another reason is, the paucity of pyrites in England.

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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, maing cracks and chasms in it some miles in length, which open at the instant of the shock, and close again in the intervals between them; nay, it is sometimes so violent that it forces the superincumbent strata, breaks them all throughout, and thereby per

fectly 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 Etna, 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 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.

Farther, 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 part 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, VOL. II.-19

as disgorging that fire which, while 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 seventyfourth part of the semi-diameter of the earth. And the vast sphere beyond 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 experiment 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 boiling water, it is very possible there may be

some whose violence, farther 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.

Chymistry 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 wet 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 Ætna. 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 tremour 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