332 ON THE MOVEMENTS OF THE EARTh's CRUST. 



of the earth's axial rotation. They state that there are various forces 

 which may be efficient in altering- it,— some make the sidereal day 

 shorter, others make it longer. The latter are preponderant, and among 

 them the tidal wave plays the greatest part, so that for this reason in 

 the course of time the sidereal day becomes always longer and longer. 

 Refrigeration is the most powerful force which contributes towards the 

 shortening of the sidereal day, but its action is calculated by Thomson 

 ( Trans. Geol. Soc. Glasgow, I. e. p. 28) at only one six-thousandths of the 

 tidal wave ; and this last action cannot be annulled by any of the other 

 forces, which act sometimes in one, sometimes in another, direction 

 (transport of material from higher to lower latitudes, or vice versa, ac- 

 cumulation of ice at the poles, etc.), and which in course of time cease 

 to act, the tidal wave acting always for millions of years in the same 

 direction (Thomson, "Geological Dynamics," Trans. Geol. Soc. Glasgow, 

 1869, vol. Ill, part 2, p. 2230 . 



In this way, therefore, the sidereal day must in course of time always 

 become longer and longer. Now what influence has this upon the 

 earth? If this were fluid throughout, it is clear that it must at once 

 change its form. According as the sidereal day became longer and the 

 centrifugal force diminished, its compression must have decreased. 

 But the old theory of a fiery fluid interior is now rejected by physicists, 

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

 Now,will this solid body retain its form without reference to the length 

 of tlie 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 earth's present form agrees, at all events in some de- 

 gree, with the length of the sidereal day. It has at present a coin- 

 l)ression which about agrees with that which it should have from cal- 

 culation with its present axial rotation. As it may now be rendered 

 l^robable that the earth, since it acquired a solid surface, has lost so 

 much of its axial roration that the sidereal day has become several 

 times longer, the circumstance that the compression suits that agreeing 

 with the axial rotation seems to show that the solid earth has really 

 changed its form. Jupiter and Saturn have a sidereal day respectively 

 of 9^ 55'" and 10'' 15'", and a compression of one-seventeenth and 

 one-tenth. In Mars, the sidereal day of which is about 24'' 37'", ob- 

 servations have not been able to prove definitely any compression. 

 There would seem, therefore, to be a connection 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 coin- 

 cides with the axial rotation. 



J. Croll ("Climate and Time," 1875, p. 335; see also Amer. Jour. Set., 



