GRADUAL PROPAGATION OF ATTRACTION. 355 



tion from the moon to the ocean. So feeble a velocity 

 was inconsistent with the mechanical explanation of at- 

 traction of which we have just spoken. The explana- 

 tion, in effect, necessarily supposes that the proper 

 motions of the celestial bodies are insensible compared 

 with the motion of the gravitative fluid. 



After having discovered that the diminution of the 

 eccentricity of the terrestrial orbit is the real cause of 

 the observed acceleration of the motion of the moon, 

 Laplace, on his part, endeavoured to ascertain whether 

 this mysterious acceleration did not depend on the 

 gradual propagation of attraction. 



The result of calculation was at first favourable to the 

 plausibility of the hypothesis. It showed that the gradual 

 propagation of the attractive force would introduce into 

 the movement of our satellite a perturbation proportional 

 to the square of the time which elapsed from the com- 

 mencement of any epoch ; that in order to represent 

 numerically the results of astronomical observations it 

 would not be necessary to assign a feeble velocity to 



t 



attraction ; that a propagation eight millions of times 

 more rapid than that of light would satisfy all the phe- 

 nomena. 



Although the true cause of the acceleration of the 

 moon is now well known, the ingenious calculation of 

 which I have just spoken does not the less on that ac- 

 count maintain its place in science. In a mathematical 

 point of view, the perturbation depending on the gradual 

 propagation of the attractive force which this calculation 

 indicates has a certain existence. The connexion be- 

 tween the velocity of perturbation and the resulting in- 

 equality is such that one of the two quantities leads to a 

 knowledge of the numerical value of the other. Now, 



