250 KINETIC THEORIES OF GRAVITATION. 



bj the theory of tbe renctions of autecedent /orces, tlie converse prob- 

 lem, to explain forces by the hypothesis of antecedent waves, is by no 

 means so simple. Accordingly, a very weak point in all undulatory 

 theories of gravitation has been an account of the origin (to say nothing 

 of the conMnuance) of the primum mohile. "On the source and main- 

 tenance of the sun's heat," Professor Challis remarks as follows: "We 

 arc led to the couelusiou that the undulations which emanate equally 

 in all directions from each atom, and constitute by their dynamic action 

 the re[)ulsion of heat, are ujainly produced by the reaction of the atoms 

 due to their inertia and impenetrability. It is plain however, from 

 this reasoning, that there must be an original and independent source 

 of undulations. Now obviously such a supply may be conceived to be 

 furnislied to the sun by undulations emanating from the stars. We have 

 ocular evidence that stars transmit light-undulations, and it is quite 

 I)ossible that they originate others not sensible to the sight."* As our 

 sun is one of the stellar frateri:ity, surely this device of mutual borrow- 

 ing is an extraordinary method of production. 



" That action at a distance is not a universal condition of force is 

 proved by the modern discovery that light and heat, which are modes 

 of force, are transmitted through space by the intervention of a me- 

 dium. If one kind of force requires a medium of transmission, why not 

 another ? Again, it is found In- ex[)erieuce that the same portion of 

 matter may attract or repel, according to circumstances. But inherent 

 force cannot possibly be so changed by circumstances. In the same 

 matter it must continue to be always the same."t 



"All physical force being pressure, there must be a medium by which 

 the pressure is executed." And the origin of this pressure is therefore, 

 the origin of "all physical force." Tlie fundamental postulate of this 

 medium is that it is a uniformly continuous elastic fluid, exerting a 

 pressure always proportional to its density. As it is contrary to prin- 

 ciple to ascribe elasticity to atomic matter, the question might arise, 

 why is it more pro[)er to ascribe this occult property to the wther ? In 

 the case of air (trf uniform temperature) the constant ratio of pressure 

 to density results we are told, from a vis a tcrgo, the vibratory action 

 of the interstitial wther. The author perceives the incongruity of deny- 

 ing to the air a quality attributed to the «ther, when the law is implic- 

 itly the same in both ; and he suggests in extenuation, " we can con- 

 ceive of the existence of another order of aether having the same relaticrn 

 to the first as that has to air, and so on ad libitum.'"X 



The very key-note of the hypothesis is dynamic letherial " pressure." 

 "All the different kinds of physical force detected by observation and 

 experiment are modifications o^ iwessurts of the £ether."§ But when we 



* Phil. Mao-., June, 1803, vol. xsv, p. 4e5. 

 tPbil. Mas-, October, lti(i:5, vol. xxvi, p. 284. 

 t Phil. Mug., .Jnue, 18(W, vol. xxxi, pp. 4U8, 469. 

 ^ Ibidem, p. 470. 



