LAPLACE. 159 



ive force would introdnce into the movement of onr satellite a pertur- 

 bation proportional to the square of the time which elapsed from the 

 coinmenct'meut 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 oji that account maintain its place in science. In a mathe- 

 matical point of view, the perturbation depending on the gradual propa. 

 gatiou of the attractive force which this calculation indicates has a cer- 

 tain existence. The connection 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 assign- 

 ing 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 i)rofess to explain the 

 force of attraction by the impulsive energy of a fluid, will see what pro- 

 digious velocities they must satisfy. 



The reader cannot fail again to remark the sagacity with which La- 

 place 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 tliem 

 numerical conclusions in presence of which the mind remains con- 

 loundcd. 



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 wlio would be inclined to avail themselves of 

 this imposing authority that the principal argument of Laplace in favor 

 of the system of emission consisted in the advantage which it afforded 

 of snbmitting every question to a process of simple and rigorous calcu- 

 lation ; whereas, on the other hand, tbe 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 phenoaieua 

 of diffraction and polarization. In other respects, the care which Laplace 

 always employed in pursuing his researches, as far as possible, to their 



