356 \ LAPLACE. 
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 eccen- 
tricity 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 ob- 
secure 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 Mécanique Céleste supposed, like 
Newton, that light consists of material molecules of ex- 
cessive tenuity and endued in empty space with a velo- 
city 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 con- 
nected the laws of simple refraction which light under- 
goes in its passage through the atmosphere, and the laws 
