Rev. E. Elll^Oii Changes in the Earth's Axis. 265 



jacent strata, depression by their contraction, tlie i-esulting change is 

 diminished to a considerable, possibly to a very great extent. If, for 

 instance, the expansion or contraction take place between the depths 

 of 10 and 50 miles, the shift of the Pole will be diminished to less 

 than a sixtieth part of its former amount. If the supposed areas 

 occupy positions other than the most effective, these changes must 

 be still further reduced, and may even vanisli altogether. 



These calculations are based on the supposition of a practically 

 rigid Earth. If its interior be fluid, Professor Twisden's tide-waves 

 still hold good, for their magnitude depends only on observed 

 quantities. Mr. Darwin's calculations involve Laplace's hypothetical 

 law of density, and obtain results true for a solid globe. But mathe- 

 maticians seem to consider that even if the interior be fluid, yet as 

 far as change of rotation is concerned, it must behave as if it were 

 rigid. Accordingly the results are unaltered. 



Mr. Darwin points out that if alterations in the Earth's surface 

 should produce a separation, however small, between the axes of 

 figure and of rotation, the mass of the Earth would be thrown into 

 a state of stress, which he suggests might be relieved by an earth- 

 quake restoring coincidence. Though the strains would be extremely 

 minute, this seems possible. He works out the results. of this, and 

 also those of the Earth being plastic, capable of yielding to any 

 stress, and deduces that the pole of rotation would describe a kind 

 of spiral round the point towards which the pole of figure was being 

 shifted by the change. This point he had shown could not be far 

 distant, and therefore the spiral could not be wide. Nevertheless 

 he seems to think that in a plastic globe this cause might lead to 

 considerable wandering of the poles. But with respect to the Earth, 

 he appeals to Sir William Thomson's demonstrations of its rigidity 

 as excluding the hypothesis of plasticity. 



Mathematicians may seem to Geologists almost churlish in this un- 

 willingness to admit a change in the Earth's axis. Geologists scarcely 

 know how much is involved in what they ask. They do not seem 

 to realize the vastness of the Earth's size, or the enormous quantity 

 of her motion. When a mass of matter is in rotation about an axis, 

 it cannot be made to rotate about a new one except by external 

 force. Internal changes cannot alter the axis, only the distribution 

 of the matter and motion about it. If the mass began to revolve 

 about a new axis, every particle would begin to move in a new 

 direction. What is there to cause this? When a cannon ball strikes 

 an iron plate obliquely, the shock may deflect it into a new direction. 

 The Earth's equator is moving faster than a cannon ball. Where is 

 the force that could deflect every portion of it and every particle of 

 the Earth into new directions of motion? The cannon ball is 

 slightly and slowly deflected by gravity. But the attraction of the 

 Sun and Moon could produce on a slightly distorted globe no effect 

 essentially different from what they produce now.^ No other force 

 exists capable of producing any effects at all. 



1 Suppose the Sun's attraction at any instant to be causing in the Earth a be- 

 ginning of a rotation about any diameter through the Equator. Twelve hours later 



