THE INTERIOR OF THE EARTH. 463 



of verity." This also explains why the duration of the sun's rotation, 

 twenty-five clays, is less than that of the revolution of the various 

 planets. And the triple ring of Saturn seems to be an ocular proof 

 of the original extension of the atmosphere of that planet, and of its 

 successive contractions. So many analogous phenomena certainly ren- 

 der Laplace's cosmogonic hypothesis highly probable. 



A final confirmation of the theory is supplied by spectrum analysis. 

 The study of the spectra of the nebulae has demonstrated that, if many 

 of them are merely agglomerations of stars, others are still gaseous 

 bodies veritable specimens of the primitive chaos, exemplifying per- 

 fectly Kant's, W. Herschel's, and Laplace's conception of the begin- 

 nings of worlds as they left the Creator's hands. Of the nebulae two 

 appear to be composed of a central globe with a ring like Saturn's, 

 and in many others it seems possible to discern the gyratory move- 

 ment by means of which planetary systems are formed. 



Of recent investigations that have served to establish the basis 

 and develop the results of Laplace's theory, we must place in the first 

 rank the valuable researches of M. Edouard Roche, on the form of the 

 heavenly bodies, which the author has recently supplemented by an 

 essay on the constitution and origin of the solar system. M. Roche 

 first demonstrates that by virtue of the particular form of the " free 

 surface " bordering the atmosphere a surface having a projecting 

 ridge at the equator as the nebula contracts a fluid stratum will slide 

 from the poles toward the equator and be thrown off over the equa- 

 torial ridge as through an opening. It is thus that an equatorial zone, 

 independent of the central body, will be formed and become an outer 

 ring. 



But the theory shows that inner rings will be formed from portions 

 of the mobile matter brought toward the equator from the poles, and it 

 is thus that Saturn's two inner rings would be formed, their radius being 

 less than twice that of the planet. The equatorial extent of the plan- 

 et's atmosphere being at present equal to 2, there can not have been a 

 ring thrown off inside this distance. Laplace's theory, not admitting 

 inner rings, accounts only for the formation of the largest of the three. 

 M. Roche also holds that the moon was formed from an inner ring, 

 and that it was developed in the bosom of the earth's atmosphere, 

 which, withdrawing little by little, left its satellite free. 



Every conception that favors Laplace's theory clearly tends to 

 confirm the hypothesis of the earth's original fluidity, but without set- 

 tling the question of the liquidity of the nucleus at the present time. 

 Let us see to what extent this obscure question has been elucidated. 



The equatorial swelling, which changes so slightly the .spherical 

 form of the globe, has nevertheless a very appreciable effect on the 

 globe's rotation on its axis. If the earth were an exact sphere and 

 entirely homogeneous, or if it were composed of homogeneous concen- 

 tric spheres, the sun's attraction would have no effect on the move- 



