KINETIC THEORIES OF GRAVITATION 265 



forms of energy. And this constancy is all that is signified by the oft- 

 quoted but not always justly apprehended " conservation of force." So 

 far from there being any fixed relation between gravitative force and the 

 conversion of motion, the ratio varies in every planet; and while the 

 height to which a pound of gunpowder would project a ball upward 

 would difler widely in different planets, the velocity of projection and 

 the returning energy of the fall would be precisely the same in all- 

 Were we to rigorously' deny that gravitation is energy, or that energy 

 is force, we could not correctly affirm the conservation of "force." The 

 thing truly conserved would be energy, and this is undoubtedly the 

 more accurate and less misleading form of expression. 



Mr. Croll says in concluding his essay : " In the case of the loaded 

 piston rising under the pressure of the steam, we have the pressure of the 

 steam and length of space both diminishing as the vis viva or mechan- 

 ical work increases. This is in harmony with the principle of conserva- 

 tion, for pressure or force diminishes as energy or work increases. But 

 in the case of gravitation matters are reversed, for the force increases^ 

 along with the work. As the weight descends and performs work, the 

 pressure of the weight, the thing which performs the work, increases 

 also; and when the weight is rising and energy diminishing, the force 

 or pressure of the weight is not increasing but actually diminishing 

 also." A very sufficient demonstration that steam and gravity are not 

 correlated ; that they are not both " forces " in the same sense of the 

 term. Similarly a bar-magnet employed in educing magnetism in- 

 another steel bar, so far from losing what the other gains, has its own 

 magnetism re-enforced by the operation. Vires acquirit eundo. 



" This difficulty," says Mr. Croll, " along with all the others which we- 

 have been consideripg, will entirely vanish if we adopt the view of gravity 

 which has been ably advocated by Faraday, Waterston, and other physi- 

 cists, viz, that it is a force pervading space external to bodies, and that on 

 their mutual approach this force is notincreased,asis generally supposed ;. 

 the bodies merely pass into a place where the force exists with greater 

 intensity ; for in such a case, the intensity of the force in the space 

 external to any body is inversely as the square of the distance from 

 the center of convergence of these lines of force. As the stone recedes 

 from the earth its vis viva is transferred to space, and exists there as 

 gravity. When the stone approaches to the earth, the force existing in 

 space is transferred back to the body, and appears again as vis viva.''''*' 



Here then is an hypothesis which based on an a priori sentiment of 

 fitness rather than on any direct induction, can be submitted to the 

 test of observation. As a fact of observation, gravitation is always 

 found to be a property of gross tangible matter, with its tensions math- 

 ematically directed to the mass-centers, and in quantity always directly 

 proportioned to the number of material molecules, agreeably to the first 

 and second propositions. As a fact of observation, the moon may pas»- 



" Pliil. ^hi^., loco cilat.. pp. 456,457. 



