Earthquake, is a sudden and violent concussion of the earth, which is generally attended with ucommon noise, both in the air and under ground; inconsequence of which, whole cities are at once levelled, as well as rocks; the course of rivers is altered; and the most dreadful devastations are thus occasioned.
There is no phenomenon in nature, more ca'culated to impress the human mind with awe, than an earthquake ; but it has not till lately been investigated with philosophical precision, and he history of these events still remains very incomplete.
Of the observations, which in-defatigable naturalists have been able to collect, the following are the principal: 1. Where there are any volcanoes or burning mountains, earthquakes may naturally be expected to occur more frequently than in other countries. 2. Earthquakes are, in general, preceded by long droughts; but they do not always happen immediately after them. 3. They are likewise frequently indicated by certain electrical appearances in the atmosphere, namely, the aurora borealis, the falling of stars, etc.
4. A short time previous to the shock., the sea swells, with a loud noise; fountains are disturbed, and become muddy ; and the irrational animals appear frightened, as if conscious of approaching calamity.
5. The air, at the time of the shock, is in general very calm and serene; but afterwards becomes dark and cloudy. 6. The concussion begins with a rumbling noise, similar to thatof carriages : a rushing sound resembling the wind is sometimes heard ; at others, explosions not unlike the tiring of cannon ; and the ground is agitated in different directions. A single shock seldom exceeds a minute in its duration ; but frequent concussions succeed each other, at short intervals, for a considerable length of time. 7. During the shock, chasms are made in mad earth, whence flames, but oftcner vast quantities of water, are discharged. Flames and smoke are also emitted from spots of ground where no chasms are perceptible; and though the abysses formed in the earth are in general not extensive, yet in violent earthquakes they are frequently so large as to bury whole cities. 8. The water of the ocean is, on such occasions, affected perhaps still move than the land ; the sea now rising to a prodigious height, now divided to a considerable depth, and emitting great quantities of air, flames, and smoke. Similar agitations occur in the waters of ponds, lakes, and even rivers.
Lastly, the effects of earthquakes are not confined to one particular district or country, and frequently extend to very distant regions; though there is no instance of the whole globe having been convulsed it the same time.
The cause of earthquakes, or the theory of this tremendous phenomenon, is but imperfectly under-stood. It is, however, certain, that they arise from the confinement of air within the bowels of the earth, where it is generated by sulphureous vapeurs acting on different metallic ores, the principal and most copious of which appears to be iron. In confirmation of this theory, we shall only observe, that that artificial earthquakes may be easily-produced, by burying equal quantities of iron-filings and sulphur, mixed in a moist state, and confined in a vessel, so as to exclude the access of external air, and prevent the escape of the inflammable gas thus generated. In a few days (and, if large quantities be employed, in a few hours) this composition grows remarkably hot, and will explode with a violence and impetuosity resembling the natural phenomenon: - but we do not advise our junior readers to attempt such dangerous experiments. As it would be deviating from our plan to enter into farther particulars, Ave can only refer the inquisitive to the 73d vol. of the "Philosophical Transactions" of the Royal Society for 1733, where they will find an ample account of the latest and most awful earthquake that has happened in Europe, within the memory of man.