THE BALANCE. 117 



weights of the bodies to be compared are in equilibrium 

 or not when suspended at opposite ends of a straight 

 lever. Since by art. 13 two equal forces applied in 

 this manner cannot be in equilibrium unless the arms 



FIG. 86 (an. proj. real size). 



of the lever are equal, the first condition which a 



balance ought to fulfil is that both arms of the 4 beam ' are 



precisely equal. But, farther, a balance ought to be 



sensitive, that is, a very small difference between the 



weights should cause a perceptible inclination of the 



beam. To secure this it is necessary in the first place 



that the beam should turn readily. In order, therefore, 



to diminish friction, the beam is supported by the lowest 



3dge of a triangular steel bar s (fig. 86), which passes 



r hrough it at the middle. This edge is made sharp like 



i knife, and rests upon supports made of a very hard 



wbstance, like agate or steel, p (fig. 86), the surfaces 



jf which are either concave or, in the best balances, 



