Hypothesis of the Internal Fluidity of the Earth. 137 



Par. 4) be an augmented velocity of rotation of the crust as 

 compared with the interior, so that whereas the latter re- 

 quired the same time as before to complete a diurnal axial 

 revolution, the former would accomplish the same in a some- 

 what shorter period. Owing, however, to the inferior surface 

 of the crust being in contact with the superior surface of 

 the internal fluid, a portion of the surplus velocity of the 

 former would be communicated to the latter. Were the 

 surfaces in contact perfectly free from inequalities, and truly 

 concentric, the surplus force of the crust would be wholly 

 expended in gradually increasing the velocity of rotation of 

 the interior ; but since, as we have seen, this is far from 

 being the case, it will at once appear evident that a portion- 

 of this surplus is converted into a gradually up-heaving force, 

 operating in a direction from east to west, or contrary to 

 that of the earth's rotation on its axis. The total amount of 

 the force of which a portion is thus converted into the " up- 

 heaving force" of the geologist, is, according to the assump- 

 tions and calculations we have employed, equivalent to the 

 momentum of the whole solid crust of our globe (say 4998* 

 millions of cubic miles) moving over a space of 471*75"f 

 yards in 24 hours. 



12. According to the hypothesis of the density at the 

 surface being 2, and at the centre 21, it appears by cal- 

 culation that at the depth of 106 \ yards the mean specific 

 gravity of the materials occupying that position would be 

 2*00028991. Suppose, then, a crust to be formed ivithout 

 contracting, of this thickness, its specific gravity as a mass 

 would be about 2-00014496. Such a crust w T ould have no 

 tendency to sink in the subjacent molten mass. But sup- 

 pose that it should part with 1500° of heat, whilst the 

 temperature of the fluid mass beneath remained undi- 

 minished, then in this case the resultant contraction and 

 consequent increase of density would be 49V us (Par. 9), and 

 the specific gravity of the mass augmented to 2'04041526, 



* 190,000,000 (square miles of surface) x 25£ miles (thickness of crust). 



t 1887 yards (arc of excess as by Par. 4) halved for mean between the upper 

 and lower surfaces of the crust, and again halved for the mean between the 

 equator, where the maximum is attained, and the poles, where the effect is zero. 



