AIM AND PROGRESS OF PHYSICAL SCIENCE. 331 



Machines, and the motive powers required for their move- 

 ment, furnish, in fact, the most familiar illustrations 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 depen- 

 dent 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 pis- 

 ton 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 throvigh 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 observed in the electro-magnetic 

 machines of our telegraphs. 



Thus, then, we obtain mechanical motive force from the 

 most varied processes of nature in the most different ways ; but 

 it will also be remarked in only a limited quantity. In doing 

 so we always consume something that nature supplies to us. In 

 the watermill we use a quantity of water collected at an eleva- 



