20 REPORT—1850. 
frequently, according to the facility with which the materials take fire, and 
also as their volume has relation with the extent of the spaces in which they 
are enclosed.” His views are nearly identical with those of Don Ulloa, but 
are more clearly expressed by the latter, who says— 
“ The bursting of a new volcano causes a violent earthquake ; this tremu- 
lous motion, which we properly call an earthquake, does not so usually 
happen in case of a second eruption, when an aperture has been before 
made, or at least the motion is comparatively small.” ‘“ Volcanoes owe their 
origin to sulphurous, nitrous, and other combustible substances in the bowels 
of the earth; these, mixed and turned into a paste, with subterraneous waters, 
ferment and take fire (this was Lemeri’s view); by dilating the contiguous 
wind or air its volume is so increased, as to produce the same effect as gun- 
powder fired in a narrow space.” “ The subterraneous noise proceeds from 
the ignition of the airs on exploding.” 
Dolomieu’s theories, as to the Calabrian earthquake of 1789, are not very 
different. ‘Interior waters, increased by those from the surface, may have 
run into the focus of ΖΕ πᾶ: they would in consequence be converted into 
very expansive vapour, and strike against every obstacle to their dilatation.” 
He has previously shown that Calabria itself is not a voleanic country; he 
therefore proceeds :— 
“ Provided these should have met with channels conducting them to the 
cavities below Calabria, they would have been capable of occasioning all 
those convulsions of which I have given a description.” 
Sir W. Hamilton concludes from all his examinations of the Calabrian 
earthquakes, that “some great chemical operation of the nature of the vol- 
canic sort was the cause.” (Phil. Trans. vol. Ixxiii.) 
Thus the older writers fix their regards wholly upon the presumed focus 
or origin of the explosion, as Dolomieu calls it, but none, except Mitchell, 
attempt to show any distinct train of causes by which the forces here origi- 
nating, in a centre of volcanic activity, are transferred and become opera- 
tive at vast distances and in lands not subject to voleanic action. Nor, it 
must be confessed, have modern authors, even Humboldt, been much more 
successful in this, or in shaping to themselves a distinct idea of what the 
nature of the earthquake shock itself is. The words—“a trembling,” “a 
vibration,” “a concussion,” “a movement,” “δὴ undulation,” are to be found 
scattered through the narratives of earthquakes, but even amongst scientific 
authors these records refer merely to the effect upon their senses, of the 
motions of the earth’s surface, and not to any definite or precise idea, either 
of the origin or the mode of propagation of the shock. 
Humboldt in his latest work, the ‘ Cosmos,’ as well as in his ‘ Personal 
Narrative,’ does not express himself with clearness upon earthquake move- 
ments. He seems disposed at one place to adopt the theory of Mitchell im- 
plicitiy ; yet at another, one fancies he has some notion of the earth-shock 
being a wave of elastic compression, and therefore propagated in a totally 
different manner from that of the subteraneous lava tidal wave, moving the _ 
solid crust above it, in which Mitchell’s theory consists: his clearest expres- 
sion of view is perhaps in the following sentence :—“ The filling up of fissures 
with crystalline matter interferes by degrees with the free escape of vapours, 
which confined become operative through their tension in three ways 
—concussively, explosively, or suddenly up and down, and as first observed 
in a large portion of Sweden, liftingly or continuously, and only i in a long 
period of time perceptibly altering the level of the sea and Jand.” Here he 
confounds as thoroughly as the ancient authors, the direct effects in perma- 
nent elevation of land by volcanic or other action from beneath, with the 
