SSO- Archdeacon Pratt on the Thickness of 



neous is self-evident, but, as in the former case, shows nothing 

 regarding solidity or fluidity, but only density. 



5, Mr. Hopkins's conclusion stands therefore unaffected, and, 

 as it appears to me, is the only physical investigation of the 

 subject upon which any reliance can be placed. The only 

 assumption is that the earth was once fluid, or sufficiently so to 

 have the arrangement of its parts regulated according to fluid 

 laws. The remarkable correspondence between the ellipticity 

 of the earth deduced upon this hypothesis and with the law of 



density — — (in itself o priori a very probable law), and the 



mean ellipticity deduced from the geodetic measurement of the 

 surface, is an immoveable argument in favour of the hypothesis. 

 And the near coincidence between the amount of precession ob- 

 served, and that calculated from the above law of density, is an 

 equally satisfactory argument that this law is the true one. 



6. That the crust cannot be thin, as some have conceived, 

 appears also from the following considerations. The Himalaya 

 Mountains and the neighbouring mountain-ground may be 

 roughly regarded as a band of superabundant matter lying on 

 the surface of the sea-level, about 200 miles wide, 1000 miles 

 long, and 2 miles high. The vast ocean stretching south of 

 India may be regarded as another vast hollow of still larger di- 

 mensions, and of a general depth equal to half the mean depth of 

 the ocean (as water is about half the density of rock), which in 

 some parts we may take to be four miles. Now the deficiency 

 of pressure arising from this hollow, and the excess of pressure of 

 the mountain-i'egion, must combine, through the intervention of 

 the fluid or lava beneath, and produce a strain upon the crust 

 which the crust might perhaps sustain if the strain acted by thrust 

 and not by tension ; but (as Mr. Airy has shown in the Philo- 

 sophical Transactions, 1855, p. 102) no rock is suificiently co- 

 herent or sufficiently free from cracks to withstand the tension 

 which the weight of the superincumbent matter would produce 

 in some parts of the crust, even if so thick as 100 miles. If, 

 then, we add to this the effect of the deficiency also arising from 

 the ocean (of which Mr. Airy takes no account), a crust as thick 

 as 200 miles would not resist the tendency to crack and open in 

 some parts ; in which case the mountain-region would subside 

 and the ocean-bed rise up, and the present aspect of the surface 

 could not continue. It has been suggested that the crust may 

 project downwards into the lava so as to be supported by buoy- 

 ancy. But this will not produce the desired effect ; for the 

 crust being formed from the fluid, will have pretty nearly the 

 aame density as the parts of the lava from which it was formed j 



