KINETIC THEORIES OF GRAVITATION. 269 



a medium," it is not easy to perceive why it should so indubitably be 

 "of the nature of pressure" rather than of tension. Is the phenomenon 

 of the terrestrial tidal wave rendered any more intelligible by referring 

 it to the action of a "pressure" from beneath, than by referring it, as 

 the "most eminent physicists" do, to the action of a lunar tension from 

 above ? 



Rejecting a pressure-medium as " purely hypothetical," the author yet 

 thinks a pressure-force in gravity " is beyond all doubt"! not perceiving 

 that the latter conception is as " purely hypothetical" as the former. A 

 speculation born of metaphysical imaginings as to the " possible," framed 

 in no physical relations to associate it with any known action, supported 

 by no fact of observation, result of experiment, or sound induction, 

 based on assumptions directly at variance with all ascertained laws or 

 conditions of gravitative action, — if suck a speculation be not "purely 

 hypothetical," in what propulsive undulations, corpuscular chaos, or 

 aetherial vortex, shall we seek to find a fitting subject for the appellation 1 ? 

 And yet hypothetical as the speculation pre-eminently and undoubtedly 

 is, — baseless, formless, insubstantial, — it comes to us with the prestige of 

 a distinguished physicist as its propounder, and of a learned association 

 as its audience and recipient. 



Leray. 1869. 



Within the last twenty years, probably more than a dozen " original " 

 discoveries of the cause of gravitation have been announced to the 

 French Academy of Sciences. Two brief essays of the same year may 

 here be noticed. A Note by P. Leray, entitled a " New Theory of Grav- 

 itation," was presented to the Academy through M. Faye September 6, 

 1869. 



" This new theory rests on the assumption of an aether — a fluid ex- 

 ceedingly rare and perfectly elastic — and on the two following principles: 

 First, that in the free aether (that is, undisturbed by the presence of 

 other bodies) there exist at every point equal currents crossing each 

 "other in all directions ; second, that in passing through bodies, the 

 currents of the aether are retarded proportionally to the thickness trav. 

 ersed, and to the mean density of the path. It may be added, that the 

 currents thus enfeebled, on passing again into the aether, recover but 

 slowly their former force, and may be considered approximately — within 

 the limits of our solar system — as preserving a constant value." This 

 gravific fluid evidently does not differ essentially from that of Lesage. 

 These simple hypotheses, says the writer, " conduct in effect to the same 

 results as the law of universal attraction, without requiring any action 

 at a distance j and give moreover the key to many phenomena which 

 this law does not explain." 



Considering first, the case of an isolated body, it is evident that the 

 currents, being equal in all directions, neutralize each other, effecting 

 therefore no change of position in the isolated body ; although by the 



