied, their momenta will be 

 equal, and the balance will remain in equilibrio. 



In order to have a balance as perfect as possible, 

 it is necessary to attend to the following circum- 

 stances : 



1. The arms of the beam ought to be exactly 

 equal, both as to weight and length. 



2. The points from which the scales are sus- 

 pended should be in a right line passing through 

 the centre of gravity of the beam ; for by this, the 

 weights will act directly against each other, and 

 no part of either will be lost on account of any 

 oblique direction. 



3. If the fulcrum, or point upon which the beam 

 turns, be placed in the centre of gravity of the 

 beam, and if the fulcrunvantUtbe points of suspen- 



