220 KINETIC THEORIES OF GRAVITATION. 



ber of such corpuscles which must thus be more or less '' spent" in ful- 

 filling their appointed function, it follows that the total activity of bom- 

 bardment on matter cannot be as vigorous now as it was a million years 

 ago, and must be still less vigorous a million years hence ; all which is 

 contrary to the unchangeable continuity of gravity affirmed by our sixth 

 condition. 



As has been well remarked by an able anonymous writer in the North 

 British Review, "The attraction of gravitation is not as the surface of 

 the bodies, but as their mass. Lesage had therefore to suppose his solid 

 bodies not solid, but excessively porous, built up of molecules like cages, 

 so that an infinite number of atoms went through and through then), 

 allowing the last layer of the sun or earth to be struck by just as many 

 atoms as the first, otherwise clearly the back part of the sun and earth 

 would gravitate more strongly than the front or nearer sides, which 

 would be struck only by the siftings of the previous layers of matter. 

 This notion involves a prodigious quantity of material in the shape of 

 flying atoms, where we j^erceive no gross matter, but very little mate- 

 rial in solid bodies, where we do find gross matter ; and it further re- 

 quires that the accumulation of atoms which strike the solid bodies 

 perpetually should be insensible."* 



Not only does the " gravific fluid " utterly fail to give an approximate 

 representation of the actual conditions of the planetary movements, but 

 as must be evident, it will not permit the continued existence of any 

 such movements. A mass moving in free space in any direction except- 

 ing directly toward a similar mass, must receive a more active shower 

 of corpuscles in its front than in its rear, and must thus be retarded by 

 a differential of energy directly proportioned to its velocity. Every 

 planet must accordingly encounter a tangential resistance to its orbital 

 motion, proportional to its own gravitation and to its velocity. 



As illustrative of the different estimates of this hypothesis formed by 

 distinguished men, the following citations may be permitted. M. Pierre 

 Prevost, professor or philosophy and general physics in the University 

 of Geneva, published two years after the death of Lesage, an account 

 of his writings, in which, after a sketch of his corpuscular hypothesis, 

 he remarks, "I pause at the foot of this mgjestic edifice with a senti- 

 ment of hope ; persuaded that the labors of the founder will not be 

 suffered to perish, and that men of genius will share with me the ad- 

 miration it has inspired."t And Professor Tait regards it as "the only 

 plausible answer to this [great problem] which has yet been pro- 

 pounded."! Sir John Herschel, on the other hand, has remarked, " The 

 hypothesis of Lesage which assumes that every point of space is pene- 

 trated at every instant of time by material particles sui generis, moving 

 in right lines in every possible direction, and impinging upon the ma- 



* North British Review, March, 1868, vol. xlviii, p. 126 of American edition. 

 \ Xoticc de la Vie et des iScrits de G.-L. Le Sage, published at Geneva in lr;05. 

 t Lectures on Physical Science, loco citat,, p. 291). 



