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 haying 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 
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, 
