14 A. BLYTT. [NO. 1. 



sidereal day became longer, and the centrifugal force dimi- 

 nished, its oblateness would necessarily become less. But, now, 

 the old doctrine of an internal igneous mass is rejected by phy- 

 sicists, and Thomson supposes that the Earth is a solid body 

 throughout. Now, would this solid body retain its form in spite 

 of the lengthening of the day, or would it yield and accom* 

 modate itself to the change? The sea would of course imme- 

 diately do so, and according as the centrifugal force dimi- 

 nished, it would sink in lower and rise in higher latitudes. We 

 know that the present form of the Earth corresponds, at any 

 rate somewhat closely, to the length of the day. At present 

 its flattening is just about what is due to its present rotation, 

 as worked out by calculation. If, now, it can be shown pro- 

 bable that the Earth, since it obtained a solid crust, has lost 

 so much of its rotation that the day has become several 

 hours longer, then the circumstance that the oblateness corre- 

 sponds to that due to the rotation, seems to show that the 

 solid Earth has really altered its form. Jupiter and Saturn 

 have sidereal days of, respectively, 9 h. 55 m. and 10 h. 

 15 m., and an oblateness of respectively fa and fa. In Mars, 

 whose day is about 24 h. 37 m., observations have not been 

 able to show, conclusively, any flattening. There therefore 

 appears to be a connection between oblateness and rotation. 

 But it may be objected that perhaps Jupiter and Saturn are 

 still melted masses. 



W. Thomson, and Tail even more, appear to be of the 

 opinion, that the Earth would not alter its form. They sup- 

 pose that it must have become solid not many millions of years 

 ago, as the flattening corresponds somewhat closely to the ro- 

 tation. 



J. Groll (Climate and Time. London 1875 p. 335, vide 

 also Am. Journ. of Science Ser. III. Vol. VII p. 457) thinks 

 that the day became lengthened so slowly, that denudation 

 would have time to act sufficiently on the Earth's form, that 

 it would come to correspond to the length of the day. As 

 the seas sink in the lower latitudes, the shores of the continents 



