KINETIC THEORIES OF GRAVITATION. 213 



If it is possible to represent in such terms the lowest assignable limit of 

 transition, it is because we are furnished with a test of planetary move- 

 ment of most marvelous delicacy in the record of eclipses occurring at 

 a particular locality 2,000 years ago ; — fixing the relation of annual revo- 

 lution to diurnal rotation with an almost absolute precision. Sir John 

 Herscbel remarks: "From such comparisons Laplace has concluded 

 that the sidereal day has not changed by so much as one-hundredth of a 

 second since the time of Hipparchus!"* This implies the absence of 

 even an infinitesimal "aberration" of the gravity radiant, or the negation 

 of any assignable interval for its full and complete action. Hence the 

 fourth category above stated. 



The same consideration serves to show that the energy of gravity has 

 undergone no abatement or change during the lapse of two thousand 

 years. Hence the sixth category. 



It is but just however, to notice here that a minute outstanding 

 anomaly of the moon, detected in recent years, and still unexplained, 

 detracts somewhat from the accuracy of the above infinitesimal measure; 

 though it does not impair the value of the general argument. Every 

 investigation, every calculation, of the astronomer, assumes the actiou 

 of gravity to be for all distances, — absolutely instantaneous. 



Villemot. 1707. 



Philippe Villemot, a French doctor of theology, and a distinguished 

 mathematician, published at Lyons in 1707 an astronomical treatise, 

 entitled Nouveau Systeme, ou XouveUe Explication du Mouvement ties 

 Planetes, in which, referring the movements of the planets to Cartesian 

 vortices, he announced the theory that their gravitation is occasioned 

 by a difference of pressure, on their outer and inner faces, of the fluid 

 constituting the solar vortex, owing to an increase of its density out- 

 ward from the sun. The general conception is obviously somewhat 

 similar to the speculation cursorily hazarded by Xewtou in 1G79, and 

 again recurred to by him (though only transiently) in 1717, or ten years 

 later than the above publication by Villemot. 



The details of this system cannot here be given, from want of access 

 to his work. The Nouveau Systeme, however, appears to have been very 

 favorably received by the author's contemporaries. 



Bernouilli. 1731. 



It is now nearly a century and a half since the elder John Beruouilli, 

 of Switzerland, the illustrious mathematician, (professor at the univer- 

 sities of Groningen and afterward of Basel,) imagiued a method of ac- 

 counting for the action of gravitation by centripetal impacts from with- 

 out. Still retaining his early prepossessions in favor of the philosophy 

 of Descartes, he devised a very curious combination of oetherial vortices 

 and Newtonian emissions. This eclectic hypothesis was promulgated 

 * Outlines of Astronomy, chap, sviii, sec. 908. 



