DISCOVERIES IN THE CELESTIAL SPACES. 735 



two-fold origin of the masses by fusion and solidification, or 

 by precipitation from the waters. The typical character and 

 mmcralogical differences of rocks, or, in, other words, the 

 associations of certain mostly crystallised substances recurring 

 in the most remote regions, are as little made a subject of 

 consideration in the Protogcea as in Hooke's geognostic views. 

 Even in the last-named writer, physical speculations on 

 the action of subterranean forces in earthquakes, in the 

 sudden upheaval of the sea's bottom and of littoral districts, and 

 in the origin of islands and mountains, hold a prominent 

 place. The nature of the organic remains of a former world 

 even led him to conjecture that the temperate zone must 

 originally have enjoyed the heat of a tropical climate. 



It still remains for us to speak of the greatest of all 

 geognostic phenomena, the mathematical figure of the earth, 

 in which we distinctly trace a reflection of the primitive 

 world in the condition of fluidity of the rotating mass, and 

 its solidification into our terrestrial spheroid. The main out- 

 lines of the figure of the earth were sketched as early as the. 

 close of the seventeenth century, although the relation be- 

 tween the polar and equatorial axes was not ascertained with 

 numerical exactness. Picard's measurement of a degree, 

 made in 1670 with instruments which he had himself im- 

 proved, is so much the more important, since it was the 

 means of inducing Newton to resume with renewed zeal his 

 theory of gravitation, (which he discovered as early as 1666, 

 but had subsequently neglected,) by offering to that profound 

 and successful investigator, the means of proving how the 

 attraction of the earth maintained the moon in its orbit, 

 whilst urged on its course by the centrifugal force. The fact 

 of the compression of the poles of Jupiter, which was much 

 earlier recognised,* had, as it is supposed, induced Newton 

 to reflect on the causes of a form which deviated so con- 

 siderably from sphericity. The experiments on the actual 

 length of the seconds pendulum by Richer at Cayenne in 

 1673, and by Varin on the western coast of Africa, had 

 been preceded by others of less decisive character, prose- 

 cuted in London, Lyons, and Bologna, at a difference of 7 of 

 latitude.f 



* Cosmos, p. 156. 



t Delambre ; Hist, de I'Astronomie mod. t. ii, p. 601 



