60 Mr. Matter on the Dynamics of Earthquakes. 
calculation respecting the waves that rolled upon the shores of Europe, Great 
Britain, Madeira, and the West Indies, &c., at the great Lisbon earthquake in 
1755. 
He finds that these waves, if produced at a point indicated by their respective 
directions of motion, and situated some thirty leagues or so at sea, about midway 
between Oporto and Lisbon, must have diverged thence with very different velo- 
cities, to have reached these several shores at the observed times; and this he 
accounts for by asserting that ‘the true reason of this disproportion in the velo- 
city of the waves seems to be in the difference of the depth of water through 
which they had passed ;”” those moving most rapidly which passed over the deep 
ocean, and those most slowly that entered the shallower seas or estuaries. 
The latter conclusion shows the highest acumen in the author, writing at a 
time when the laws of tidal waves were so ill understood, and had engaged so 
little attention. 
To return to the question, why are displaced bodies (such as the stones of 
the Calabrian obelisks) not replaced, more or less completely, by the back stroke, 
or return motion, of the wave ? 
Starting from the proposition that earthquake motion consists in the transit of 
an elastic wave, or a succession of these, through the solid crust of the globe, pro- 
duced by any original impulse, which may be either percussive, as when vast masses 
of rock suddenly give way, and are broken up by the pressure of elastic or liquid 
matter from below, or such as operates (if it were possible, as Michell assumes) 
when vast cavities, filled with high-pressure steam, or water in an incandescent 
state, are broken up, and the steam instantly condensed, and its place supplied with 
cold water, producing a shock, such as we observe on a minute scale when steam 
is blown from a pipe into cold water; or of the nature of that called into action 
when masses of material, resisting the ventage of volcanoes, are at last blown 
away, and produce a mighty recoil by the explosion, as in the case recorded by 
Humboldt, of a mass of rock weighing above two hundred tons, which was shot 
from the crater of Cotopaxi to a distance of nine miles; which of these, or what 
other sufficient cause, whether existing in nature or not, may be assigned for the 
original impulse, it is unnecessary for us here to consider: only let an impulse of 
some sort be granted, and we are to follow out its consequences according to the 
known laws of waves or pulses in zriform, liquid, and solid bodies. 
