KINETIC THEORIES OF GRAVITATION. 263 



tact, aiul there is no reason, apart from specific experience, to regard the 

 one as in any respect less probable than the other." * 



In our profound ignorance of all beyond the ascertained facts of grav- 

 itation, there could be no objection whatever to the substitution of the 

 word "tend" for the word "attract," did it adequately express the 

 observed fact that A induces in B a " tendency" to approach ; the quan- 

 tity of tendency in B being found to be proportional not merely to its 

 own mass, but notably to the mass of the distant body A. 



Mr. Croll proceeds : " The attraction theory is also in opposition to 

 the principle of the conservation of force, as has been shown, I think 

 clearly, by Faraday. When a stone for example is thrown upward 

 from the earth, it not only loses all its motion, but it loses its attraction 

 in proportion to the square of its distance from the center of the earth. 

 What becomes of the motion imparted to the stone ? It is not trans- 

 formed into attraction, for the attraction diminishes as well as the motion. 

 When the stone again falls to the earth it gains both motion and attrac- 

 tion. In the former case, the attraction is said to consume the motion, 

 and instead of becoming stronger becomes weaker in consequence ; and 

 in the latter case it imparts this same motion, and yet after imparting 

 the motion, it is actually found not only not to have lost but to have 

 gained force thereby. Faraday justly asks what becomes of the force or 

 motion imparted to the stone ! It is not converted into attraction, for 

 the attraction becomes less instead of greater in consequence. And in 

 the case of the falling stone, where does the motion come from ? If the 

 motion arises from the attraction of the earth, then there must be a 

 -certain amount of this attractive force converted into motion; and if so, 

 the attractive force should be so far reduced; but instead of this it is 

 actually increased. There is therefore no account given of what be- 

 comes of the motion externally imparted to the stone when thrown 

 upward, or whence the increase both of attraction and motion is derived 

 as it descends. If the attraction theory be correct, then there is a des- 

 truction of force in the one case and a creation of force in the other ; and 

 if so, then the conservation of force is violated." t 



Although this is merely a more prolix statement of the objection 

 urged by Faraday, it may be again specifically answered. It will be 

 found on careful examination, that the whole difficulty really proceeds 

 from an hypothesis tacitly adopted by the writer, while ostensibly 

 opposing " certain hypothetical elements in the theory." From the nec- 

 essary limitations of language, we have constantly to make words do 

 double or multiple duty in carrying different ideas; and to many this is 

 a source of cou.staut confusion and miscouceptiou. The tacit assump- 

 tion underlying this supposed violation of the conserv^ation of " force " 

 is that the force of gravity is the same in kind as the force of the fall- 



* Examiuatiou of Sir William Haaiilton's Philosophy, by John Stnart Mill, chap, 

 .xxiv, vol. ii, p. 245, of Am. editiou. 



t L. E. D. Phil. Mag., 1837, vol. xxxiv, p. 4.51. 



