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-[* 
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 w^ould be 
2 '00028991. Suppose, then, a crust to be formed luithout 
contracting, of this thickness, its specific gravity as a mass 
would be about 2-00014496. Such a crust would 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 4 a-Ves (Par. 9), and 
the specific gravity of the mass augmented to 2 '04041526, 
* 196,000,000 (square miles of surface) x 25|- miles (tliiclmess 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. 
