THE LEVER. 85 



This is a somewhat altered form of enunciating a 

 law of mechanics, usually called the principle of virtual 

 velocities. 



The distances from the fulcrum at which forces are 

 applied are called the arms of the lever. In a straight 

 lever the spaces described are proportional to the length 

 of the arms ; such a lever is therefore in equilibrium 

 if the forces are inversely proportional to the arms of the 

 lever. The lever shown in fig, 57 enables us to make 

 experiments with various lengths of the arms; for 

 distance, 70 gr at 14 cm and 98 gr at 10 cm distance from the 

 middle, &c. 



The applications of the lever are exceedingly 

 numerous. There are three kinds of lever, dis- 

 tinguished from each other by the position of the fulcrum 

 with reference to the power employed to move the lever 

 md the resistance to be overcome by it. In fig. 59, 



FIG. 59 (i real size}. 



B, a crowbar is used in two ways as a lever, to raise 

 r eight. At one end muscular force is applied in the 

 jtion indicated by the arrow. In raising the weight 

 the fulcrum is between the power and the resistance ; 

 ih a lever is called a lever of the first kind ; but in 

 sing the weight J., the other extremity of the lever 

 on the ground, and so becomes the fulcrum about 

 dch the bar is turned ; here the resistance to be over- 

 ie is between the fulcrum and the power ; this is a 



