ESTIMATING THE rO\YEIl OF LEVEKS. 49 



kind. The joint of the bono is the fulcrum; the strong 

 muscle or tendon attached to the bone near the joint is 

 the power ; and the weight of the limb, with whatever re- 

 sistance it overcomes, is the weight. A great advantage 

 results from this contrivance, because a slight contraction 

 of the muscle gives a swift motion to the limb, so import- 

 ant in walking and running, and in the use of the arms. 



ESTniATIXG THE POWEK OF LEVEES. 



The power of any lever is easily calculated by measuring 



the length of its two arms, that is, the two parts into 



Fig. 4G. which it is divided by 



-t^-- the weight, fulcrum. 



*' r" '" ;^^ " " ' ^y^JT-' ' ^'^"^"'^^ and power. In a 

 f ^ ^^,::^'^''^"' '^N ^ lever of the first kind, 



--.-''''' if the weight and 



Lexer of the first icind. p^^,^^ 1^^ equally dis- 



tant from the fulcrum, they will move through equal dis- 

 tances, and nothing will be gained ; that is, a power of 

 100 pounds will lift a weight of 100 pounds only. If the 

 power be twice as far as the weight, its force will be 

 doubled ; if three times, it will be tripled ; and so forth. 

 In a lever of the second kind, if the weight be equidistant 

 between the fulcrum Fig. 47. 



and power, the power ^p"~--rjrr-_ 

 will move through | "< ~~~-~~---c--_ 



twice the distance of 1 1* i^ 



the Aveight, and the ^ 



power of the instru- Lever of the second Unci. 



ment will therefore be doubled ; if twice as far, it will be 

 tripled, and so on, as shown in the annexed figures. The 

 same mode of reasoning will explain precisely to what 

 extent the force is diminished in levers of the third kind. 



These rules will show in what manner a load borne on 

 a pole is to be placed between two persons carrying it, 



