THE LEVER. 41 



from the axis of motion. The power and weight are said to 

 balance each other, or to be in equilibrium, when the effort 

 of the one to produce motion in one direction, is equal to 

 the effort of the other to produce it in the opposite direction. 

 The power of a machine is calculated, when it is in a state 

 of equilibrium, that is, when the power just balances the re- 

 sistance opposed, and the momentum of each is equal. 



The lever is any inflexible bar of iron, wood, or other ma- 

 terial, which serves to raise weights, while it is supported at 

 a point by a prop or fulcrum, on which, as the centre of mo- 

 tion, all the other parts turn. There are three different kinds 

 of levers. The Jirst kind has the fulcrum between the 

 weight and the power, as in steelyards and scissors. It is 

 the most common kind, and is chiefly used ror loosening 



rge ijj^ks ; or for raising great weights to small heights, 

 orderto place ropes under them. Let it be required to 



aise a body which weighs ten hundred pounds, by the 

 ^trength of a man equal to a hundred pounds weight. Now 

 asNthe man's strength is only equal to the tenth part of the 

 weight of the body to be raised, the arm of the lever, to 

 which his strength id to be applied, must be ten times as 

 long as the other, in order that the power and weight may 

 be in equilibrium".. A balance is a lever of this kind, with 

 equal arms ; but if one arm be four times the length of the 

 other, then it is a lever which gains power in the proportion 

 of four to one, and a single pound weight, put into the scale 

 which is suspended from the long arm, will balance four 

 poflnds in the other. The second kind of lever is when the 

 prop is at one end, the power at the other, and the weight 

 between them. It explains why two men carrying a burden 

 upon a pole, may bear unequal shares according to their 

 strength, by placing it nearer to the one than the other. He, 

 to whom the burden is five times the nearest, will have to 

 bear five times as much weight as the other. In the case 

 of two horses of unequal strength the beam may be so di- 

 vided, that they shall draw in proportion to their respective 

 ability. The third kind of lever is when the prop is at one 

 end, the weight at the other, and the power applied between 

 them. To this kind are generally referred the bones of a 

 man's arm, for when he lifts a weight by the hand, the mus- 

 cle 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 

 4* 



