MECHANICS. 107 



SECT. IV. 



i 



OF MECHANICAL AGENTS. 



177. A- FUNDAMENTAL distinction among me- 

 chanical agents, or the powers which put bodies in 

 motion, consists in this, that in some the intensity 

 of their action, or the acceleration they produce in 

 a given time, is the same whether the hody acted on 

 be at rest or in motion; in others, it is greatest 

 when the body acted on is at rest, and becomes less 

 as its velocity increases. 



Gravity is the only force which is certainly known to 

 act with equal intensity on bodies in motion and 

 at rest. Magnetism has probably the same proper- 

 ty. Every other power acts more forcibly on a body 

 at rest than on one that has already acquired motion 

 in the direction in which it acts. This happens with 

 respect to the elasticity of springs, the impulse of 

 fluids, and the strength of animals, the only powers, 

 except gravity, which are employed to put machines 

 in motion. The knowledge of the laws, according to 

 which the action of these powers diminishes, as the 

 motion they communicate increases, is of great im- 

 portance in mechanics. 



178, The 



