208 SEE— TEMPERATURE, SECULAR COOLING [April 20, 



secular contraction of the globe? Does not the absence of such 

 movement indicate that the effect of such cooling and contraction 

 is wholly inappreciable? 



It may be remarked also that the supposed sensible shrinkage 

 is contrary to our hypothesis, based on what appear to be highly 

 probable conditions, namely that no external work is done, but only 

 an interchange effected between the central and peripheral parts of 

 an essentially adiabatic system. On these several grounds there- 

 fore it is impossible to entertain the view that the shrinkage or 

 movement of matter deep down in the earth ever exerts a sensible 

 influence at the surface. The detachment of the main body of the 

 matter composing the moon from the vast depression now occupied 

 by the Pacific Ocean, with opposite tidal fragments from the Atlan- 

 tic, is a much more probable view of the origin of the principal 

 oceanic basins. The levels were no doubt much restored by the 

 plasticity of the material and the subsequent action of the water and 

 atmosphere, but earthquake forces on the other hand have largely 

 counteracted this tendency to uniformity.^ 

 § 6. Why the Figure of the Earth can never Have Tended to the 



Tetrahedral Form. 



It is not without considerable surprise that thoughtful students 

 of the physics of the earth have noticed in recent years the recurrence 

 of the suggestion that the original form of the earth was that of a 

 tetrahedron. It is difficult to understand what there is in this im- 

 probable conjecture to render it attractive to some minds. But 

 such an hypothesis may no doubt be compared with Kepler's specu- 

 lations on the regular solids, before the discovery of the true laws of 

 the planetary motions. Kepler's hypothesis was set forth in the 

 Mysterium Cosmo graphicum; 1596, and as given by Whewell in the 

 " History of the Inductive Sciences," Vol. L, p. 416, is as follows : 



"The orbit of the earth is a circle; round the sphere to which this circle 

 belongs describe a dodecahedron ; the sphere including this will give the 

 orbit of Mars. Round Mars describe a tetrahedron ; the circle including this 



^ Since this paper was completed I have received from Professor W. H. 

 Pickering a copy of a suggestive paper, just published in the Journal of 

 Geology, in which he deals with the origin of the moon. Many of his views 

 are similar to those here adopted. The suggestion that the moon originated 

 in the Pacific Ocean is due to Rev. O. Fisher, who published it in Nature, 

 Jan., 1882, Vol. XXV, p. 243, and again in the second edition of the " Physics 

 of the Earth's Crust," Chap. XXV. 



