124 THE MECHANICAL FUNCTIONS. 



5. Muscular Power. 



IN Machines contrived by human skill the chief 

 art consists in devising expedients for regulating 

 and directing the given moving power, so that 

 it may bear, in the proper degree, and in the 

 proper order, upon some particular objects, and 

 produce some particular effect. The whole of 

 the apparatus employed with this intention, how- 

 ever numerous may be its parts, however various 

 the forms of its wheels, its levers, or its pulleys, 

 and however complicated may be their con- 

 nexions, resolves itself into a series of interme- 

 diate instruments for the transferrence of motion 

 from the source of power, or the point where its 

 action is impressed, to the parts which are de- 

 signed ultimately to receive the action of the 

 force employed. It is an established principle 

 in physics, that mere machinery is incapable of 

 generating mechanical force; and that such 

 force must always be originally derived from 

 some extraneous source. Some impulse from 

 without, whether it be the pressure of the wind, 

 the fall of a stream of water, or the action of 

 men or horses, or any other kind of foreign 

 agency, must be resorted to, both to set the 

 engine in motion, and to continue its move- 

 ments when they are once begun. Nor is the 



