Historical Eulogy on the Marquis De Laplace. 5 



He had at first examined whether we could explain the accelera- 

 tion of the moon's motion on supposing that the action of gravity is 

 ECt'instantaneous but subject to a successive transmission, like that 

 of I'a'.ht. In this way, he could not discover the true cause. At last, 

 a new research served his genius better. On the 19th of March, 

 1787, he gave to the Academy of Sciences a solution clear, and un- 

 attended with this insurmountable difficulty. He proved very dis- 

 tinctly that the acceleration observed is a necessary effect of univer- 

 sal gravitation. 



This great discovery enlightened then the most important' points 

 of the system of the world. In effect, the same theory made him 

 know that, if the action of gravitation upon the stars is not instanta- 

 neous, we must suppose it propagated more than fifty million times 

 faster than light, the well known velocity of which is seventy thou- 

 sand leagues a second. 



He concluded still, from his theory of the lunar motions, that the 

 medium in which the stars move, opposes only to the course of the 

 planets a resistance, as it were, insensible ; for this cause would 

 above all things act upon the motion of the moon, and it produces 

 upon it no observable effect. 



The discussion of the motions of this star is pregnant with re- 

 markable consequences. We can conclude from it, for example, 

 that the motion of rotation of the earth around its axis is invariable. 

 The length of the day has not changed one hundredth part of a 

 se'cond, for two thousand years. It is worthy of notice, too, that an 

 astronomer would have no need of leaving his observatory in order 

 to measure the distance from the earth to the sun. It would suffice 

 him to observe, constantly, the variations of the lunar motion : from 

 these he could conclude this distance with exactness. 



A consequence still more striking is that which relates to the figure 

 of the earth; for the form itself oi the terrestrial globe is imprinted 

 upon certain inequalities of the course of the moon. These inequal- 

 ities could not exist if the earth were perfectly spherical.. We can 

 determine the amount of the earth's applatisement, (oblateness,) by 

 observing the lunar motions only. The results which we have de- 

 duced from them, agree with the actual measures that have been ob- 

 tained by the great geodesical voyages to the equator, to the North- 

 ern Regions, to India, and to divers other countries. 



It is to Laplace, above all, that we owe this astonishing perfection 

 of modern theories. 



