Mechanic Powers. 249 



than what is sufficient to constitute a balance be- 

 tween the weight and the power. 



Some philosophers have considered the wheel 

 and axle, and the system of pulleys, as only mo- 

 difications of the lever; and the wedge and the 

 screw as (modifications of the inclined plane. 

 If this be admitted, we shall then have, instead 

 of six, only two mechanical powers. The mo- 

 difications and combinations of these are, how- 

 ever, almost endless, and wonders are performed, 

 when to these means of increasing force are 

 added the most powerful agents in nature, wind, 

 water, and steam, as exemplified in the wind- 

 mill, the water-mill, and, above all, the steam- 

 engine. If the simple and obvious principles I 

 have here elucidated shall assist the student in ' 

 estimating the advantage of the more common 

 machines, and stimulate him to pursue his re- 

 searches into the manner of operation of the 

 more complex engines to which I have just ad- 

 verted, these explications will not have been 

 given in vain. 



M 4 



