AIM AND PEOGRESS OF PHYSICAL SCIENCE. 377 



Machines, and the motive powers required for their 

 movement, furnish, in fact, the most familiar illustra- 

 tions of the uniformity of all natural forces expressed by 

 the law of the conservation of force. Any machine which 

 is to be set in motion requires a mechanical motive 

 power. Whence this power is derived or what its form, 

 is of no consequence, provided only it be sufficiently 

 great and act continuously. At one time we employ a 

 steam-engine, at another a water-wheel or turbine, here 

 horses or oxen at a whim, there a windmill, or if but 

 little power is required, the human arm, a raised weight, 

 or an electro-magnetic engine. The choice of the machine 

 is merely dependent on the amount of power we would 

 use, or the force of circumstance. In the watermill the 

 weight of the water flowing down the hills is the agent ; 

 it is lifted to the hills by a meteorological process, and 

 becomes the source of motive power for the mill. In the 

 windmill it is the vis viva of the moving air which 

 drives round the sails ; this motion also is due to a 

 meteorological operation of the atmosphere. In the steam- 

 engine we have the tension of the heated vapour which 

 drives the piston to and fro ; this is engendered by the 

 heat arising from the combustion of the coal in the fire- 

 box, in other words, by a chemical process ; and in this 

 case the latter action is the source of the motive power. 

 If it be a horse or the human arm which is at work, we 

 have the muscles stimulated through the nerves, directly 

 producing the mechanical force. In order, however, that 

 the living body may generate muscular power it must be 

 nourished and breathe. The food it takes separates again 

 from it, after having combined with the oxygen inhaled 

 from the air, to form carbonic acid and water. Here 

 again, then, a chemical process is an essential element to 

 maintain muscular power. A similar state of things is ob- 

 served in the electro-magnetic machines of our telegraphs. 



