1844] WRITINGS OF JOSEPH HENRY. 221 



more restricted sense than force, which is applied as a more 

 general term, to whatever tends to produce or resist motion. 

 The following list of mechanical powers he believed would 

 be found to include all the prime movers employed at the 

 present time, either directly or indirectly, in producing 

 mechanical changes in matter, and all these could be referred 

 to two sources: 



! Water power "j 

 Tide power j- Referable to celestial disturbance. 

 Wind power J 



f Steam and other powers "J Referable to that which 

 developed by combustion. |- is called vital or or- 

 Animal power. J ganic action. 



These natural motive principles are not always directly 

 employed in producing work, but are sometimes used to 

 develop other power by disturbing the natural equilibrium 

 of other forces, and in this way they give rise to a class of 

 mechanical motors which may be called intermediate powers. 

 It will be evident on a little reflection that the forces of 

 gravity, cohesion and chemical attraction, with those of the 

 " imponderable " agents of nature, so far as they belong to the 

 earth, all tend to produce a state of stable or permanent 

 equilibrium at the surface of our planet — that in all cases 

 before the energies of these forces can be exhibited, the dis- 

 turbing effect of some extraneous force is required, hence 

 these principles in themselves are not the primary sources of 

 power, but are merely secondary agents in producing 

 mechanical effects, or in other words it will be found that 

 while the approximate source of every power is the force 

 exerted by matter in its passage from an unstable to a stable 

 state of equilibrium, yet in all cases it may be referred beyond 

 this to a force which disturbed a previously existing quies- 

 cence. As an example we may take the case of water power, 

 in which the mechanical effects are approximately due to the 

 return of the water to a state of stable equilibrium on the 

 surface of the ocean, but the cause of the continued motion 

 is the force which produced the original disturbance, and 

 which elevates the liquid in the form of vapor. Also in the 



