Feb. 9, 1871] 


the case, and therefore a simple calculation will show that at a 
depth of about twenty-five geographical miles from the surface 
downwards a temperature of about 3,000° F. should be 
attained, which would represent a heat at which iron melts, or 
one sufficient to keep Java in a state of perfect molten liquidity 
at the surface of the earth. As it must be remembered, how- 
ever, that at this depth the substance of the earth would be 
exposed to the pressure of the superincumbent mass, and as it 
has been shown by experiment that many substances become 
more refractory—z.e., require a greater degree of heat to melt 
them—when exposed to pressure than when at the surface, the 
above calculation will have to be modified considerably in order 
to meet this condition. Unfortunately, we have not as yet 
sufficient data at command to enable us to estimate the true ratio 
in which the melting points of such rocks would become elevated 
by pressure; yet we may safely take it for granted, after allowing 
far more than the maximum rate of increase, deduced from the 
experiments of Bunsen and Hopkins, that we should not require 
to sink so deep again in order to attain a temperature fully 
sufficient to keep such substances in a state of fusion, or, in other 
words, this deduction necessitates the supposition that the solid 
rock crust of the earth cannot, at the utmost, be more than 50 
miles in thickness. 
Tf we now reason from the above data as premisses, it will follow 
as a natural consequence that our globe must in reality be a sphere 
of molten matter surrounded by an external shell or crust of 
solid matter, of very insignificant thickness when compared to 
the diameter of the entire globe itself; or, in other words, this 
deduction represents exactly such a state of things as would 
ensue in the event of a sphere of molten matter becoming con- 
solidated on its exterior by the cooling action of the external 
atmosphere ; and the figure of the earth itself, which is an ellip- 
soid of revolution, z.2., a sphere somewhat flattened in at the 
poles, but bulging out at the equator, being that which a plastic 
mass revolving round its own axis would assume, is regarded by 
natural philosophers in general as all but conclusive evidence, 
that the earth at an early period of its history must have been in 
a fluid condition. 
Although the doctrine that the earth is a molten sphere sur- 
rounded by athin crust of solid matter was all but universally 
taught by geologists, there have of late years been brought 
forward several arguments to the contrary, which apparently are 
more in favour of its being a solid or nearly solid mass through- 
out, and these arguments are fully entitled to our consideration, 
as our object is not to defend any particular theory, but to 
arrive as nearly as we can at the truth. I will, therefore, in 
the first place proceed to scrutinise all which has been brought 
forward-in opposition to the older hypothesis, and then to con- 
sider whether any other explanation yet advanced is more in 
accordance with the facts of the case. 
First of all we have to answer the question as to whether it is 
possible for such a thin crust to remain solid, and not at once 
to become melted up and absorbed into the much greater mass of 
molten matter beneath it? This would doubtless be the case, if 
the central fluid mass had any means of keeping up its high 
temperature, independently of the amount of heat it actually 
possessed when it originally assumed the form of an igneous 
globe. This question, however, in reality answers itself in the 
negative, since it is evident that no crust could even commence 
to form on the surface, unless the sphere itself was at the moment 
actually giving off more heat from its outer surface to the sur- 
rounding atmosphere than it could supply from its more central 
parts, in order to keep the whole in a perfectly fluid condition, 
so that when once such a crust, however thin, had formed upon the 
surface, it is self-evident that it could not again become melted up 
or re-absorbed into the fluid mass below. 
This external process of solidification due to refrigeration 
would then continue going on from the outside inwards, until 
a thickness of crust had been attained sufficient to arrest or neu- 
tralise (owing to its bad conductibility of heat) both the cooling 
action of the surrounding air and the loss of more heat from the 
molten mass within; and thus a stage would soon be arrived at 
when both these actions would so counterbalance one another 
that the further cooling down of the earth could be all but 
arrested: a condition ruling at the present time, since the earth- 
surface at this moment, so far from receiving any or more than a 
minute amount of heat from the interior, appears to depend 
entirely, as regards its temperature, upon the heat which it 
receives from the sun’s rays. 
We haye next to consider the argument that, if the earth’s ex- 
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297 

terior was in reality only such a thin covering or crust, like the shell 
of an egg, to which it has often been likened, that such a thick- 
ness would be altogether insufficient to give to it that stability 
which we know it to possess, and that consequently it could 
never sustain the enormous weight of its mountain ranges, such 
as, for example, the Himalayas of Asia or the Andes of America, 
which are, as it were, masses of rock piled up high above its mean 
surface-level. 
At first sight, this style of reasoning not only appears plausible, 
but even seems to threaten to upset the entire hypothesis alto- 
gether. It requires but little sober consideration, however, to 
prove that it is more, so to speak, sensational in character than 
actually founded on the facts of the case ; for it is only requisite 
for us to be able to form in our minds some tangible idea of the 
relative proportion which the size of even the highest mountain 
bears to that of the entire globe itself, to convince us, if such a 
crust could once form and support itself, that it could with ease 
support the weight of the mountains also. The great Himalaya 
chain of mountains rises to a maximum altitude of 31,860 feet, 
or six miles above the level of the sea; and if the earth 
could be seen reduced in scale down to the size of an orange, to all 
intents and purposes it would look like an almost smooth ball, since 
even the highest mountains and deepest valleys upon its surface 
would present to the eye no greater inequalities in outline than the 
little pimples and hollows on the outside of the skin of an ordinary 
orange. If this thin crust of the earth can support itself, it is 
not at all likely to be crushed in by the comparatively speaking 
insignificant weight of our greatest mountain chains, for in point 
of fact it would be quite as unreasonable to maintain such a 
supposition, as to declare that the shell of a hen’s egg would 
be crushed in by simply laying a piece of a similar egg-shell upon 
its outside. 
That a very thin spheroidal crust or shell enclosing a body of 
liquid matter such as an ordinary fowl’s egg, does possess in it- 
self an enormous degree of stability and power to resist pressure 
from without, is easily demonstrated by merely loading a small 
portion of its surface with weights as long as it does not give 
way under them, Even when placed on its side (or least strong 
position) it was found that a portion of the shell only one quarter 
of an inch square would sustain several pounds weight without 
showing any symptoms of either cracking or crushing ; or, in 
other words, this simple experiment indicates that if the external 
crust of the earth was but as thick and strong in proportion as 
an egg-shell, it would be fully capable of sustaining masses 
equal in volume and weight to many Himalayas piled up one 
atop of another, without any danger whatever to its stability. 
The next argument which has been advanced against the pro- 
bability of the major part of the earth’s substance being in a 
fluid condition, is one based altogether upon astronomical consi- 
derations. It having been demonstrated when two clocks are 
set agoing, the pendulums of which are similar to one another 
in all respects except that whilst the bob of the one is solid, that 
of the other is hollow and filled with mercury, that the latter will 
swing somewhat faster, and consequently the clock gain time upon 
the former. The late Mr. Hopkins, of Cambridge, applied this 
observation to the consideration of movements of the earth in 
space, and by a very elaborate course of mathematical reasoning 
and calculation, demonstrated that the earth, if not quite solid, 
must be nearly so, since according to his results, if it was merely 
a comparatively thin shell filled with liquid matter, the ratio of 
certain of its movements (the precession or nutation) would differ 
considerably from what they are actually known to be, and these 
conclusions appeared to be confirmed by the subsequent calcu- 
lations of Sir William Thomson and Archdeacon Pratt. Although 
grave doubts suggested themselves as to the correctness of the 
values used in these calculations for two of their most important 
elements, viz., the condensing action of pressure and the ex- 
panding action of the very high temperatures within the globe— 
both of which have not as yet been determined with any cer- 
tainty, and although it might also be surmised that the condi- 
tions of a pendulum bob of polished glass filled with heavy 
slippery mercury swinging at the end of a rod must be extremely 
different from those of a nearly spherical globe filled with viscid 
sticky lava revolving around its own axis; still geologists felt 
themselves quite unable to answer the arguments of the astro- 
nomers and mathematicians, and since none of them appeared to 
be sufficiently versed in either astronomy or mathematics to be 
able to submit the method of reasoning or the calculations to any 
strict scrutiny, they felt themselves, reluctantly no doubt, com- 
pelled to bow to the decision of such eminent authorities, 
