ME. EOBEBT MALLET ON VOLCANIC ENEEGY. 
165 
or Solid enough to resist as an equilibrated dome), the more rapid contraction of the 
nucleus causes it to tend to shrink away from the interior surface of the crust-shell, and 
to leave the latter partially unsupported ; or, if we assume adhesion between the two at 
the surface of contact, then the contracting nucleus as it shrinks tends to pull all parts 
of the spherical crust-shell along with it, radially towards the centre. 
In either case the direct result is to produce mutual pressures by tangential strain in 
all parts of the crust-shell, which, as being sufficiently thick and rigid, resists these 
forces as a dome. 
62. If the dome be incapable of resisting the tangential pressures thus produced, it 
must crush by the mutual compression of its parts ; and if not everywhere homogeneous, 
or if the pressure be greater in some places than in others, it will crush partially along 
the weakest places or those most strained. 
But as the material of the rigid dome or shell is, like all other matter, compressible as 
well as amenable to shearing-strain, so these tangential pressures must be attended 
with compressions, and therefore with motion in the particles of the material. 
63. The “work” produced by these mutual pressures and motion is transformed into 
heat , which either heats the whole spherical shell uniformly, if the compressions or work be 
uniformly diffused through its volume, or heats certain points or lines or planes 
within it more intensely if the work due to intervening volumes be more or less 
transferred to those places. Let us fix this in the mind by an illustration. 
Suppose we have an egg with its hard covering shell and softer interior, and that by 
some means we could cause the soft interior to diminish uniformly in volume so as to 
shrink aw 7 ay (or tend to do so) from the interior of the shell, and thus to leave the latter 
exposed to the pressure of the atmosphere. As soon as this takes place tangential 
pressures are produced within the shell, so that (if we imagine it a form of equilibrium) 
its particles mutually approach each other. The temperature of the whole shell will 
therefore necessarily rise, by the internal work thus produced ; and, for illustration, we 
may suppose the increase of temperature the same in all parts of the shell. 
But let us now assume that the shell had been previously cracked through, along 
various irregular lines upon its surface (as we see eggs in boiling often are), or had like 
lines of weakness in it. As the compression will be greatest along those lines where 
the resistance to it is least, so the chief amount of the work of compression will be 
transferred to those lines of fracture or of weakness, and the increase of temperature 
produced by the greater part of the internal work will cause the parts of the shell 
about these places to become much hotter than the intervening parts of the unbroken 
shell. What the pressure of the atmosphere upon the unsupported egg-shell here does, 
is done by gravitation of the crust-shell itself, and attraction of the nucleus in the case 
of our globe. 
64. Another and nearer illustration, indeed an almost parallel case, is presented by the 
phenomena that actually attend the cooling of large spherical or cylindrical masses of 
iron. A moderately sized sphere of cast iron which has been cast in an iron mould so 
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