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POPULAR SCIENCE REVIEW. 
correct and justifiable reasoning to assume that such is actually 
the case : and it follows, therefore, as a natural consequence, 
that a temperature above 3,000° degrees — representing a heat 
sufficient to melt rocks like granite, &c. — will be found at a 
depth of some forty miles from the surface (more or less, 
according to the rate of increase used in the calculation), or, in 
other words, that at that comparatively small depth an internal 
mass of molten matter exists in the interior of the earth. 
Coming now to the consideration of the second hypothesis, 
which represents the earth as being nearly, if not quite, solid to 
the core, we find that it is founded upon such purely astrono- 
mical evidence as to altogether ignore any data which the 
geologist can eliminate from the direct study of the earth itself. 
The late Mr. Hopkins of Cambridge, who first advanced this 
hypothesis, appears to have arrived at this conclusion, from 
observing, when two clock pendulums are set agoing equal in all 
other 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 than the former. 
Applying this observation to the consideration of the move- 
ments of the earth in space, Mr. Hopkins, by an extremely 
elaborate course of mathematical reasoning and calculation, 
demonstrated that the earth must be nearly, if not quite, solid ; 
since, if it was merely a comparatively thin shell, filled with 
liquid matter, the ratio of certain of its movements (precession 
and nutation) would differ very considerably from what they are 
actually known to be — conclusions which subsequently were un- 
derstood to be further confirmed and verified by the arguments 
and calculations of Sir William Thomson and Archdeacon Pratt. 
Although it might have been surmised that the conditions of 
a pendulum-bob of polished glass, filled with equally slippery 
non-adhesive mercury and swinging at the end of a rod, must be 
very different from those of our nearly spherical globe, filled 
with molten, but viscid or sticky, lava and revolving upon its 
own axis, geologists at once felt themselves put to utter con- 
fusion by this “ dictum ” of the astronomers and mathematicians ; 
and, being none of them sufficiently versed in either astronomy 
or mathematics as to be able to submit the reasoning or calcu- 
lations to any exact scrutiny, felt themselves — reluctantly, no 
doubt — compelled to bow to the decision of such eminent 
authorities. 
8o stood the matter until last summer, when fortunately M. 
Delaunay, an authority equally eminent as mathematician and 
astronomer, was induced to undertake the reconsideration of 
the problem ; a labour which has not only resulted in altogether 
reversing the above decision and demonstrating the complete 
fallacy of the premises upon which the reasoning was founded. 
