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ON THE FACTS OF EARTHQUAKE PHENOMENA. 79 
_ But before quitting the region of conjecture, I would add a few words upon 
the probable nature of that force, or mode of application of that force, upon 
which the earthquake shock, the actual stroke, depends. We have seen 
sufficiently that this force must be an wnpulse, it must be of the nature of a 
blow, percussive; hence it is not produced by the direct action of the elevatory 
force itself, which acts slowly, liftingly (erhobenlich), as Humboldt says, and 
hydrostatieally. It may result occasionally from fractures, produced by the 
steady pressure of this evenly acting agency, yet these cannot be the usual 
or principal action, but only subordinate. 
Now the almost universal succession of phenomena recorded in earth- 
quakes is, first a trembling, then a severe ‘shock, or several in quick suc- 
cession, and then a trembling, gradually but rapidly becoming insensible. 
It would be possible to fill page after page with accounts of earthquake 
shocks, which all ring the changes, on a tremor beginning gently and in- 
creasing rapidly, then one or more violent shocks, like blows, and afterwards 
a trembling again, gradually dying away, rarely indeed the shock first. 
Thus to take one example for all, from Dr, Patrick Russell’s account of the 
earthquake in Syria, on 25th November 1759 :—“ About half-an-hour after 
seven at night the earthquake came on; the motion at first was gently tre- 
mulous, inereasing by degrees until the vibrations became more distinct and 
at the same time so strong as to shake the walls of the hause with consi- 
derable violence; they again became more gentle, and thus changed alter- 
nately seyeral times during the shock, which lasted in all about two minutes.” 
In general the average of numerous narratives seems to give from three 
or four to fifteen seconds as the duration of the great shocks; from two to 
ten or fifteen minutes for that of the powerful vibratory shakings; and an 
unlimited, or at least uncertain time, for slighter tremors afterwards. What 
sort of impulse then will be competent to account for this general order of 
succession? I belieye it will be found either in the sudden bringing into 
contact under pressure of large ignited surfaces with cold water, or the 
blowing, through and into, cold water, of volumes of steam under pressure, 
and this steam suddenly condensed therein. 
When an irruption of igneous matter takes place beneath the sea-hottom, 
the first action must be to open up large clefts or fissures in its rocky ma- 
terial, or to lift and remoye its incoherent portions, such as mud, gravel, &c. 
The first portions of water that gain access thus to the ignited surfaces, 
repelled by their heat, are brought into that peculiar state which Boutigny 
and others have called spheroidal. While in this condition their intestine 
motion may be great, but little steam is generated; and while this is the 
ease, no impulses will ever be conveyed to a distance, but only those trem- 
blings or vibrations which precede the shock, and which with wonderful 
acuteness Aristotle calls βράσται, ebullitions like those of a boiling ealdron ; 
but no sooner has the surface of lava become cooled to the point at which 
repulsion ceases, and the water, altering its state, comes into close contact 
with the heating surfaces, than a vast yolume of steam is evolved explosively, 
and, blown off into the deep and cold water of the sea, is as instantly con- 
densed; and thus a blow or impulse (or several of these), of the most tre- 
mendous sort, is given at the volcanic focus, and being transferred outwardly 
in all directions, is transmitted as the earthquake shock; but the surfaces 
of ignited material, now cooled down below the point at which steam can 
be generated rapidly, merely keep up a gentler ebullition, which is trans- 
mitted as the trembling after the shock, dying away as the mass grows cold, 
or again repeating all the phases, as the surfaces again become heated by 
conduction from the fervid magazine in the interior of the lava mass. 
Of course this may be endlessly varied. The first great blow may break 
