44 GEOGNOSY. [Book I 
is almost wholly covered with water appears explicable only on the 
assumption of an excess of density in the mass of that half of the 
planet. The existence of such a vast sheet of water as that of the 
Pacific Ocean is to be accounted for, says Archdeacon Pratt, by the 
presence of “some excess of matter in the solid parts of the earth 
between the Pacific Ocean and the earth’s centre, which retains the 
water in its place, otherwise the ocean would flow away to the other 
parts of the earth.” ? The same writer points out that a deflection 
of the plumb-line towards the sea, which has in a number of cases 
been observed, indicates that “the density of the crust beneath the 
mountains must be less than that below the plains, and still less than 
that below the ocean-bed.” * Apart therefore from the depressions 
of the earth’s surface in which the oceans lie, we must regard the 
internal density, whether of crust or nucleus, to be somewhat 
irregularly arranged,—there being an excess of heavy materials in 
the water hemisphere and beneath the ocean-beds as compared with 
the continental masses. : 
It has been argued from the difference between the specific 
gravity of the whole globe and that of the crust, that the 
interior must consist of heavier material, and may be metallic. 
But the effect of the enormous internal pressure, it might be sup- 
posed, should make the density of the nucleus much higher, even 
if the interior consisted of matter which on the surface would be no 
heavier than that of the crust. In fact,.we might on the contrary 
argue for the probable comparative lightness of the substance 
composing the nucleus. That the total density of the planet does 
not greatly exceed its observed amount may indicate that some 
antagonistic force counteracts the effects of pressure. The only 
force we can suppose capable of so acting is heat, though to what 
extent this counterbalancing takes place is still unknown. It 
must be admitted that we are still in ignorance of the law that 
regulates the compression of solids under such vast pressures as 
must exist within the earth’s interior, We know that gases and 
vapours may be compressed into fluids, sometimes even into solids, 
and that in the fluid condition another law of compressibility begins. 
We know also from experiment that some substances have their 
melting point raised by pressure.? It may be that the same effect. 
takes place within the earth; that pressure increasing inward to — 
the centre of the globe, while augmenting the density of each 
successive shell, may retain the whole in a solid condition, yet at 
temperatures far above the normal melting points at the surface. 
Hence on this view of the matter it is possible that the difference 
between the density of the whole globe and that of the crust may be 
entirely due to pressure and not to any essential difference of 
' Ligure of the Larth, 4th edit., p. 286, 
ms Op. cit. p. 200. See also Herschel, Phys, Geog.; and O. Fisher, Cambridge Phil. 
Trans. xii,, part ii. 
Under a pressure of 792 atmospheres, spermaceti has its melting point raised from 51° 
to 80°2°, and wax from 64°5° to 80°2°, 


