136 LAPLACE. 



beeu deinoustrated. The demoustratiou of the Newtonian conception 

 of tlie precession of the equinoxes was, then, a great discovery, and it 

 is to D'Aleinbert that the glory of it is due.* The illustrious geometer 

 gave a complete explanation of the general movement in virtue of 

 wbich the terrestrial axis returns to the same stars in a period of about 

 20,01)0 years. He also connected with the theory of gravitation the 

 perturbation of precession discovered by Bradley, that remarkable 

 oscillation which the earth's axis experiences continually during its 

 movement of progression, and the period of which, amounting to about 

 eighteen years, is exactly equal to the time which the intersection of the 

 moon's orbit with the ecliptic employs in describing the 360° of the 

 entire circumference. 



Geometers and astronomers are justly occupied as much with the 

 figure and physical constitution which the earth might have had in 

 remote ages as with its present figure and constitution. 



As soon as our countryman Richer discovered that a body, whatever 

 be its nature, weighs less when it is transported nearer the equatorial 

 regions, everybody perceived that the earth, if it was originally fiuid, 

 ought to bulge out at the equator. Huyghens and Newton did more ; 

 they calculated the difference between the greatest and least axes, the 

 excess of the equatorial diameter over the line of the poles.t 



The calculation of Huyghens was founded upon hypothetic properties 

 of the attractive force which were wholly inadmissible ; that of Newton 

 upon a theorem which he ought to have demonstrated. The theory of 



* It must be admitted that M. Ara^o has here imperfectly represented Newton's labors 

 on the great problem of the precession of the equinoxes. The immortal author of the 

 Principia did not merely conjecture that the conical motion of the earth's axis is due to the 

 disturbing action of the sun and moon upon the matter accumulated around the earth's 

 equator; he demonstrated by a very beautiful and satisfactory process that the movement 

 must necessarily arise from that cause ; and although the means of investigation, in his 

 time, were inadequate to a rigorous computation of the quantitative effect, still his researches 

 on the subject have been always regarded as affording one of the most striking proofs of 

 sagacity which is to be found in all his works. — Translator. 



t It would appear that Hooko had conjectured that the figure of tlie earth might be spher- 

 oidal before Newton or IIu3'gliens turned their attention to the subject. At a meeting of the 

 Koyal Societj' on the 2>*th of February, lC7d, a discussion arose respecting the figure of Mer- 

 cury, which M. Gallet, of Avignon, had remarked to be oval on tlie occasion of the planet's 

 transit across the sim's disk on theTtu of November, 1G77. Hooke was inclined to suppose 

 that the phenomenon was real, and that it was due to the whirling of the planet on an axis 

 " which made it somewhat of the shape of a turnip, or of a solid made by an ellipsis turned 

 round upon its shorter diameter." At the meeting of the society on the 7th of March, the 

 subject was again discussed. In reply to the objection offered to his hypothesis on the ground 

 of the planet being a solid body, Ilooke remarked that " although it might now be solid, yet 

 that at tlio beginning it might have been fluid enough to receive that shape ; and that al- 

 tiiough this supposition should not be granted, it would be probable enough that it would 

 really run into that shape and make the same appearance ; and that it is not improbable but 

 that the water here upon the earth might do it in some measure by the influence of the diur- 

 nal motion, which, compounded with that of the moon, he conceived to be the cause of the 

 tides." (Journal Book of the Royal Society, vol. vi, p. GO.) Richer returned from Cayenne 



