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- 

 scure 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 lisrht consists of material molecules of ex- 



J <^-> 



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 ; 

 w r hereas, 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 



