198 Baron Fourier's Historical Eloge of the 



any sensible resistance to the motions of the planets ; for this 

 cause would particularly affect the motion of the moon, where- 

 as it produces no perceptible effect. 



The discussion of the motions of this planet is pregnant 

 with remarkable consequences. We may conclude from it, 

 for example, that the motion of rotation of the earth about 

 its axis is invariable. The length of the day has not varied 

 the 100th part of a second for 2000 years. It is remarkable 

 that an astronomer need not go out of his observatory to 

 measure the distance of the earth from the sun. It would be 

 sufficient to observe carefully the variations of the lunar mo- 

 tions, and from this he would deduce with certainty the dis- 

 tance required. 



A still more striking consequence is that which relates to 

 the figure of the earth ; for the form even of the terrestrial 

 globe is impressed on certain inequalities of the lunar orbit. 

 These inequalities would not have taken place if the earth had 

 been a perfect sphere. We may determine the compression 

 at the poles of the globe by the observation of the lunar mo- 

 tions alone, and the results hence deduced agree with the real 

 measures which have been obtained by the great trigonometrical 

 surveys at the equator, in the northern regions, in India, and 

 in different countries. 



It is to Laplace that we especially owe this astonishing per- 

 fection of modern theories. 



I cannot undertake to recount at present the series of his 

 labours, and the discoveries to which they have led. The 

 simple enumeration of them, however rapid it may be, would 

 exceed the limits which I am obliged to prescribe to myself. 

 Beside these researches on the secular equation of the moon, 

 and the no less important and difficult discovery of the cause 

 of the great inequalities of Jupiter and Saturn, we may men- 

 tion those admirable theorems on the libration of the satellites 

 of Jupiter. To these we may add his analytical inquiries re- 

 specting the tides, — a subject which he has pursued to an im- 

 mense extent. 



There is scarcely a point of physical astronomy of any im- 

 portance that he did not study with the most profound atten- 

 tion ; and he submitted to calculation most of the physical con- 



