THE CONNECTION AND EQUIVALENCE 

 OF FORCES. 



IT is well known that our machines create no power, but 

 only return what they have received. The motion of a 

 clock is produced by a weight or a spring ; but it is the power 

 of the human arm applied to stretch the spring or elevate the 

 weight, which is expended in the movement of the wheels and 

 pendulum in twenty-four hours, or in eight or fourteen days. 



A water-wheel sets in motion, in a mill, one or more mill- 

 stones ; in a foundry, one or more hammers ; in saltworks 

 or mines it pumps or raises weights to certain heights ; in 

 factories, it communicates movements to looms, spinning ma- 

 chines, and rollers. In all these instances, the work per- 

 formed by the water-wheel is due to the force exerted by the 

 falling water on the buckets, which sets the wheel in motion ; 

 and this force must be greater than the resistance presented 

 by the different machines in operation. The performance of 

 the machine is measurable by this force. 



The work of a steam-engine is executed by the movement 

 of a piston upwards and downwards by the pressure of steam, 

 just as a water-wheel is moved by the pressure of water. 

 The cause of this pressure is heat, which is derived from the 

 chemical process of combustion, and is absorbed by water. 

 By this heat, steam is produced, and the necessary expansion 



