Rev. 0. Fisher — Rigidity of the Earth. 251 



This statement possibly refers to Professor Hecker's observations on 

 the bodily tides of the earth. I understand the word ' solid ' as 

 opposed to 'fluid', not as equivalent to 'rigid'. Is it not possible 

 that there may be a liquid substratum to the earth's crust, and the 

 earth may nevertheless be rigid with respect to the external disturbing 

 forces of the attraction of the moon and sun as evidenced by tides? 

 By way of illustration, we are quite unconscious of the rotation of 

 the earth on which we stand. 13ut if we could observe the earth 

 with a telescope from the moon, the efPect of rotation would be very 

 marked by the rapid passage of objects near our equator across the 

 field of view. Is it not possible that the earth's rotation may impart 

 to it a ' gyroscopic ' quasi rigidity, which may enable it to withstand 

 the deforming influence of external forces, although at the same 

 time forces internal to the earth will be unaffected by it. 



Professor Perry, in his little book on spinning-tops, illustrates 

 how "rapid motion gives a peculiar quasi rigidity to flexible and 

 ■even to fluid things. Here," says he, " is a disk of quite thin paper ; 

 and when I set it in rapid rotation you observe that it resists the 

 force exerted by the blow of my flst as if it were a disk of steel. 

 Hear how it resounds when I strike it with a stick. Where has its 

 flexibility gone? " 



In the highly interesting and instructive paper which Sir Gr. H. 

 Darwin read at the Geodetic Conference of 1909, giving an account 

 of Hecker's observations of the tidal deformation of the earth, he 

 reproduced Hecker's curve, which represents the rigidity both in 

 north and south, and east and west directions. The radius of this 

 •curve is proportional to the rigidity, and shows in a remarkable 

 manner that the rigidity is much greater in the east and west direction 

 than in the north and south. And in the discussion that followed 

 Darwin said that he " considered as worthy of consideration Professor 

 Hecker's explanation of the remai-kable absence of symmetry in the 

 path of the vertical" (that is, the difference of rigidity in different 

 directions, Hecker's explanation depending upon the geographical 

 situation of Potsdam), " but he suggested an alternative possibility. 

 The curve was much compressed in the north and south direction, 

 showing that the earth has much greater rigidity east and west than 

 north and south. It is possible to explain this to some extent by the 

 earth's rotation. Lord Kelvin introduced the idea of ' gyroscopic ', that 

 is of greater rigidity east and west due to rotation. Whether this is 

 a sufficient explanation cannot be said, because no one haa succeeded 

 in solving completely the gravitation problem of a rotating elastic 

 globe " ; ^ nor yet, I believe, of a partially liquid one. Although it 

 may be that rotation imparts a greater rigidity in the equatorial 

 direction, yet Professor Perry's experiment quoted above shows that 

 there is great rigidity imparted in the axial direction also. If rotation 

 is competent to impart the needful excess of rigidity, it may be 

 pertinent to ask, why not the whole of it ? 



I do not remember ever to have seen changes of level accounted for 

 on the hypothesis of a solid earth. There also appears to be some 



1 Nature, vol. Ixxxi, p. 427, October 7, 1909. 



