68 MECHANICAL POWERS. 



one end, the power at the other end, and the weight to 

 be raised between the two. This is exemplified by a man 

 drawing a plug ; he thrusts the end of the handspike 

 through the ring, and resting it on the ground, he ele- 

 vates the end he holds, and thus draws the plug. The 

 advantage gained is in the same proportion as in the 

 former lever. Cutting-knives used by patten-makers, 

 doors, oars of a boat, the rudder of a vessel, &c., are 

 levers of this kind. A pair of bellows is a compound 

 lever of the second kind. This lever shows how two 

 men carrying a weight on a pole, as brewers' men do a 

 cask, or chairmen a sedan-chair, may bear an unequal 

 proportion of the weight. If the weight be exactly in 

 the middle of the two, each will support an equal share of 

 the burden ; but if it be nearer the one than the other, 

 he to whom it is nearest will have the heaviest weight, 

 and that in inverse proportion to his distance from it, 

 as compared with that of the other man. 



The third kind of lever is when the power is between 

 the fulcrum and the weight. A ladder raised by a man 

 against a wall is a lerer of this kind, and so is the human 

 arm, and the limbs of animals generally. In this kind 

 of lever there is a loss of power in proportion to the dis- 

 tance between the power and the fulcrum, compared with 

 the distance between the power and the weight. The 

 muscles of the human arm for this reason exert a force 

 of one hundred pounds to raise a weight of ten pounds, 

 the distance from the acting muscles to the elbow being 

 about one-tenth of the distance from the muscles to the 

 hand. The disadvantage is here made up by the com- 

 pactness and convenience of the motion, the muscles 

 being sufficiently powerful to produce the effect required. 



The Wheel and Axle consist of a wheel fixed to the 

 end of an axle, so that they both turn round together. 

 This power is sometimes named the perpetual lever, it 

 being in reality a lever on whose arms the power and 

 weight may always act perpendicularly, although the 

 lever turns round its fulcrum. Cranes are of this nature; 

 church-bells are moved by the wheel and axle, as also 

 are the helms of ships. The advantage gained is as the 

 diameter or circumference of the wheel is to the diameter 



