28 MECHANIC POWERS. 



the power. Thus, let E (fig. 9.) be the prop of 

 the lever E F, and W a weight of one pound, 

 placed three times as far from the prop as the 

 power P acts at F, by the cord going over the 

 fixed pulley D : in this case, the pow r er must be 

 equal to three pounds, in order to support the 

 weight of one pound. 



To this sort of lever are generally referred the 

 bones of a man's arm ; for when he lifts a weight 

 by the hand, the muscle that exerts its force to 

 raise that weight is fixed to the bone about one- 

 tenth part as far below the elbow as the hand is. 

 And the elbow being the centre round which the 

 lower part of the arm turns, the muscle must there- 

 fore exert a force ten times as great as the weight 

 that is raised. 



Since in this kind of lever there is a loss of power, 

 it is used only through convenience ; as in the 

 case of raising a ladder, which being fixed at one 

 one end, is by the strength of a man's arms reared 

 against a wall. 



What is called the hammer-lever differs in no- 

 thing but its form from a lever of the first kind. 

 Its name is derived from its use, that of drawing a 

 nail out of wood by a hammer. 

 . Suppose the shaft of a hammer to be five times 

 as long as the iron part which draws the nail, the 

 lower part resting on the board as a fulcrum ; 

 then, by pulling backwards the end of the shaft, a 

 man will draw a nail with one-fifth part of the 

 power that he must use to pull it out straight with 

 a pair of pincers ; in which case, the nail would 

 move as fast as his hand ; but with the hammer, 

 the hand moves five times as much as the nail, by 

 the time that the nail is drawn out. 



