J 58 LAPLACE. 



sequently reinvented and perfected by Lesage. These ideas, in effect, 

 had been communicated to him before they were published to the world. 



Accoiding to Lesage, there are iu the regions of space bodies moving 

 in every possible direction, and with excessive rapidity. The author 

 ap])licd to these the name of ultramundane corpuscules. Their totality 

 constituted the gravitative fluid, if indeed the designation of a fluid be 

 applicable to an assemblage of particles having no mutual connection. 



A single body placed in the midst of such an ocean of movable par- 

 ticles would remain at rest although it were impelled equally iu every 

 direction. On the other hand, two bodies ought to advance toward each 

 other, since they would serve the purpose of mutual screens, since the 

 surfaces facing each other would no longer be hit in tiie direction of their 

 line of junction by the ultra-mundane particles, since there would then 

 exist currents, the effect of which would no longer be neutralized by 

 opposite currents. It will be easily seen, besides, that two bodies 

 ])lunged into the gravitative fluid would tend to approach each other 

 with an intensity which woukl vary in the inverse proportiou of the 

 square of the distance. 



If attraction is the result of the impulse of a fluid, its action ought 

 to employ a finite time in traversing the immense spaces which separate 

 the celestial bodies. If the sun, then, were suddenly extinguished, the 

 earth, alter the catastrophe, would, mathematically speaking, still con- 

 tinue for some time to experience its attractive influence. The contrary 

 would happen on the occasion of the sudden birth of a planet : a cer- 

 tain time would elapse before the attractive force of th« uew 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 veloc- 

 ity of propagation. Daniel Bernoulli, for example, iu attempting to 

 explain how the spring-tide arrives upon our coasts a day and a half 

 after the syzygies, that is to say, a day and a half after the epochs when 

 the sun and moon are most favorably 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 ex- 

 planaVion of attraction, of which we have just spoken. The exi)lanation, 

 iQ 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 orl)it is the real cause of the observed acceleration of the 

 motion of the moon, Laplace, on his part, endeavored to ascertain 

 whether this mysterious acceleration did not depend on the gradual 

 propagation of attraction. 



The result of calculation was at first favorable to the plausibility of 

 the hypothesis. It showed that the gradual propagation of the attract- 



