That there are many substances exist ing within the depth of the earth, which, coalesced, produce fire, cannot be disput ed, but that they exist in such amazing quantities as to afford fire for centuries, seems at least problematical ; there are therefore but two ways of accounting for their continuance, either that the volatile effluvia of the ignited matter, collected on the sides of volcanic caverns, becomes new fuel ; or that heat being necessary for the various properties of the globe, a self-existent fire, coeval with the creation.
has been placed within it by the wisdom of the Creator, an idea not of more doubt ful evidence than the existence of the electric fluid in visible, or any other of the phenomena of nature by which we are surrounded. (See Vozcirso.) Admit ting these' premises, another cause of earthquakes occurs ; it is well known, that volcanos communicate with the sea, by the frequent discharge of saline wa ter and tufa, or the slime and mud of its bottom, through their craters ; this inter fering with the operations of the fire, vast bodies of steam must ensue, than which nothing can be more powerful and insi nuating; this rushing by the force of vio lent explosions through every aperture of the various stratas of the surrounding earth, must occasion those horizontal and perpendicularmovements and tremblings, so terrific to the inhabitants above ; be sides, hot steams impregnated with sulphurous vapours often attend earth quakes.
Homer, whose knowledge was exten sive, seems to have been aware that the sea caused earthquakes, several instances of which might be quoted from the Iliad : " But Neptune rising from the seas profound, The God whose earthquakes rock the solid ground." Book xiii. 1. 67.
And in the xxth book, line 77.
" Beneath stern Neptune shakes the solid ground ; The forests wave, the mountains nod around ; Through all their summits tremble Ida's woods, And from their sources boil her hundred floods." Indeed, the sudden eruption of stones and calcined matter seem unquestionably the efiect of water flying off in steam, and carrying every loose object with it. The sea, or water of any great extent, always indicates the commencement of an earth quake, before it is otherwise perceived ; this circumstance (loth not proceed from any cause peculiar to the component parts of the water, but merely from the motion of the earth under the bottom, which is not felt by a person on the adjacent shore, probably from its gliding steadily in one direction, and returning in the same man ner ; but water, ever seeking a level, will rise at the remotest influence from the land, as that inclines towards it, and then rush precipitately back to its pre vious level, as far as it can be attained, before another inclination of the earth prevents it. The sea is observed to re
tire before the eruptions of Vesuvius, which is evidently caused by the ris ing of the earth during the first efforts of the matter endeavouring to escape out of the mountain ; when that is dis charged, the water flows bs,ck again im petuously, plainly indicating that . the earth has again sunk to its original place ; the same effect was noticed at Lisbon, as far as related to the dreadful agita tion of the sea, from which vessels re ceived violent 'shocks at fifty leagues distance; indeed, the effects of that earth quake, so fatal to the city above named in 1755, were felt throughout Europe and America.
Having detailed some of the probable causes of earthquakes, it will be pro per to mention the indications of their . approach in those countries where they are most prevalent, which is in Mexico, Peru, Jamaica, and the neighbouring islands ; Italy, particularly in Sicily ; Asia Minor, and Portugal ; they are felt in al most every other country, but so slightly as seldom to occasion serious injury. Be ginning near their visible causes, where they are necessarily most frequent, it may be observed, that when a long inter val has occurred from the last eruption of a volcano, there is just reason for alarm that the succeeding will be introduced by violent concussions of the earth. There are some phenomena which attend earth quakes of decided certainty, others may happen accidentally near the time of their approach, and be attributed erroneously to them ; such as very dry and hot sea sons, which undoubtedly take place fre quently where shocks are but little known, and dark atmospheres caused by unusual vapours ; of the latter, many in stances are recorded without the least calamity following ; indeed, the case of Lisbon is directly in point against this be• i ng universally an indication of earthquake, for the morning of that dreadful day was particularly fine, and the sun shone with the utmost brilliancy. Neither is the sud den ebbing and flowing of the sea, or rivers, always to be depended upon as the forerunner of convulsions of the earth, though it is uniformly the consequence of them, as such effects have been observed without any assignable cause. Electrical phenomena sornetimes attend them, in violent streams of lightning, the aurora borealis, meteors, &c. but as these are common appearances, they afford no just cause of alarm, if a trembling of the earth doth not very soon succeed.