350 COSMOS. 



es by fusion and solidification, or by precipitation from the 

 waters. The typical character and mineralogical differences 

 of rocks, or, in other words, the associations of certain mostly 

 crystallized 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 organ- 

 ic 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 geog- 

 nostic 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 solid- 

 ification into our terrestrial spheroid. The main outlines of 

 the figure of the Earth were sketched as early as the close of 

 the seventeenth century, although the relation between 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 improved, is so much 

 the more important, since it was the means of inducing New- 

 ton 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 inves- 

 tigator the means of proving how the attraction of the Earth 

 maintained the Moon in its orbit, while urged on its course 

 by the centrifugal force. The fact of the compression of the 

 poles of Jupiter, which was much earlier recognized,* had, as 

 it is supposed, induced Newton to reflect on the causes of a 

 form* which deviated so considerably 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, prosecuted in London, Lyons, and Bologna at a 

 difference of 7° of latitude.! 



The decrease of gravity from the poles to the equator, which 

 even Picard had long denied, was now generally admitted. 

 Newton recognized the polar compression^ and the spheroidal 

 form of the earth as a consequence of its rotation ; and ho 



* Cosmos, vol. i., p. 164. 



t Delambre. Hist, de VAstronomie Mod., t. ii., p. 60 J 



