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 as sembling 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 Etna, 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

semidiameter of the earth. And the vast sphere be yond this depth, in diameter 6,451,538 fathoms, may probably be only filled with air; which will be here greatly condensed, and much heavier than the heaviest bodies we know in nature. But it is found by experi ment, that, the more air is compressed, the more does the same degree of heat increase its spring, and the more capable does it render it of a violent effect; and that, for instance, the degree of heat of boiling water increases the spring of the air above what it has in its natural state, in our climate, by a quantity equal to a third of the weight wherewith it is pressed. Whence we may conclude, that a degree of heat, which on the surface of the earth will only have a moderate effect, may be capable of a very violent one below. And, as we are assured, that there are in nature degrees of heat much more considerable than that of boiling water, it is very possible there may be some whose violence, further assisted by the exceeding weight of the air, may be more than sufficient to break and overturn this solid orb of 43,528 fathoms; whose weight, compared to that of the included air, would be but a trifle.

Chemistry furnishes us a method of making artificial earthquakes, which shall have all the great effects of natural ones; which, as it may illustrate the process of nature in the production of these terrible phenomena under ground, we shall here add.

To twenty pounds of iron filings, add as many of sulphur; mix, work, and temper the whole together with a little water, so as to form a mass, half moist and half dry. This being buried three or four feet under ground, in six or seven hours time, will have a prodigious effect; the earth will begin to tremble, crack, and smoke, and fire and flame burst through.

Such is the effect even of the two cold bodies, in

cold ground; there only wants a sufficient quantity of this mixture to produce a true Etna. If it were supposed to burst out under the sea, it would produce a spout; and, if it were in the clouds, the effect would be thunder and lightning.

An earthquake is defined to be a vehement shake, or agitation, of some considerable place, or part of the earth, from natural causes; attended with a huge noise like thunder, and frequently with an eruption of water, or fire, or smoke, or winds, &c.

They are the greatest and most formidable phenomena of nature. Aristotle and Pliny distinguish two kinds, with respect to the manner of the shake, viz. a tremor and a pulsation; the first being horizontal, in alternate vibrations, compared to the shaking of a person in an ague. The second perpendicular, up and down, their motion resembling that of boiling.

Agricola increases the number, and makes four kinds; which Albertus Magnus again reduces to three, viz. inclination, when the earth librates alternately from right to left, by which mountains have been sometimes brought to meet, and clash against each other; pulsation, when it beats up and down, like an artery; and trembling, when it shakes and totters every way, like a flame.

The Philosophical Transactions furnish us with abundance of histories of earthquakes; particularly one at Oxford, in 1665, by Dr. Wallis and Mr. Boyle. Another at the same place in 1683, by Mr. Pigot. Another in Sicily, in 1692-3, by Mr. Hartop, Father Alessandro Burgos and Vin. Bonajutus, which last is one of the most terrible ones in all history.

It shook the whole island; and not only that, but Naples and Malta shared in the shock. It was of the second kind mentioned by Aristotle and Pliny, viz. a

perpendicular pulsation or succussion. It was impossible, says the noble Bonajutus, for anybody in this country to keep on their legs, on the dancing earth; nay, those that lay on the ground, were tossed from side to side, as on a rolling billow; high walls leaped from their foundations several paces.

The mischief it did is amazing; almost all the buildings in the countries were thrown down. Fifty-four cities and towns, besides an incredible number of villages, were either destroyed or greatly damaged. We shall only instance the fate of Catania, one of the most famous, ancient, and flourishing cities in the kingdom; the residence of several monarchs, and a university. "This once famous, now unhappy Catania," to use words of Father Burgos, "had the greatest share in the tragedy. Father Antonio Serovita, being on his way thither, and at the distance of a few miles, observed a black cloud, like night, hovering over the city; and there arose from the mouth of Mongibello great spires of flame, which spread all around. The sea all of a sudden began to roar, and rise in billows; and there was a blow, as if all the artillery in the world had been at once discharged. The birds flew about astonished, the cattle in the fields ran crying, &c. His and his companion's horse stopped short, trembling; so that they were forced to alight. They were no sooner off, but they were lifted from the ground above two palms. When casting his eyes towards Catania, he with amazement saw nothing but a thick cloud of dust in the air. This was the scene of their calamity; for of the magnificent Catania, there is not the least footstep to be seen." Bonajutus assures us, that, of 18,914 inhabitants, 18,000 perished therein. The same author, from a computation of the inhabitants, before and after