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frequently take their rise from the sea. According to the description of the structure of the earth before given, any combustible stratum must lie at greater depths in places under the ocean than elsewhere; hence far more extensive fires may subsist there than where the quantity of matter over them is less; for any vapour raised from such fires, having both a stronger roof over it, and being pressed by a greater weight, beside the additional weight of the water, will not only be less at liberty to expand itself, and consequently of less bulk, but it will also be easily driven away towards the parts round about, where the superincumbent matter is less, and therefore lighter. On the other hand, any vapour raised from fires, where the superincumbent matter is lighter, finding a weaker roof over it, and being not so easily driven away under strata that are thicker and heavier, will be very apt to break through, and open a mouth to a volcano; and it must necessarily do this long before the fires can have spread themselves sufficiently to be nearly equal to those which may subsist in places that lie deeper. All this seems to be greatly confirmed by the situation of volcanos, which are almost always found on the tops of mountains, and those often some of the highest in the world.
If then the largest fires are to be supposed to subsist under the ocean, it is no wonder that the most extensive earthquakes should take their rise from thence: the great earthquake of Lisbon has been shown to have done so; and that the cause of it was also at a greater depth than that of many others, appears from the greater velocity with which it was propagated.
The great earthquake that destroyed Lima and Callao, in 1746, seems also to have come from the sea; for several of the ports on the coast were overwhelmed by a great wave, which did not arrive till 4 or 5 minutes after the earthquake began, and which was preceded by a retreat of the waters as well as that at Lisbon. Against this it may perhaps be alleged, that there were 4 volcanos broke out suddenly in the neighbouring mountains, when this earthquake happened, and that the fires of these might be the occasion of it. This however is not very probable; for, to omit the argument of the wave and previous retreat of the waters, already mentioned, it is not very likely that more than one fire was concerned: besides, the vapour, opening itself a passage at these places, could not well be supposed, if it took its rise from thence, to spread itself far; especially towards the sea, where it is manifest that the strata over it were of great thickness, as appears from the great velocity with which the earthquake was propagated there; the shocks also continued with equal, or nearly equal, violence, for some months after the openings were made; whereas, if these fires had been the cause of them, they must immediately have ceased on the fires finding a vent, as it has happened in other cases. It is therefore much more probable that a very large quantity of vapour, taking its rise from some far
more extensive fire under the sea, spread itself from thence, and as it passed in places where the roof over it was naturally much thinner, as well as greatly weakened by the undermining of these fires, it opened itself a passage and burst forth.
As the most extensive earthquakes generally proceed from the lowest countries, but especially from the sea, so those of a smaller extent are generally found among the mountains: hence it almost always happens that earthquakes which are felt near the sea, if at all-violent, are felt also in the higher lands; whereas there are many among the hills, and those very violent ones, which never extend themselves to the lower countries. Thus we are told that at Jamaica "shakes often happen in the country, not felt at Port Royal: and sometimes are felt by those that live in and at the foot of the mountains, and by nobody else." On the other hand, the earthquake that destroyed Port Royal extended itself all over the island: and the same was observed of a smaller earthquake that happened there in 1687-8; which latter undoubtedly came from the sea, as appears by Sir Hans Sloane's account of it.
Earthquakes of small extent are also very common among the mountains of Peru and Chili. Antonio d'Ulloa says, "Whilst we were preparing for our departure from the mountain Chichi-Choco, there was an earthquake which was felt 4 leagues round about: our field tent was tossed to and fro by it, and the earth had a motion like that of waves; this earthquake however was one of the smallest that commonly happen in that country." The same author tells us, in another place, that "during his stay at the city of Quito, or in the neighbourhood of it, there were two earthquakes, violent enough to overturn some houses in the country, which buried several persons under their ruins.”
Sect. 5.-It is generally found that earthquakes in hilly countries are much more violent than those which happen elsewhere; and this is observed to be the case, as well when they take their rise from the lower countries as among the hills themselves. This appearance being so easily to be accounted for, from the structure of the earth already described, Mr. M. contents himself with establishing the certainty of a fact which tends so greatly to confirm it.
The earthquakes that have infested some of the towns in the neighbourhood of Quito, have not only been incomparably more violent than that which destroyed Lisbon, but they seem to have exceeded that also which destroyed Lima and Callao. In Lisbon, many of the houses were left standing, though few of them were less than 4 or 5 stories high. At Lima also, it is only said that "all the buildings, great and small, or at least the greatest part of them, were destroyed." Callao likewise, as it appears from the accounts we have of it, had many houses left unhurt by the earthquake, till the wave came which overwhelmed the whole town, and threw down every thing that lay in its way. All
these effects seem to be greatly short of those produced by an earthquake that happened at Latacunga, in the year 1698, when the whole town, consisting of more than 600 houses, was entirely destroyed in less than 3 minutes time, a part of one only escaping; notwithstanding that the houses there are never built more than one story high in order, if possible, to avoid these dangers. Ambato, a village about the same size as Latacunga, together with a great part of Riobamba, another town in the same neighbourhood, were also entirely destroyed, or received considerable damage from it. At the same time a volcano burst out suddenly in the neighbouring mountain of Carguayraso: and “near Ambato the earth opened in several places, and there yet remains to the south of that town a cleft of 4 or 5 feet broad, and about a league in length, lying north and south; there are also several other like clefts on the other side of the river." The city of Quito was affected at the same time, but received no damage, though it is no more than 42 geographical miles from Latacunga, not far from which the greatest violence of the shock seems to have exerted itself. These towns are supposed to stand by far the highest of any in the world, being as high above the level of the sea as the tops of some of the highest mountains in Europe; and the ground on which Riobamba stands, wants but 90 yards of being 3 times as high as Snowdon, the highest mountain in Wales.
The country on which these towns stand serves as a base, from which arise another set of high lands and mountains, which are much the highest in the known world. Among these mountains there are no less than 6 volcanos, if not more, within an extent of 120 miles long, and less than 30 broad, the lowest of which exceeds the height of Riobamba by above of a mile, and the highest by more than twice that quantity. Now, as the earthquakes have been more violent at the foot of these mountains than in the lower lands, so they have been still more violent towards the tops of them: this is sufficiently manifest from the many rents made in them, and the rocks that have been broken off from them on such occasions: but it appears still more manifestly, and beyond all dispute, in the bursting forth of volcanos, which are almost always at the very summit of the mountains, where they are found. In these instances, the earth, stones, &c. which lay over the fire, are generally scattered by the violence of the vapour, that breaks its way through, to the distance of some miles round about.
The great earthquake of the 1st of November 1755, was also more violent among the mountains than at the city of Lisbon. We are told that "the mountains of Arrabida, Estrella, Julio, Marvan, and Cintra, being some of the largest in Portugal, were impetuously shaken, as it were, from their very foundations; and most of them opened at their summits, split and rent in a wonderful manner, and huge masses of them were thrown down into the subjacent vallies."
The same was observed at Jamaica likewise. In the earthquake that destroyed Port-Royal in 1692, we are told, that " more houses were left standing at that town than in all the island besides. It was so violent in other places that people were violently thrown down on the ground, where they lay with their legs and arms spread out, to prevent being tumbled about by the incredible motion of the earth. It scarcely left a planter's house or sugar-work standing all over the island: I think it left not a house standing at Passage-fort, and but one in all Liganee, and none in St. Iago, except a few low houses, built by the wary Spaniards. In Clarendon precinct the earth gaped, and spouted up with a prodigious force great quantities of water into the air, 12 miles from the sea; and all over the island there were abundance of openings of the earth, many thousands. But in the mountains, are said to be the most violent shakes of all; and it is a generally received opinion, that the nearer to the mountains the greater the shake; and that the cause thereof, whatever it is, lies there. Indeed they are strangely torn and rent, especially the blue, and other highest mountains, which seem to be the greatest sufferers, and which, during the time that the great shakes continued, bellowed out prodigious loud noises and echoings.
"Not far from Yallowes, a mountain, after having made several moves, overwhelmed a whole family, and a great part of a plantation lying a mile off; and a large high mountain near Portmorant, near a day's journey over, is said to be quite swallowed up.
"In the blue mountains, whence came those dreadful roarings, may reasonably be supposed to be many strange alterations of the like nature; but those wild desert places being very rarely, or never, visited by any body, we are yet ignorant of what happened there; but whereas they used to afford a find green prospect, now one half part of them, at least, seem to be wholly deprived of their natural verdure."
SECT. VI. Mr. M. has supposed that fires lying at the greatest depths generally produce the most extensive earthquakes; we must however except from this rule those cases where the depths are very great: for, as the weight of 3 miles perpendicular of common earth is capable of absolutely repressing the vapour of inflamed gunpowder, so we may well suppose that there may be a quantity of earth sufficient to repress the vapour of water, and keep it within its original limits, though ever so much heated. Now, whenever this is the case, it is manifest that it can produce no effect: or it may happen, that though the quantity of earth may not be sufficient absolutely to repress the vapour, yet it may so great, as to suffer it to expand but very little: in this case an earthquake arising from it would be but of small extent; the wave-like motion would be little or none; the vibratory motion would be felt every where; and the propagation of the motion would be very quick. This last circumstance being almost the only
one, by which these earthquakes can be known from those which owe their origin to shallower fires, it must be very difficult to distinguish them with certainty, as it is almost impossible to distinguish the difference of the time of their happening in different places, when the whole perhaps is comprehended within the space of 2 or 3 minutes; possibly however some of the earthquakes which we have had in England may have been of this class.
Sect. 7.-If we would inquire into the place of the origin of any particular earthquake we have the following grounds to go upon: 1st, The different directions in which it arrives at several distant places: if lines be drawn in these directions, the place of their common intersection must be nearly the place sought but this is liable to great difficulties; for there must necessarily be great uncertainty in observations which cannot, at best, be made with any great precision, and which are generally made by minds too little at ease to be nice observers of what passes; also the directions themselves may be somewhat varied, by the inequalities in the weight of the superincumbent matter, under which the vapour passes, as well as by other causes. 2ndly, We may form some judgment concerning the place of the origin of a particular earthquake from the time of its arrival at different places; but this also is liable to great difficulties. In both these methods however we may come to a much greater degree of exactness by taking a medium among a variety of accounts, as they are related by different observers. But, 3dly, we may come to the greatest' degree of exactness in those cases where earthquakes have their source from under the ocean; for in these instances the proportional distance of different places from that source may be very nearly ascertained by the interval between the earthquake and the succeeding wave: and this is the more to be depended on, as people are much less likely to be mistaken in determining the time between two events which follow one another at a small interval than in observing the precise time of the happening of some single event.
Let us now, by way of example, endeavour to inquire into the situation of the cause that gave rise to the earthquake of Nov. 1st, 1755, the place of which seems to have been under the ocean, somewhere between the latitudes of Lisbon and Oporto, (though probably somewhat nearer to the former,) and at the distance perhaps of 10 or 15 leagues from the coast. For, 1st, the direction in which the earthquake arrived at Lisbon was from the north-west; at Madeira it came from the north-east; and in England it came from the south-west; all of which perfectly agree with the place assumed. 2dly, The times in which the earthquake arrived at different places agree also with the same point. And, 3dly, the interval between these, and the time of the arrival of the subsequent wave, concur in confirming it. That all this might the better appear, Mr. M. subjoined the following table, assuming the point,