O.V THE THEORIES OF ELEVATIO.V AXD EAIITIIQITAKES. 49 
Sir James Hall*, in which he succeeded in fusing several calcareous sub¬ 
stances. But in these cases the effect of pressure was manifested in the 
prevention or the escape of the carbonic acid, which being thus retained 
appears to have acted as a flux on tho other portions of the substances 
wJuch were experimented on. Thus fusion was in this instance promoted 
instead of bang impeded by the pressure necessary to retain portions of tlie 
subs^ce which would otherwise have been volatilized; but nothing was 
aecidw m these experiments as to the consequences of increasing the pres¬ 
sure tar beyond that point, whicli is here the real question at issue. More¬ 
over, the substance acted on was so entirely different in its chemical con- 
stitution from the general igneous masses which probably fonu so large a 
portion of the globe, that no inference can be drawn respecting the effect of 
gijrat pressure in the solidifying of those masses, from the oxiicriments of 
which we are speaking. 
It should also be remarked with respect to the reasoning above given, 
that I have token no account of that property of the fluid mw in conse- 
queuce of which the interior generation of elastic gases is constantly pro¬ 
ceeding, a property which I assume to belong to the primmval fluid mass, 
by virtue of tlie analogy which it is assumed to bear to existing volcanic 
i r . eflcct of this production of gases would be to retard 
e solidjDMtion. It would Icngtlien tho duration of tlie process of cooling 
by circulation, and after that process had ceased with reference to the parti¬ 
cles ot the mass itself, the gases would still impede the solidification by pre¬ 
venting the entire interna! rejmse of the mass. 1 have stated in general 
terms that no incrustation of the surface would take place so long as the 
cooling by circulation continued, provided the loss of heat were accom- 
panied by an increase of density. In considering the process of solidiflca- 
lon, owever, more in detail, os I shall do in tim sequel, we ought to regard 
tins as on y approxunately true, since there is little doubt but thM the cold 
or tlie external atmosphere would frei>zc tho outer surface hofure the circu- 
Jation had ceased m the interior of the moss. But before entering into any 
detailed consideration of the process of solidification, I shall speak of the 
evi en^ we possess, or may hope to obtain, respecting the general i-xtcnt to 
which It has already proceeded. ^ 
15. Thickness oj ihe Earth's Solid Cmst .—It has frequently boon assumed 
y speculative geologists, not only that the centrnl portion of the earth is 
nmu, ^ we have shown it may be, but also that the solid crust ivJiioh enve- 
pes It dora not exceed a few leagues in tliickness; and on this iissuniptlon 
eon^ ot volcanos, p w'c have seen, have been fminded. In seeking for 
^ rminate evidence on this point, on which our previous reasoning 
us in periect uncertainly, it occurred to me some years ago, that it 
^ht possibly be lound in the delicate but well-defined phamomena of iire- 
«f nutation, of whieli 1 have already had occasion to sjK’ak. To 
e more distinctly the problem wliich it was necessary to solve for 
3 purpose, I may remind the reader that during the annual revolution of 
e earth, its does not preserve a perfect parallelism in its different posi- 
ODs, so that the point of the heavens to which it is directed at any time is 
no exactly the same os that to which it pointed at any previous epoch 
en the earth was at the same point of her orbit. This angular motion 
o he axis, though very slow, has been determined with great accuracy by 
roDomical observation. I have alrt-ady stated it to be due to the cir¬ 
cumstance of the resulting force of attructiou of the sun and moon on the 
1847. 
* Transactions of the Royal Society of Edinburgh, vol. vi. p. 71- 
