THE LEVER. 65 



1. Many examples occur in practice of levers of the 

 first kind. A crowbar, used to raise stones from the 

 earth, is an instance of this sort ; so is a handspike of 

 any kind used in the same way. A hammer for draw- 

 ing a nail operates as a lever of the first kind, the heel 

 being the fulcrum, the nail the weight, and the hand 

 the power ; the distance through wliich the handle 

 passes being several times greater than that of the 

 claws, the force exerted on the nail is increased in like 

 proportion. A pair of scissors consists of two levers, 

 the rivet being the fulcrum ; and in using them, as ev- 

 ery one has observed, a greater cutting force is exert- 

 ed near the rivets than toward the pomts. This is ow- 

 ing to the power being expended through a greater dis- 

 tance near the points, according to the rule ah-eady 

 explained. Pincers, nippers, and other similar instru- 

 ments are also double levers of the first kind. 



A common steelyard is another example, the shding 

 weight becoming gradually more effective as it is moved 

 further from the fulcrum or hook supporting the instru- 

 ment. The brake or handle of a pump is a lever of 

 this class, the pump-rod and piston being the weight. 

 The common balance is still another, the two arms 

 being exactly equal, so that 

 one weight will always bal- 

 ance the other, and on this 

 its usefulness and accuracy 

 entirely depends. The most 

 sensitive balances have light 

 beams with long arms, and 

 the turning-point of hardened steel or agate, in the 

 form of a tliin wedsre, on which the balance turns al- 



