INERTIA, GRAVITY, AND CONSERVATION. 369 



be due to the force exerted, and, if the conservation principle 

 be true, must have consumed an equivalent proportion of the 

 cause of attraction ; and yet, according to the definition of 

 gravity, the attractive force is not diminished thereby, but 

 increased four-fold, the force growing up within itself the 

 more rapidly, the more it is occupied in producing other force. 

 On the other hand, if mechanical force from without be used 

 to separate the particles to twice their distance, this force is 

 not stored up in momentum or by inertia, but disappears ; and 

 three-fourths of the attractive force at the first distance disap- 

 pears with it. How can this be ? 



We know not the physical condition or action from which 

 inertia results ; but inertia is always a pure case of the con- 

 servation of force. It has a strict relation to gravity, as ap- 

 pears by the proportionate amount of the force which gravity 

 can communicate to the inert body ; but it appears to have 

 the same strict relation to other forces acting at a distance as 

 those of magnetism or electricity, when they are so applied 

 by the tangential balance as to act independent of the gravi- 

 tating force. It has the like strict relation to force communi- 

 cated by impact, pull, or in any other way. It enables a 

 body to take up and conserve a given amount of force until 

 that force is transferred to other bodies, or changed into an 

 equivalent of some other form ; that is all that we perceive 

 in it ; and we cannot find a more striking instance amongst 

 natural, or possible phenomena, of the necessity of the con- 

 servation of force as a law of nature ; or one more in con- 

 trast with the assumed rariable condition of the gravitating 

 force supposed to reside in the particles of matter 



Even gravity itself furnishes the strictest proof of the con- 

 servation of force in this, that its power is unchangeable for 

 the same distance ; and is by that in striking contrast with 

 the variation which we assume in regard to the cause of grav- 

 ity, to account for the results at different distances. 



It will not be imagined for a moment that I am opposed 

 16* 



