74 OUTLINES OF NATURAL PHILOSOPHY. 



lever is made to turn another ; and then an equili- 

 brium takes place, when the weight is to the power 

 as the product of all the arms, taken alternately, 

 beginning with that to which the power is applied, 

 to the product of all the other arms. 



e. The common apparatus for raising a draw-bridge, is 

 a combination of a lever of the first kind, with a 

 lever of the second ; the two levers are parallel, and 

 remain in equilibria in all situations. 



f. In the construction of the animal body, this me- 

 chanical power is much employed. The bone is the 

 lever, the muscle is the power, or rather the me- 

 dium through which the power acts, and the joint 

 is the fulcrum. The insertion of the muscle, or the 

 point to which the power is applied, is usually 

 nearer the joint or fulcrum than the weight to be 

 raised is. A different structure might have produ- 

 ced more strength, but would have diminished the 

 activity of the animal, and the velocity of its mo- 

 tion. 



Of the Balance. 



133. The balance is a lever with equal arms, 

 used for comparing weights with one another, and, 

 when well constructed, must have the following 

 properties : 1. It should rest in a horizontal posi- 

 tion, when loaded with equal weights. 2. It should 

 have great sensibility, that is, the addition of a 

 small weight in either scale should disturb the 

 equilibrium, and make the beam incline sensibly 

 from the horizontal position. 3. It should have 



great 



