The compass of this earthquake may be reckon'd to take up 300 miles
diameter, as a circle. Now, we cannot conceive, how any subterraneous
vapour can produce such an effect, as instantaneously to demolish all
these cities; and that such an accident should never happen after.
That the whole country of _Asia minor_ should not at the same time be
destroy'd, its mountains be renversed, its fountains, springs, and rivers
broken up and ruin'd for ever. Instead whereof we find nothing suffered,
but those cities; no kind of alteration in the surface of the country;
it remains the same as it were in the beginning of time. In 1586 an
earthquake in _Peru_, that extended 900 miles.
From these considerations, I cannot persuade myself, to enter into the
opinion of vapours, and eruptions being the cause sought for; and, after
we have treated the argument in a superficial view, we must go a little
deeper.
If we would consider things like philosophers, let us propose to ourselves
this problem: Where is the power to be plac'd, that is requir'd to move a
surface of earth 30 miles in diameter?
To answer this, consult the ingineers, and those that make mines in the
sieges of towns; they will acquaint us, that the effect of mines is
produced in form of an inverted cone. And that a diameter of 30 miles, in
the base, will require an _axis_ of 15 or 20 miles to operate upon that
base, so as to shake it, at least. Now the vapours, or whatever power we
propose to operate, according to the foregoing requisite, in order to form
the appearance of an earthquake, must be 15 or 20 miles deep in the earth.
But what mind can conceive, that any natural power is able to move an
inverted cone of solid earth, whose base is 30 miles diameter, whose axis
20? or was it possible; would not the whole texture of that body of earth
be quite disturb'd and shatter'd, especially in regard to its springs and
fountains? but nothing like this is ever found to be the consequence of an
earthquake, tho' fatal to cities.
Apply this reasoning to the earthquake of _Asia minor_, and this vigorous
principle at the _apex_ of the cone must lie, at least, 200 Miles deep in
the ground. Enough to show the absurdity of any moving power plac'd under
the Earth! A cone of 300 miles diameter at base, 200 miles _axis_: I dare
be bold to say, that all the gun-powder made since its invention, if put
together and fired, would not be able to move it; how much less pent up
vapours? what must we say of a circle of 900 miles diameter?
But, could that be admitted as possible, would any one be persuaded,
that such a subterraneous tumult, of so vast an extent, will be no ways
injurious to the internal system of springs and fountains, and that this
shall often be repeated without the least damage? We may as well imagine,
that we can stab a man 100 times and never touch vein or artery.
Since I gave in my two papers to the Royal Society, a letter of Mr.
_Flamsted_'s has been printed, which abundantly confirms my sentiments.
The whole drift of it is, to show how invalid is the vulgar idea
conceiv'd, of earthquakes arising from subterraneous vapours and
eruptions: That the earth itself is not moved to any depth, and that the
shock must arise from the atmosphere. The circumstances which he has
judiciously collected, are extremely agreeable to mine; many of them the
very same, strongly confirming my _hypothesis_: And had that great man
known the properties of electricity, which we are now masters of, he would
have prevented me in this affair.
"Considering (says he) what variety of substances, sand, gravel, stones,
rock, minerals, clay, and mold, our earth is compounded of, and how little
nitre, or explosive matter, a large quantity thereof will afford; I cannot
think, where we can find matter enough to move so vast a bulk of earth,
as all the South parts of _England_, all the _Netherlands_, with part
of _Germany_, all _France_, and perhaps _Italy_, (which were shock'd at
once the 8th of _September_ last 1692;) or part of _Asia_, and near all
_Europe_, which trembled together the same day, 91 years before.
"But, allowing there may have been sufficient matter prepared for these
purposes, I can hardly think, there are continued cavities, at any
reasonable depth, all under _Europe_, wherein an explosion being made,
might shake the whole at once, and yet make no clefts, or separations,
in those parts where the minerals and mountainous rocks part from the
light mold and clay. If an hundred barrels of gun-powder could be fixed
in some cave, a thousand yards under ground; allowing the force of the
explosion sufficient to raise all the weight of earth incumbent on the
cavern; it would certainly break the loose mold from any large solid rock
we may conceive adjacent, and leave at least some clefts behind it. But we
seldom or never hear of such clefts, made in such places, when earthquakes
happen."
Again, he writes thus: "I cannot apprehend, (if all earthquakes must
be made by explosions in subterraneous caverns) why sometimes a large
country, or whole continent, should be thereby shook all at once; why
there should be no eruptions in the neighbourhood?"
From all circumstances consider'd, he concludes, that the abstruse,
effective cause of them comes from the air; and that a calm is necessary
before an earthquake. And these two particulars are likewise Dr. _Hales_'s
positions: "The earth-lightning, as he calls it, is first kindled on the
surface, and not at great depths, as has been thought; whose explosion is
the immediate cause of an earthquake. He says, long, dry, hot seasons,
are usually the preparatory forerunners of earthquakes." From all these
considerations I conclude; earthquakes are not caus'd by subterraneous
vapors.