DENSITY OF THE EARTH. 25 



pendulum became, in Eicher's comparison of the march of 

 the same astronomical clock at Paris and at Cayenne (in 

 1(572), the first experimental proof of the variation in the 

 force of gravity in different latitudes. Picard had, indeed, 

 been occupied in the preparations for this important expe- 

 dition, but he does not on that account attribute to himself 

 the merit of the first proposition. Richer left Paris in 

 October 1671, and Picard, in the description of his mea- 

 surement of a degree, published in the same year (1671), 

 remarked simply ( 14 ), that " at a meeting of the Academy, 

 one of the members expressed an opinion, that, on account 

 of the Earth's rotation, weights might be found to be less 

 heavy, or have less gravity, at the equator than at the pole." 

 He adds doubtingly, " that, indeed, according to some 

 observations made in London, Lyons, and Bologna, it would 

 seem as it the seconds pendulum required to be shortened 

 in approaching the equator : but that, on the other hand, 

 he is not sufficiently convinced of the accuracy of those 

 measurements, because, at the Hague, the length of the 

 seconds pendulum was found to be quite the same as at 

 Paris, notwithstanding the difference of latitude." "We 

 unfortunately cannot learn when Newton first obtained the 

 knowledge, which was to him so important, of Eicher's 

 pendulum results, obtained in 1672 but first published in 

 a printed form in 1679 ; or of Cassiai's discovery, in 1666, 

 of the ellipticity of Jupiter; or, at least, we cannot learn it 

 with the same exactness and certainty which we possess in 

 regard to his very late knowledge of Picard's arc. At a 

 period when, with so happy an emulation, theoretical views 

 stimulated to the undertaking of observations, and the 

 results of observation reacted in their turn on theory, the 



