296 Mr. 0. Fisher — Changes of Latitude on the Earth's Surface. 



over whatever it be that underlies thera. It is obvious that a more 

 or less perfectly fluid substratum is required to give this capability 

 of slipping. Indeed the nearest analogy to the formation of a chain 

 of mountains seems to be found in the crushing together of the 

 adjacent edges of two sheets of floating ice. 



I have thus, I think, shown that the supposition of a liquid 

 layer beneath the crust of the earth is not a supposition that 

 is so wholly untenable as has been represented, and that, as far 

 as authority (that blind guide) can determine the question, we 

 have it on both sides. I recur therefore to my original proposition, 

 that physicists should be moved to consider what elements of 

 change in the position of the earth's crust as regards latitudes 

 might be introduced under the influences acting upon it, by the 

 existence of a considerable degree of mobility between the crust 

 and a solid nucleus owing to the intervention of a liquid stratum. 

 The solidity of the crust being supposed due to cooling, and that 

 of the nucleus to pressure, I would consider the crust thin, and the 

 fluid substratum also thin. I would consider the crust and the 

 fluid of sensibly the same thicknesses at the equator as elsewhere, 

 taking the equatorial bulge as existing in the solid nucleus. This 

 supposition would seem admissible, because we have volcanos in 

 low as well as in high latitudes, showing that, if they be connected 

 with a liquid substratum, that cannot be much more deep down in the 

 one region than in the other. Moreover, the ratio of the centrifugal 

 force to gravity being small, it is not likely that the upper limit 

 of the pressure necessary to induce solidity should be at an ap- 

 preciably greater depth below the surface at the equator than at the 

 poles. I would consider the crust flexible, but for tlie purposes of 

 the problem not horizontally compressible further than would allow 

 it to fit the spheroid in any new position so that it should move all 

 together. Then I would inquire, a portion of it having become 

 thickened by assemblages of foldings near together, in other words 

 by the elevation of continents, what effect these would have upon 

 the position of the crust as regards the axis of rotation of the 

 whole, supposed fixed in space. 



This is, in short, Dr. Evans's original problem, which he en- 

 deavoured to solve by experimental means. 



But the question I would put is, whether, supposing the crust 

 free to move under forces as small as those which this change of 

 form would bring to bear upon it, the effect would be (not that 

 the protuberances would be brought towards the equator, as in Dr. 

 Evans's wheel), but that the crust would tend to be carried into 

 such a position that the axis of symmetry of the solid nucleus and 

 of the crust together {i.e. their axis of greatest moment of inertia) 

 would tend to coincide with the axis of rotation. If this were 

 so, the nucleus being spheroidal, the crust would eventually become 

 so placed with regard to the axis of rotation that that would 

 coincide with its axis of principal moment of inertia. If, how- 

 ever, the crust were free to obey this tendency only to a certain 

 degree, being checked, partly by the rigidity of its parts not 



