the celestial bodies. If the sun, then, were suddenly extinguished, the
earth after the catastrophe would, mathematically speaking, still
continue for some time to experience its attractive influence. The
contrary would happen on the occasion of the sudden birth of a planet; a
certain time would elapse before the attractive force of the new body
would make itself felt on the earth.

Several geometers of the last century were of opinion that the force of
attraction is not transmitted instantaneously from one body to another;
they even assigned to it a comparatively inconsiderable velocity of
propagation. Daniel Bernoulli, for example, in attempting to explain how
the spring tide arrives upon our coasts a day and a half after the
sizygees, that is to say, a day and a half after the epochs when the sun
and moon are most favourably situated for the production of this
magnificent phenomenon, assumed that the disturbing force required all
this time (a day and a half) for its propagation from the moon to the
ocean. So feeble a velocity was inconsistent with the mechanical
explanation of attraction of which we have just spoken. The explanation,
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
commencement 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 phenomena.

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 account maintain its place in science. In a
mathematical point of view, the perturbation depending on the gradual
propagation of the at