86 Mr. Matter on the Dynamics of Earthquakes. 
away, a light of insufferable brightness, piercing the night, beamed up from these 
awful chasms, and heralded to upper air, the hissing lava. The molten mass, 
however, slowly welled up within the fissures, until its hydrostatic pressure at 
length gave it vent, and it flowed out at a distance of some miles at Monte Rossi. 
Here, then, where the mountain was ripped open from base to summit, no vast 
volumes of pent-up gases were liberated; no sudden depression of surface fol- 
lowed; nor am I aware that any such phenomena have been observed in other 
instances of eruption. We may, therefore, affirm that there is no ground in 
observed facts, for supposing large tracts of the earth’s crust ever to float upon, 
or be elevated by, subterraneous seas of elastic gaseous matter. 
There is, on the contrary, every ground for believing that the whole earth’s 
crust does rest upon liquid melted matter, and that this crust, although of enor- 
mous thickness, is extremely thin in proportion to the vast depth from the surface 
to the centre of our planet. We cannot say what the absolute thickness is, but it 
probably is not less than forty-five miles. Now granting, argwmenti gratia, 
that an original impulse is given to this solid crust, floating upon liquid lava, let 
us follow out the consequences, and see how far the production and propagation 
of such a wave as Michell has assumed, is possible in a crust resting on liquid 
matter, so as to agree with the known laws of wave motion, and the observed 
conditions of earthquake shocks. 
His theory is, that the thin crust having received the original impulse at a 
certain point, undulates, by the passage of a wave not in the elastic plate itself, 
analogous to the vibration of a stretched string, or rather of a sonorous plate, but 
that the crust is forced to undulate by the passage below it of a wave of the 
fluid upon which it rests, so that the surface of the crust assumes the form, and 
follows the motion, of the undulating fluid below, in the same way as the carpet 
of his experiment follows the undulation of the wave of air below it. Now, we have 
shewn that the rate of transit of the great earth wave, or true earthquake shock, 
is immense, that it, at least, equals the velocity of sound in the same solids. 
The question then is, can a wave propagated under the conditions thus assigned 
by Michell, have any such velocity of transit ? 
Admitting, for a moment, that the crust of our earth bears any analogy to the 
flexible carpet of his experiment,—admitting that the enormous shell of, at least, 
forty-five miles in average thickness, could have flexibility enough to follow the 
