ON THE movp:ments of the earth's crust. 333 



1876, ser. 3, vol, xii, p. 457) thinks that the sidereal day lengtheDs so 

 slowly that deuadation will have time to adjust the form of the earth 

 so as to coincide with the length of the sidereal day. Just as the sea 

 sinks under low latitudes, the continents in the same latitudes will also 

 become lower by denudation, but under higher latitudes the rising sea 

 will protect the land instead of denuding it; and in this way the earth 

 must then, by denudation alone, acquire a form always suitable to its 

 axial rotation. But this is evidently erroneous. Imagine the earth 

 formed of ellipsoidal layers with increasing solidity inwards. When the 

 centrifugal force diminished, equilibrium would be disturbed throughout 

 the whole mass, and in the interior tension would constantly increase. 

 Nay, not even at the surface can denudation alter the compression. 

 For we know from the recent investigations of the deep sea that in 

 this deep sea, far from the continents, no products of weathering are 

 present; only volcanic ashes and cosmical dust are deposited. Tlius 

 denudation is not even capable of obliterating the inequalities of the 

 surface, still less the internal tension i)roduced by the lengthening of 

 the sidereal day. And as the day has become considerably longer, the 

 sea ought to be collected towards the poles and the land under the 

 equator, in case tbe solid earth had not changed its form. 



Others thiuk that the earth may actually change its form. The first 

 who expressed this opinion, so far as I can find, is Herbert Sj)encer. In 

 the Philosophical Magazine (1847, vol. xxx, p. 194) he published a 

 small memoir, entitled '' The Form of the Earth no proof of original 

 Fluidity," in which he maintains that even the solid earth may cha'.jge 

 its form, according as the centrifugal force changes. When a body in- 

 creases in size, the power ot resistance to external forces increases only 

 as the square of the dimensions, while the wasting and destructive 

 forces (weight, centrifugal force) increase in the same proportion as the 

 mass of the body, and therefore as the cube of the dimensions. As the 

 size increases we therefore come to a point at which even the most solid 

 body must yield to the forces. We must therefore assume, says Spencer, 

 that the earth, by reason of its size, must yield and change its form, in 

 case the centrifugal force, for example, changes ; for the most solid 

 matter known to us, exposed to the same forces which act upon the 

 earth, would overstep the bounds of solidity before attaining a tliousand- 

 )niiliontli part of the earth's size. This argument, in Professor Schi^tz's 

 opinion, is not tenable. At any rate, I believe that Spencer is the first 

 Avho expressed the opinion that ev'en a solid earth can change its form. 

 In the above-cited discourse of 1800,* Peirce says that the lengthening 

 of the sidereal day may be sui)posed to have altered the form of the 

 solid earth. And Principal Dawson, in his '' Story of the Earth and 

 Man" (ed. 0, 1887, p. 291), says that this alteration of form by reason 

 of the lengthening of the sidereal day must have taken place at longer 

 or shorter intervals. So long as the crust of the earth did not yield, 



* See ante, p. 329, foot-note. 



