54 
MR. HOPKINS’S RESEARCHES IN PHYSICAL GEOLOGY. 
forms may have existed on its surface. My previous investigations have now demon- 
strated the truth of this conclusion as applicable to the earth from the first formation 
of its external solid shell. All such hypotheses, therefore, as have sometimes been 
made with respect to a change in the position of the earth’s axis are entirely excluded, 
whether we suppose the interior of the earth to be now, or to have been heretofore, 
solid or fluid. A fact also, in itself not uninteresting, is thus established in the ear- 
liest history of our globe. 
Nor would it have been possible, I conceive, to arrive at this result by any general 
considerations immediately derivable from the nature of our problem, and independent 
of its complete solution. The investigations contained in my two preceding memoirs 
were, in fact, commenced under the impression that the solution of this problem of 
the precession and nutation of the earth’s axis on the hypothesis of the interior fluidity 
of the earth, would probably lead to results different from those which had been long 
before obtained on the supposition of the earth’s entire solidity. This impression was 
founded on the consideration of the great difference between the direct action of a 
force on a solid, and that on a fluid mass, in its tendency to produce rotatory mo- 
tion. We have seen, in fact, that the disturbing forces of the sun and moon do not 
tend to produce directly any motion in the interior fluid, in which the rotatory motion 
causing precession and nutation, is produced indirectly by the effect of the above 
forces on the position of the solid shell. A modification is thus produced in the effects 
of the centrifugal force, which (as appears from the results of our investigations) 
compensates for the want of any direct effect from the action of the disturbing forces. 
This compensation will scarcely, perhaps, be deemed less curious than many of those 
which have been recognized in the solar system, and by which, amidst apparently con- 
flicting causes, its harmony and permanency are so beautifully preserved. 
§. Condition respecting the Temperature of Fusion for the matter composing the Earth, 
in order that its actual Temperature may be due to its original Heat. 
12. There is also another conclusion to be drawn from our investigations which it 
may be worth while to notice. It has been assumed in these memoirs that pressure 
is effective in producing solidification ; it has been already remarked, however, that 
should that not be the case, our conclusions respecting the thickness of the earth’s 
crust will still, d fortiori, be true. Our determination, therefore, of the least limit to 
that thickness is independent of this unknown effect of pressure, or, in other words, of 
the experimental determination of the temperatures of fusion for different substances 
under high pressures. With the aid of a proper series of experiments on this point, a 
direct method of arriving at an approximation to the thickness of the crust of the 
globe, or rather, to its least limit, might be easily explained. I shall not here, how- 
ever, enter into any discussion on this subject. The conclusion to which I would 
now direct attention is this — the present temperature of the interior of the earth can- 
not be due to its original heat, if the temperature of fusion for the matter composing 
