1906-7. TRANSACTIONS. 133 



northern hemisphere and taper towards the south, which too 

 agrees with our geography. Inversely, the oceans should con- 

 tract towards the north, a condition fairly well borne out. 



Furthermore, the north polar area being represented by a 

 surface and the south polar one by a corner, it would follow that 

 the flattening of the earth in the southern hemisphere would be 

 less than in the northern; and again, gravity would increase less 

 rapidly towa,rds the south pole than towards the north pole. 

 Both these considerations have been confirmed by geodetic and 

 pendulum observations. 



If the tetrahedral theory was effective at the early stage of 

 the earth's existence, in giving us many of our mountain systems 

 and our polar physical conditions, to-day with a pretty rigid crust 

 its effect must be vanishingly small and unrecognizable as due to 

 that theorA^ 



We may refer to another theory of the figure of the earth, 

 contained in a paper presented by J. H. Jeans to the Royal Society 

 in 1902. This theory shows, under certain assumptions, that the 

 earth was pear-shaped at a certain stage of its existence, and 

 contracting assumed the spherical form. I cannot in this place 

 pursue this subject. of the figure of the earth any further; it was 

 only alluded to to show one of the factors — the contracting forces, 

 ever active, whereby strains and stresses are set-up, and without 

 which no earthquakes are possible. 



Fisher in his " Physics of the Earth's Crust " 1889, combats 

 the theory of mountain building as being due to the secular cooling 

 of the earth and the accompanying contractions, but this would 

 not preclude smaller motions to which earthquakes m.ay be re- 

 legated. Arrhenius considers the crust of the earth compara- 

 tively thin; at a depth of about 40 miles to merge into a hot fluid 

 mass, the magma, due to the increasing temperature. From the 

 deepest boring on the earth the increase of temperature is about 

 1°F. for 51 feet, or say 100°F. per mile. Beyond a depth of about 

 200 miles the magma assumes the gaseous form. He writes 

 "the earth as well as the sun contracts, whereby heat is envolved 

 and the contraction partly arrested or decreased. Nevertheless 

 the earth slowly shrinks. This pertains especially to the interior 

 of the earth, for the temperature of the surface is almost wholly 

 due to radiation from the sun, and in a small degree upon the 

 character of the atmosphere. It may be assumed that, broadly 

 speaking, the radiation of the sun and the nature of the atmos- 



