OF THE MECHANICAL POWERS. 55 



as the distance of the power from the prop is to the dis- LECT. 

 tance of the weight from the prop, the power andv^x^^ 

 weight will exactly balance or counterpoise each other : 

 and as a common lever has next to no friction on its 

 prop, a very little additional power will be sufficient 

 to raise the weight. 



There are four kinds of levers. 1. The common 

 sort, where the prop is placed between the weight and 

 the power ; but much nearer to the weight than to the 

 power. 2. When the prop is at one end of the lever, 

 the power at the other, and the weight between them. 

 3. When the prop is at one end, the weight at the 

 other, and the power applied between them. 4. The 

 bended lever, which differs only in form from the first 

 sort, but not in property. Those of the first and 

 second kind are often used in mechanical engines; 

 but there are few instances in which the third sort is 

 used. 



A common balance is by some reckoned a lever of theThe 6a- 

 first kind; but as both its ends are at equal distances e * 

 from its center of motion, they move with equal veloci- 

 ties ; and therefore, as it gives no mechanical advan- 

 tage, it cannot properly be reckoned among the me- 

 chanical powers. B 



A lever of The first 



the first kind ^_ _^_^ ^ , , f , f , %^j of 

 is represent- 

 ed by the bar 

 ABC, sup- 

 ported by the 



Note 32. The opposite arms of a balance are sometimes made un- 

 equal for fraudulent purposes, the axis being placed nearer the sub- 

 stance to be weighed, than to the weight, which will thus obtain a 

 decided mechanical advantage. In that case, however, the fraud 

 may be readily detected by changing the contents of the two scales ; 

 so that the mechanical advantage which was in the first instance given 

 to the scale intended to contain the goods about to be weighed, 

 will be transferred to the opposite end of the lever, and the dispro- 

 portion will then become apparent. 



