OF THE MECHANICAL POWERS. 59 



traction may be as much nearer to the stronger horse LECT 

 than to the weaker, as the strength of the former ex- ^-v^, 

 ceeds that of the latter. 



To this kind of lever may be reduced oars, rudders 

 of ships, doors turning upon hinges, cutting knives, 

 which are fixed at the point of the blade, and the like, 



If in this lever we suppose the power and weight to The third 

 change places, so that the power may be between the 

 weight and the prop, it will become a lever of the third 

 kind : in which, that there may be a balance between the 

 power and the weight, the intensity of the power must 

 exceed the intensity of the weight, just as much as the 

 distance of the weight from the prop exceeds the dis- 

 tance of the power from it. Thus, let E be the prop of 

 the lever AB s-\r> 



prop as the power P acts at F, by the cord C going 

 over the fixed pulley D ; in this case, the power must 

 be equal to three pounds, in order to support the 

 weight. 



To this sort of lever are generally referred the bones 

 of a man's arm : for when we lift 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 center 

 round which the lower part of the arm turns, the muscle 

 must therefore exert a force ten times as great as the 

 weight that is raised. 



As this kind of lever is a disadvantage to the moving 



Note 33. The actual loss of power is greater than what oar Author 

 has stated, and yet this apparent disadvantage is abundantly compen- 

 sated for by the greater compactness and usefulness of the limb 



