42 T. E. TWISDEN ON POSSIBLE DISPLACEMENTS 



total effect of all these displacements put together. The accumu- 

 lation of these small displacements is therefore the only new point 

 for discussion. 



17. It may be assumed that, at present, and within the more re- 

 cent geological periods, the earth is, in the main, a solid body*, the 

 interior being intensely hot, but kept solid by the pressure of the 

 superincumbent parts. This general statement is, of course, not 

 inconsistent with there being tracts of molten matter at various 

 places within the solid mass of the earth. 



18. This being so, the notion of there being any considerable 

 transfer of matter by change of place from one region to another 

 below the surface may be dismissed without discussion. But what 

 would be equivalent to such a transfer might arise from contraction 

 due to internal cooling. If the contraction, however, were uniform 

 throughout the mass of the earth no change would be produced in 

 the position of the axis of figure. If the contraction were not uni- 

 form, there might be such a change ; but it will be observed that 

 contraction is a case of transfer of matter from one position to a 

 neighbouring position, and consequently (art. 9) is not of a kind 

 favourable for producing a large effect. If a considerable effect of 

 the kind intended were produced by contraction, there would have to 

 be an inequality of contraction on a very large scale. 



19. In illustration of this point I may mention the results ob- 

 tained in the following case : — Suppose a solid body of uniform den- 

 sity of the same size and shape as the earth, and consider an octant 

 of its volume which has its curved surface in the northern hemi- 

 sphere comprised between the equator and the meridians of 45° E. 

 and 45° W. of Greenwich. Suppose that, the other parts of the solid 

 continuing uncontracted, this solid octant undergoes a uniform 

 contraction of volume which has the effect of lowering its surface 

 by two miles below the general surface of the solid. This con- 

 traction would bring the axis of figure forward towards Greenwich 

 by about 1° from its original position. This case will sufficiently 

 illustrate the point that inequality of contraction would have to be 

 on a very large scale to produce a cousiderable result of the kind 

 contemplated. Whether any slow deformation of the earth could 

 be effected on a large scale by unequal contraction in the process of 

 cooling would depend, I suppose, on the kinds of matter composing 

 the inner parts of the earth, and on their arrangement. In the ab- 

 sence of all information on these points the question hardly pre- 

 sents itself for serious discussion. The existence of inequality of 

 contraction is purely hypothetical ; and nothing is known of the cir- 

 cumstances which would render such an hypothesis probable or the 

 reverse. 



20. There remains for discussion the case of matter transferred 

 on the surface of the earth by water. Here, at all events, we have 

 a vera causa, a process actually in operation which might conceivably 

 produce the desired effect. The question is whether there is any 



* Proc. Boy. Soc. vol. xii. pp. 103, 104. 



