316 LAPLACE. 



ceived that the earth, if it was originally fluid, 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.* 



The calculation of Huyghens was founded upon hypo- 



* It would appear that Hooke had conjectured that the figure of 

 the earth might be spheroidal before Xewton or Huyghens turned 

 their attention to the subject. At a meeting of the Royal Society on 

 the 28th of February, 1678, a discussion arose respecting the figure of 

 Mercury which M. Gallet of Avignon had remarked to be oval on the 

 occasion of the planet's transit across the sun's disk on the 7th of 

 November, 1077. 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, Hooke remarked that 

 " although it might now be solid, yet that at the beginning it might 

 have been fluid enough to receive that shape ; and that although 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 hereupon the earth might do 

 it in some measure by the influence of the diurnal motion, which, com- 

 pounded with that of the moon, he conceived to be the cause of the Tides.' 1 '' 

 (Journal Book of the Royal Society, vol. vi. p. 60.) Richer returned 

 from Cayenne in the year 1674, but the account of his observations 

 with the pendulum during his residence there, was not published 

 until 1679, nor is there to be found any allusion to them during the 

 intermediate interval, either in the volumes of the Academy of Sci- 

 ences or any other publication. We have no means of ascertaining 

 how Xewton was first induced to suppose that the figure of the earth 

 is spheroidal, but we know, upon his own authority, that %s early as 

 the year 1667, or 1668, he was led to consider the effects of the cen- 

 trifugal force in diminishing the weight of bodies at the equator. 

 With respect to Huyghens, he appears to have formed a conjecture 

 respecting the spheroidal figure of the earth independently of Newton ; 

 but his method for computing the ellipticity is founded upon that given 

 in the Princip.ia. Translator. 



