i9i4-] "A KINETIC THEORY OF GRAVITATION." 125 



every month. Perhaps the gravitational disturbance I have sug- 

 gested may aid in explaining some of her more obscure motions; 

 and I hope it will be found to have a slight accelerating tendency so 

 as to compensate the slight retarding tendency of which I shall treat 

 in the second division of this paper. 



2. Transmission of Gravitation Cannot be Instantaneous. 



Laplace at first sought to explain the secular acceleration of the 

 moon's mean motion by ascribing to gravitation a finite velocity of 

 propagation. Later he said :- " The time of its transmission, if it 

 were sensible to us, would be particularly evinced in the acceleration 

 of the moon's motion. I suggested this as a means of explaining the 

 acceleration which is observed in this motion ; and I have found that 

 in order to satisfy observations we must ascribe to the force of grav- 

 ity, a velocity seven millions of times greater than that of a ray of 

 light. As the cause of the secular equation of the moon (c) is now 

 well understood, we may affirm that the attraction is transmitted 

 fifty millions of times more rapidly than light. We can therefore 

 assume, without any apprehension of error, that its transmission is 

 instantaneous." 



1 doubt if anyone who has bestowed careful thought on the sub- 

 ject, in the light of present-day physics, really believes this. To 

 me, it is inconceivable that my change of position, as I walk across 

 a room, is felt among the fixed stars while I am still walking; but 

 the justly great name and fame of Laplace has stamped this dogma 

 with the seal of authority, and for more than a century it has blocked 

 the path of fruitful thought on the physics of gravitation. 



Doubtless Laplace made no serious mathematical mistake in 

 reaching his remarkable conclusion, but perhaps he erred in his 

 choice of premises. He postulated^ a " force " or " gravific fluid," 

 " which rushes towards the sun with an immense rapidity ; the re- 

 sistance which the planet experiences from this current in the direc- 

 tion of the tangent, he conceives to produce a perturbation in the 

 elliptic motion, like to the aberration of light." He then applied this 



2 " System of the World," Harte's translation, Vol. 2, p. 322. 



3 Harte's translation, Vol. 2, notes, p. 490. 



