EARTHQUAKE. The dreadful conse quences attending sudden and violent tremblings of the earth, are sufficient reasons for attempting toaccount for their causes, but, as from the nature of those it is utterly impossible to ascertain them accurately, rational conjecture must be accepted in place of actual observation. Every writer on the effects of volcanic eruptions concurs in attributing earth quakes to their internal operations, which become less frequent in their neighbour hood after violent explosions through the craters ; indeed, it must be obvious on the first thought, that the furious ef forts of fire, hot air, and steam, exclu sive of electric matter, cannot but pro duce convulsive motion in the substances which confine them, and of more or less extent and violence, in proportion to the cause. When a free passage is ob tained, the agitation gradually subsides, and is only increased at intervals by the escape of accumulated matter. Sir Wil liam Hamilton, who carefully examined all the visible phenomena produced by the earthquakes of 1783, in Calabria, is de cidedly of opinion that they originated from under the sea, situated between the coast of Calabria and the island of Strom boli, or near the city of Oppida, which he illustrates, by forming a circle round those points, and remarking that the most de structive effects were there, whence they became gradually less ruinous. Mount Stromboli had been unusually quiet for some time previous to this calamitous pe riod, a circumstance highly favourable to his conjecture, founded besides on the experience of remote ages, when, ac cording to Strabo, Campania was sub ject to frequent and dreadful earth quakes, which were less common and ruinous after Vesuvius had become the means of dispersing the confined mat ter. Since that fortunate wra, the vicini ty of the above mountain has suffered more from the lava and ashes than earth quakes.
If stronger proofs were necessary, that volcanic eruptions cause this phenome non, the rising of hills near /Etna, or any other volcano, and the Lipari Islands, are sufficient demonstrations how severely the earth must be convulsed by subter raneous fires ; those consequences are not, however, always visible, earthquakes often occurring without any other effect than the overturning of slightly built edifices, an undulating or shaking motion, and a deadened sound ; but it by no means follows, that the origin of such were not to be attributed to fire, the va pours caused by which may roll through cavities, actually pass from a very great distance to a volcanic mountain, or es cape unperceived through the sea or ob scure vents.
The electric fluid being known to reside in the earth in very considerable quan tities, and always seeking an equilibrium, great abundance may sometimes collect through various causes in a particular spot, whence it will be attracted to ano ther less charged with this astonishing fluid ; when the tremendous conflict be tween it and the air is remembered dur ing a storm in our atmosphere, we must readily admit that it may produce a strong concussion in the earth, and probably be the origin of the slight earthquakes peculiar to some countries.
Another cause of inconsiderable tremb. bogs may, perhaps, proceed from the ope rations of subterraneous streams, which, rushing through caverns, and undermin ing vast bodies of earth and stone, those fall and shake the neighbouring parts in proportion to their bulk and weight. When the motion of the sea, during an earthquake, had destroyed the support of Port Royal, in Jamaica, the town sunk into it ; in the same manner new cavities occurring through the fall of earth, the surface must necessarily sink in the same degree, if it is within the influence of the cause, Frezier is of opinion that earthquakes should be ascribed to an effect of the wa ters, which appear to moisten the earth, in passages similar to the veins of living bodies. " Now the waters may occasion earthquakes after several manners, either by dissolving the salt scattered through the earth, or by penetrating through po rous lands, mixed with stones which they insensibly loosen ; and the fall or removal thereof must cause a stroke or shock, such as is felt in earthquakes. Lastly, the wa ter penetrating some sulphurous bodies must there cause a fermentation, and then the heat produces foul exhalations, which infect the air when they open the earth." This extract from a narrative of the dreadful earthquakes at Lima in 1746, is illustrated by the experiment of M. Le mery, related in the Memoirs of the French Academy of Sciences for 1700 ; that gentleman having mixed equal quan tities of filings of iron and sulphur, and tempered them with water into the con sistence of paste, buried them ; they some time afterwards agitated the earth, and finally burst into a flame. To con firm this effect of fire, however generated in the bowels of the earth, we shall quote the following paragraph from that accu rate modern observer Spallanzani, who was indebted to Professor Bottis for the facts contained in it, which relate to the production of seven small mountains by the eruption of Vesuvius in 1760. After repeated concussions of the earth, which were felt fifteen miles round Vesuvius, the sides of the fiery mountain opened in the territory of the Torre del Greco, and fifteen volcanos appeared, eight of which were soon after covered by a torrent of lava, which rushed from one of them ; the other seven remaining entire, and incessantly ejecting from their mouths vast quantities of ignited substances, which, falling almost perpendicularly around the volcanos, produced, in the short space of ten days, seven small mountains, of various heights, disposed in a right line. During these ejections, the noise which accompanied them some times resembled that of violent thunder, and at others the discharge of a number of cannons. Several of the burning stones, even the largest, were thrown to the height of 960 feet, and some fell at a considerable distance from the mouths whence they were thrown. These eruc tations shook all the neighbouring coun try, and the parings of the mountain were dreadful to the inhabitants.