178 
ME. EOBEBT MALLET ON VOLCANIC ENEEGY. 
90. He may be permitted, in corroboration of the general truth of wliat has been just 
advanced, to refer to an extremely able memoir (which scarcely, if at all, seems to be 
known to the physical geologists of Great Britain) by Signor Prof. Giuseppe Bellt, of 
the University of Pavia, entitled “ Pensieri sulla consistenza ecc. della crosta solida 
terrestre,” published in vol. ii. New Series, of the 4to Journals of the Institute of 
Lombardy, in 1850. In note 1 to p. 6 of the memoir, pp. 3-14, entitled “ Sulla 
resistenza della crosta terrestre alia compressione,” Prof. Belli has given a long and 
able investigation of very much the same question as we have here considered, and 
arrived at results completely confirmatory, though by an entirely different path 
of investigation. 
He assumes, on certain grounds, the solid shell to be rather more than 30 Italian 
geographical miles (60,000 metres) in thickness, that it is subject to its own weight, its 
density being taken at 2 '5 to 3'0 as compared to water, arising from the attraction of 
the nucleus, whose density he takes at about 5 '6, which he further supposes, though in 
contact with the interior of the shell, to offer no resistance to the descent of the latter 
through a very small distance ; i. e. he supposes, as has been done here, the nucleus 
to shrink away by cooling from the crust above it. 
Supposing the whole globe cut by a plane passing through the equator, he investigates 
the pressure by which gravitation will urge the annular surfaces of the two opposed 
hemispheres against each other, and arrives at the conclusion that it would be equal 
to the pressure of a tower ( torre ) or hollow cylinder of material equal in density to 
the crust and standing upon the annular surface of 30 miles’ width, of 1716 Italian 
geographical miles in height — that is to say, in height equal to about half the radius of 
the earth. 
Whence he shows that no known material could sustain such a strain ; that were the 
crust of cast iron it could only support two °f the crushing force to which the gravita- 
tion of the whole system exposes it, if of porphyry yyy, if °f wrought iron if of 
granite toTti an( l if of ( marmor ) primary limestone °f the whole. 
He concludes, therefore, that the solid shell does not support itself as an equilibrated 
dome, but that, in fact, it is almost wholly supported by the fluid nucleus upon which 
it floats ; and he then attempts to show that the rising of lava in volcanic vents is a 
consequence of the partial or unequal sinking of the discontinuous fragments of the 
shell into the liquid of the nucleus. It is to be regretted that Belli, after having made 
so good a commencement, should have been misled by his supposition of a very thin 
crust and a liquid nucleus into an entirely wrong track, and so come to adopt a view of 
the mechanism of volcanic action often proposed before him, and which the writer 
believes to be untenable. It may be desirable to show, however, that by Lagrange’s 
method we can treat the question of the crushing of the crust from the same point of 
view as Belli has done, and arrive at substantially a like result. 
We have shown that, for the unit of length of a section of the shell, 
T=!PE; 
