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 an illusion; and although Mr. Waterstou 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 

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



Challis. 1859. 



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

 iu the prosecution of a " Mathematical Theory of Heat," published in 

 the Philosophical Magazine for March, 1859, advauced 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 the dynamical relations and movements of different substances, 

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

 rather against the surfaces of the atoms, together with the reaction of 

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

 form and magnitudes. The rather 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 kiuds of force thau these two, the one an active force resi- 

 dent iu the rather, aud the other a passive reaction of the atoms." 



After a formidable array of partial differential equations, the author 

 concludes : " Having now shown that waves of large breadth attract a small 

 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 difficulty in forming a theory of attractive aud repulsive forces 

 seems to be overcome." t It is supposed by Professor Challis that by 

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

 greatly iu their order of magnitude and velocity maybe simultaneously 

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

 manifestations of force; aud 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 partly in accord with the strikiug 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 thau pressure, the 

 actio in distans does not admit of being explaiued by any previous or 



* Loco citat., p. 335. 



t L. E. D. Phil. Mag., November, 1659, vol. xviii, p. 334. 



