tractive force which this calculation indicates has
a certain existence. The connexion between the velocity of perturbation
and the resulting inequality is such that one of the two quantities
leads to a knowledge of the numerical value of the other. Now, upon
assigning to the inequality the greatest value which is consistent with
the observations after they have been corrected for the effect due to
the variation of the eccentricity of the terrestrial orbit, we find the
velocity of the attractive force to be fifty millions of times the
velocity of light!

If it be borne in mind, that this number is an inferior limit, and that
the velocity of the rays of light amounts to 77,000 leagues (192,000
English miles) per second, the philosophers who profess to explain the
force of attraction by the impulsive energy of a fluid, will see what
prodigious velocities they must satisfy.

The reader cannot fail again to remark the sagacity with which Laplace
singled out the phenomena which were best adapted for throwing light
upon the most obscure points of celestial physics; nor the success with
which he explored their various parts, and deduced from them numerical
conclusions in presence of which the mind remains confounded.

The author of the _Mecanique Celeste_ supposed, like Newton, that light
consists of material molecules of excessive tenuity and endued in empty
space with a velocity of 77,000 leagues in a second. However, it is
right to warn those who would be inclined to avail themselves of this
imposing authority, that the principal argument of Laplace, in favour of
the system of emission, consisted in the advantage which it afforded of
submitting every question to a process of simple and rigorous
calculation; whereas, on the other hand, the theory of undulations has
always offered immense difficulties to analysts. It was natural that a
geometer who had so elegantly connected the laws of simple refraction
which light undergoes in its passage through the atmosphere, and the
laws of double refraction which it is subject to in the course of its
passage through certain crystals, with the action of attractive and
repulsive forces, should not have abandoned this route, before he
recognized the impossibility of arriving by the same path, at plausible
explanations of the phenomena of diffraction and polarization. In other
respects, the care which Laplace always employed, in pursuing his
researches, as far as possible, to their numer