414 A. Blytt on the probable Cause of 



and Thomson assumes that the earth is on the whole a solid 

 body. Now will this solid body retain its form without refer- 

 ence to the length of the sidereal day, or will it yield and 

 accommodate itself? The sea, as a matter of course, will at 

 once yield, and, as the centrifugal force decreases, it will sink 

 under lower and rise under higher latitudes. We know that 

 the eartVs present form agrees, at all events in some degree, 

 with the length of the sidereal day. It has at present a com- 

 l^ression M'hich about agrees with that which it should have 

 from calculation, with its present axial rotation. As it may 

 now be rendered probable that the earth, since it acquired a 

 solid surface, has lost so much of its axial rotation that the 

 sidereal day has become several times longer, the circumstance 

 that the compression suits that agreeing with the axial rota- 

 tion seems to show that the solid earth has really changed its 

 form. Jupiter and Saturn have a sidereal day respectively 

 of 9 h. 55 m. and 10 h. 15 m., and a compression of -^ and 

 j^. In Mars, the sidereal day of which is about 24 h. 37 m., 

 observations have not been able to prove definitely any com- 

 pression. There would seem, therefore, to be a connexion 

 between compression and axial rotation. But it may indeed 

 be objected that Jupiter and Saturn are still possibly melted 

 masses. 



W. Thomson and Tait seem to be of opinion that the 

 earth will not change its form. They assume that it must 

 have become solid not so many millions of years since, seeing 

 that the compression nearly coincides with the axial rotation. 



J. Croll (' Climate and Time,^ 1875, p. 335 ; see also Amer. 

 Journ. Sci. ser. 3, vol. xii. 1876, p. 457) thinks that the 

 sidereal day lengthens so slowly that denudation 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 

 Avay the earth must then, by denudation alone, acquire a form 

 always suitable to its axial rotation. But this is evidently 

 eri'oneous. Imagine the earth formed of ellipsoidal layers 

 wdth 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 investiga- 

 tions of the deep sea, that in this deep sea, fiir from the con- 

 tinents, no products of weathering are present : only volcanic 

 ashes and cosmical dust are deposited. Thus denudation is 



