Chap. 7.] Atticn of the Human Arm. 59 



we elevate a weight with the hand, this weight may be 

 conficlered as fixed to the arm of a lever, whofe prop 

 is at the elbow, and whofe length is confequently equal 

 to the diftance between the elbow and the weight. 

 But this weight is fuftained in this ftate by the action 

 of the mufcles, the direction of which is very oblique 

 to the arm of the lever, and confequently the diftance 

 of the moving power from the prop is much lefs than 

 the diftance of the weight from the fame prop. Hence 

 the effort of the mufcles muft in this cafe be much 

 greater than the weight or refiftance. To account for 

 this ftructure, it muft be remarked, that the nearer a 

 power applied to a lever is to the prop, fo much the 

 lefs is the fpace it a6ts in when it raifes the weight. 

 Now the fpace which the power had to occupy was 

 what Providence had moft to regard in the ftructure 

 of our bodies. It is on this account that he has placed 

 the direction of the mufcles at a fmall diftance from 

 the prop, but he has wifely made them ftrong in pro- 

 portion. 



The prop of a lever may be regarded as a third 

 power, which keeps in equilibrio the motive force and 

 the refiftance, or which concurs with the one to ena- 

 ble it to fuftain the effort of the other. 



In a lever of the firft order, the prop C (fig. 10.) 

 which is placed between the power D and the refift- 

 ance E, fupports a force equal to the abfolute weight 

 or effort of the two forces, when thefe forces are ap- 

 plied in a direction parallel to each other, and the 

 force exerted upon the prop C 3 is in the direction 

 C I parallel to that of the two forces. But if the 

 power I Q_(fig. ii.) and the refiftance K N are in a 

 direction which inclines them towards each other, the 

 prop L is charged with lefs than the fum total of the 

 two forces, and lefs in proportion as this inclination is 

 greater, and the force which is exerted upon the 



prop 



