Rev. E. Hill— On Changes in the Earth's Axis. 263 



the body. The orange would be impaled differently on the knitting 

 needle. The Poles would not be at the spots where they now are. 

 The parallels of latitude would lie on different circles, for latitude 

 depends on distance from the pole of rotation. The light and heat 

 received by all places on the same latitude would be equal, and as 

 much as that latitude nbw receives, but anj' one place would not 

 receive the same as before. The latitudes of places would be changed. 

 There would be as hot a Torrid Zone, but it would occupy a different 

 belt of the surface. There would be as wide an Arctic Circle, but its 

 centre would be at another spot. The changes in climate would not 

 in this case be the same on all sides of the Pole. On the side nearer 

 the new Pole they would be made more arctic, more temperate on 

 the side remote. Had the Pole been thus shifted in a past age, traces 

 should somewhere remain of the then ice-cap. Geologists however 

 say that at present none are known. 



Professor Evans, in his presidential address to the Geological 

 Society, asked mathematicians whether the elevation of a certain 

 belt of land would not carry the Earth's axis of figure 15° or 20° 

 away from its present position, and whether ultimately the axis of 

 rotation would not again coincide with the axis of figure. That is, 

 he supposes the earth deformed, and asks whether this will not 

 cause such a change in the rotation axis as will end in a tilt of the 

 earth as final result. 



Professor Twisden's paper, printed in the last number (133) of 

 the Quart. Journ. of the Geol. Soc, answers this question. He takes 

 Professor Evans' supposition of an elevated zone of land, and calcu- 

 lates the deformation to which it is equivalent. He shows that 

 instead of shifting the axis of figure 15° or 20° from its present 

 position, it would produce only about 10' of angular displacement, 

 and that to obtain so great a change as even one degree, the zone of 

 land must be elevated no less than five miles. He then discusses 

 what would result from a separation between the axis of figure and 

 rotation. He points out a startling consequence, namely, that two 

 A'ast tide-waves would sweep the earth, submerging the Equator 

 eveiy 150 days to a depth of six miles or more. He adds some 

 interesting remarks on the tendencies to alteration which the present 

 river-systems may be producing, pointing out that not only are their 

 effects extremely minute, but that many of them tend to compensate 

 for each other. 



The condition necessary to produce Professor Twisden's tide- 

 waves is that the Earth's axis of figure should occupy a position 

 different from that of her axis of rotation. Now expansion and 

 contraction, upheaval and depression, denudation and deposition, 

 must necessarily deform the Earth and displace her axis of figure. 

 Bat this alteration will be produced gradually. What will happen 

 meanwhile to the axis of rotation ? Can it be supposed to remain 

 unchanged ? To this question an answer is provided by Mr. 

 Darwin's paper. 



The conclusions of this paper are reached by rather complicated 

 analysis. A simpler process suggested by Sir William Thomson is 



