KINETIC THEORIES OF GRAVITATION. 247 



whole suggestion is however, so indefinite that it must be accounted less 

 a coherent hypothesis than a mere speculation, — a cast among the possi- 

 bilities. To refer the great fact of gravitation to some unimagined and 

 unimaginable aether-motion, the special arrangements of which for effect- 

 ing the desired purpose " elude research," is not to proffer an explana- 

 tion, but to indulge in au illU'Sion ; and although Mr. Waterston has in 

 terms recognized all of the six propositions (excepting the last one) an- 

 nounced as the necessary conditions of the problem,* he has failed to 

 shjw that one of these conditions can be satisfied by his speculations. 



Challis. 1859. 



Professor James Challis, of the University of Cambridge, England, 

 in the prosecution of a " MatLjeniatical Theory of Heat," published in 

 the Philosophical Magazine for March, 1859, advanced in November of 

 the same year, to a " Mathematical Theory of Attractive Forces," based 

 on the assumption " that all substances consist of minute spherical atoms 

 of different but constant magnitudes, and of the same intrinsic inertia; 

 and that thedynamital relations and movements of different substances, 

 and of their constituent atoms, are determined by the pressures of the 

 cether against the surfaces of the atoms, togetiier with the reaction of 

 the atoms against such pressure by reason of the constancy of their 

 form and magnitudes. The aither is assumed to be a uniform elastic 

 fluid medium pervading all space not occupied by atoms, and varying 

 in pressure proportionally to variations of its density. The theory recog- 

 nizes no other kinds of force than these two, the one an active force resi- 

 dent in the ijether, and the other a passive reaction of the atoms." 



After a formidable array of partial diff'erential equations, the author 

 concludes: "Havingnow shown that wavesoflargebreadthattraciasmall 

 spherical body toward their origin, and having previously shown that 

 waves of small breadth may repel such a body in the contrary- direction, 

 the main difiiculty in forming a theory of attractive and repulsive forces 

 seems to be overcome."! It is su{)posed by Professor Challis that by 

 the disturbance of a material element, a series of undulations differing 

 greatly in their order of magnitude and velocity maybe simultaneously 

 propagated in the setherial medium, giving rise to as many different 

 manifestations of force ; and that according to their relative wave- 

 length, some of these will produce a permanent motion of translation 

 on molecules of determinate mass subjected to their influence. This 

 is i)artly in accord with the striking experiments of Guyot previously 

 referred to. 



In a following paper the author undertook ''A Theory of the Force 

 of Gravity;" remarking that, "As we have no conception from personal 

 experience and sensation of any other species of force tliAn pressure, the 

 actio in distans does not admit of being explained by any ])revious or 



* Loco dial., p. 335. 



t L. E, D. Phil. Mag., November, 1S5[», vol. xviii, p. 334. 



