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The difference between these three terrible phenomena, he takes only to consist in this; that this sulphur, in the former, is fired in the air, and in the latter under ground. Which is a notion that Pliny had, long before
“ Quid enim," says he, “ aliud est in terrâ tremor, quam in nube tonitru?”
This he thinks abundantly indicated by the same sulphurous smell being found in any thing burnt with lightning, and in the waters, &c. cast up in earthquakes, and even in the air before and after them.
Add, that they agree in the manner of the noise, which is carried on, as in a train fired; the one, rolling and rattling through the air, takes fire as the vapors chance to drive; as the other fired under ground, in like manner, moves with a desultory noise.
Thunder, which is the effect of the trembling of the air, caused by the same vapors dispersed through it, has force enough to shake our houses; and why there may not be thunder and lightning under ground, in some vast repositories there, I see no reason ; especially if we reflect, that the matter which composes the noisy vapor above · us is in much larger quantities under ground.
That the earth abounds in cavities, everybody allows; and that these subterraneous cavities are, at certain times and in certain seasons, full of inflammable vapors, the damps in mines sufficiently witness, which fired do every thing as in an earthquake, save in a lesser degree.
Add, that the pyrites alone, of all the known miner als, yields this inflammable vapor, is highly probable ; for that no mineral or ore, whatsoever, is sulphurous, but as it is wholly, or in part, a pyrites; and that there is but one species of brimstone, which the pyrites naturally and only yields. The sulphur vive, or natural brimstone, which is found in and about the burning mountains, is certainly the effects of sublimation; and those great quantities of it, said to be found about the skirts of volcanoes, is only an argument of the long duration and vehemence of those fires. Possibly the pyrites of the volcanoes, or burning mountains, may be more sulphurous than ours; and indeed it is plain, that some of ours in England are very lean, and hold but little sulphur; others again, very much; which may be one reason why England is so little troubled with earthquakes, and . Italy, and almost all round the Mediterranean Sea, so very much; though another reason is, the paucity of pyrites in England.
Comparing our earthquakes, thunder, and lightning, with theirs, it is observed, that there it lightens almost daily, especially in summer-time, here seldom; there thunder and lightning is of long duration, here it is soon over; there the earthquakes are frequent, long, and terrible, with many paroxysms in a day, and that for many days; here very short, a few minutes, and scarce perceptible. To this purpose the subterraneous caverns in England are small and few compared to the vast vaults in those parts of the world; which is evident from the sudden disappearance of whole mountains and islands.
Dr. Woodward gives us another theory of earthquakes. He endeavours to show, that the subterraneous heat, or fire, (which is continually elevating water out of the abyss, to furnish the earth with rain, dew, springs, and rivers,) being stopped in any part of the earth, and so diverted from its ordinary course, by some accidental glut or obstruction in the pores or passages through which it used to ascend to the surface, becomes, by such means, preternaturally assembled in a greater quantity than usual into one place, and therefore causeth a great rarefaction and intumescence of the water of the abyss; putting it into great commotions and disorders, and at the same time making the like effort on the earth; which being expanded upon the face of the abyss, occasions that agitation and concussion we call an earthquake.
This effort in some earthquakes, he observes, is so vehement, that it splits and tears the earth, making cracks and chasms in it some miles in length, which open at the instant of the shock, and close again in the intervals betwixt them; nay, it is sometimes so violent, that it forces the superincumbent strata, breaks them all throughout, and thereby perfectly undermines and ruins the foundation of them; so that, these failing, the whole tract, as soon as the shock is over, sinks down into the abyss, and is swallowed up by it; the water thereof immediately rising up and forming a lake in the place where the said tract before was. That, this effort being made in all directions indifferently, the fire, dilating and expanding on all hands, and endeavouring to get room and make its way through all obstacles, falls as foul on the waters of the abyss beneath, as on the earth above, forcing it forth, which way soever it can find vent or passage, as well through its ordinary exits, wells, springs, and the outlets of rivers, as through the chasms then newly opened; through the camini or spiracles of Ætna, or other neighbouring volcanoes ; and those hiatuses at the bottom of the sea, whereby the abyss below opens into it and communicates with it. That, as the water resident in the abyss is, in all parts of it, stored with a considerable quantity of heat, and more especially in those where those extraordinary aggregations of this fire happen, so likewise is the water which is thus forced out of it; insomuch that, when thrown forth and mixed with the waters of wells or
springs of rivers and the sea, it renders them very sensibly hot.
He adds, that, though the abyss be liable to those commotions in all parts, yet the effects are nowhere very remarkable except in those countries, which are mountainous, and consequently stony or cavernous underneath; and especially where the disposition of the strata is such, that those caverns open into the abyss, and so freely admit and entertain the fire, which assembling therein, is the cause of the shock; it naturally steering its course that way where it finds the readiest reception, which is towards those caverns. Besides that those parts of the earth which abound with strata of stone or marble, making the strongest opposition to this effort, are the most furiously shattered, and suffer much more by it, than those which consist of gravel, sand, and the like laxer matter, which more easily give way, and make not so great resistance. But, above all, those countries which yield great store of sulphur and nitre, are by far the most injured by earthquakes; those minerals constituting in the earth a kind of natural gunpowder, which, taking fire upon this assemblage and approach of it, occasions that murmuring noise, that subterraneous thunder, which is heard rumbling in the bowels of the earth during earthquakes, and, by the assistance of its explosive power, renders the shock much greater, so as sometimes to make miserable havoc and destruction.
And it is for this reason, that Italy, Sicily, Anatolia, and some parts of Greece have been so long and often alarmed and harassed by earthquakes; these countries being all mountainous and cavernous, abounding with stone and marble, and affording sulphur and nitre in great plenty
Further, that Ætna, Vesuvius, Hecla, and the other
volcanoes are only so many spiracles, serving for the discharge of this subterraneous fire, when it is thus preternaturally assembled. That where there happens to be such a structure and conformation of the interior parts of the earth, as that the fire may pass freely, and without impediment, from the caverns wherein it assembles unto those spiracles, it then readily gets out from time to time, without shaking or disturbing the earth; but, where such communication is wanting, or passage not sufficiently large and open, so that it cannot come at the spiracles, it heaves up and shocks the earth with greater or lesser impetuosity, according to the quantity of fire thus assembled, till it has made its way to the mouth of the volcano. That therefore there are scarce any countries much annoyed by earthquakes, but have one of these fiery vents; which are constantly in flames when any earthquake happens; as disgorging that fire, which, whilst underneath, was the cause of the disaster. Lastly, that, were it not for these diverticula, it would rage in the bowels of the earth much more furiously, and make greater havoc than it doth.
We have seen what fire and water may do; and that either of them are sufficient for all the phenomena of earthquakes; if they should both fail, we have a third agent, scarce inferior to either of them; the reader must not be surprised when we tell him it is air.
Monsieur Amontons, in his Mémoires de l'Académie des Sciences, An 1703, has an express discourse to prove, that, on the foot of the new experiments of the weight and spring of the air, a moderate degree of heat may bring the air into a condition capable of causing earthquakes. It is shown, that, at the depth of 43,528 fathoms below the surface of the earth, air is only one fourth less heavy than mercury. Now, this depth of 43,528 fathoms is only a seventy-fourth part of the