16^ GENERAL FORM AND STRUCTURE OF THE EARTH. 



rotation. Now, Jupiter, which turns on its axis in ten hours, is much 

 more oblate than the earth. The flattening of the earth is only about 

 3^-g- of its diameter, while that of Jupiter is about -^- s . 3. The forms 

 of the earth and of Jupiter have been calculated ; the data of calcula- 

 tion being the former fluidity, the time of rotation, and an assumed 

 rate of increasing density from surface to center ; and the calculated 

 form comes out nearly the same as the measured form. 



The force of this argument, however, has been, to say the least, 

 greatly exaggerated. The oblateness of the earth and planets, as has 

 been shown by Playfair and Herschel,* only proves that they have 

 assumed their form under the influence of rotation — that they are 

 spheroids of rotation — but not that they have ever been in a fluid con- 

 dition. For since a rotating body, whatever be its form, always tends 

 to assume an oblate spheroid form, and since the materials on the sur- 

 face of the earth are in continual motion, being shifted hither and 

 thither under the influence of atmospheric and aqueous agencies, it is 

 evident that the final and total result of such motions must be in the 

 course of infinite ages to bring the earth to the only form of equilib- 

 rium of a rotating body, viz., an oblate spheroid. If, for example, the 

 earth were a rigid sphere, standing still and covered with water, and then 

 set rotating, the waters would gather into an equatorial ocean, and the 

 land be left as polar continents. But this condition would not remain ; 

 for atmospheric and aqueous agencies, if unopposed, would eventually 

 cut down the polar continents and deposit them as sediments in the 

 equatorial seas, and the solid earth would thus become an oblate sphe- 

 roid. This final effect of degrading agencies would not be opposed by 

 igneous agencies, as the action of these is irregular, and does not tend 

 to any particular form of the earth. Yet this applies only to the gen- 

 eral spheroidal form ; for Hennessey has shown f that although the 

 spheroidal form would be assumed either by fluidity or by abrasion, yet 

 the degree of ellipticity of the spheroid would be different, and probably 

 sensibly different in two cases, being greater in the former ; and that 

 the actual form of the earth more nearly approaches this greater degree. 



Therefore, although there are many reasons, drawn both from geol- 

 ogy and from the nebular hypothesis, for believing that the earth was 

 once in an incandescent fluid condition, and that it then assumed an 

 oblate spheroid form in obedience to the laws of equilibrium of fluids ; 

 yet this form alone must not be assumed as demonstrative proof of 

 such original condition, since a similar form would be produced by 

 causes now in operation on the earth-surface, whatever may have been 

 its original form and condition. Moreover, it is evident that the exact 



* Lyell, Principles of Geology, vol. ii, p. 199. 



f Philosophical Magazine, vol. vii, p. 67, 1879, vol. x, p. 119, 1880, and vol. xi, p. 

 283, 1881. 



