As no record exists at Cumana, and its archives, owing to the continual devastations of the termites, or white ants, contain no document that goes back farther than a hundred and fifty years, we are unacquainted with the precise dates of the ancient earthquakes. We only know, that, in times nearer our own, the year 1776 was at once the most fatal to the colonists, and the most remarkable for the physical history of the country. The city of Cumana was entirely destroyed, the houses were overturned in the space of a few minutes, and the shocks were hourly repeated during fourteen months. In several parts of the province the earth opened, and threw out sulphureous waters. These irruptions were very frequent in a plain extending towards Casanay, two leagues east of the town of Cariaco, and known by the name of the hollow ground (tierra hueca), because it appears entirely undermined by thermal springs. During the years 1766 and 1767, the inhabitants of Cumana encamped in their streets; and they began to rebuild their houses only when the earthquakes recurred once a month. What was felt at Quito, immediately after the great catastrophe of February 1797, took place on these coasts. While the ground was in a state of continual oscillation, the atmosphere seemed to dissolve itself into water.
Tradition states that in the earthquake of 1766, as well as in another remarkable one in 1794, the shocks were mere horizontal oscillations; it was only on the disastrous 14th of December, 1797, that for the first time at Cumana the motion was felt by an upheaving of the ground. More than four-fifths of the city were then entirely destroyed; and the shock, attended by a very loud subterraneous noise, resembled, as at Riobamba, the explosion of a mine at a great depth. Happily the most violent shock was preceded by a slight undulating motion, so that most of the inhabitants were enabled to escape into the streets, and a small number only perished of those who had assembled in the churches. It is a generally received opinion at Cumana, that the most destructive earthquakes are announced by very feeble oscillations, and by a hollow sound, which does not escape the observation of persons habituated to this kind of phenomenon. In those fatal moments the cries of 'misericordia! tembla! tembla!'* are everywhere heard (* "Mercy! the earthquake! the earthquake!"—See Tschudi's Travels in Peru page 170.); and it rarely happens that a false alarm is given by a native. Those who are most apprehensive attentively observe the motions of dogs, goats, and swine. The last-mentioned animals, endowed with delicate olfactory nerves, and accustomed to turn up the earth, give warning of approaching danger by their restlessness and their cries. We shall not attempt to decide, whether, being nearer the surface of the ground, they are the first to hear the subterraneous noise; or whether their organs receive the impression of some gaseous emanation which issues from the earth. We cannot deny the possibility of this latter cause. During my abode at Peru, a fact was observed in the inland country, which has an analogy with this kind of phenomenon, and which is not unfrequent. At the end of violent earthquakes, the herbs that cover the savannahs of Tucuman acquired noxious properties; an epidemic disorder broke out among the cattle, and a great number of them appeared stupified or suffocated by the deleterious vapours exhaled from the ground.
At Cumana, half an hour before the catastrophe of the 14th of December, 1797, a strong smell of sulphur was perceived near the hill of the convent of San Francisco; and on the same spot the subterraneous noise, which seemed to proceed from south-east to north-west, was loudest. At the same time flames appeared on the banks of the Manzanares, near the hospital of the Capuchins, and in the gulf of Cariaco, near Mariguitar. This last phenomenon, so extraordinary in a country not volcanic, is pretty frequent in the Alpine calcareous mountains near Cumanacoa, in the valley of Bordones, in the island of Margareta, and amidst the Llanos or savannahs of New Andalusia. In these savannahs, flakes of fire rising to a considerable height, are seen for hours together in the dryest places; and it is asserted, that, on examining the ground no crevice is perceptible. This fire, which resembles the springs of hydrogen, or Salse, of Modena, or what is called the will-o'-the-wisp of our marshes, does not burn the grass; because, no doubt, the column of gas, which develops itself, is mixed with azote and carbonic acid, and does not burn at its basis. The people, although less superstitious here than in Spain, call these reddish flames by the singular name of 'the soul of the tyrant Aguirre;' imagining that the spectre of Lopez Aguirre, harassed by remorse, wanders over these countries sullied by his crimes.* (* When at Cumana, or in the island of Margareta, the people pronounce the words el tirano (the tyrant), it is always to denote the hated Lopez d'Aguirre, who, after having taken part, in 1560, in the revolt of Fernando de Guzman against Pedro de Ursua, governor of the Omeguas and Dorado, voluntarily took the title of traidor, or traitor. He descended the river Amazon with his band, and reached by a communication of the rivers of Guyana the island of Margareta. The port of Paraguache still bears, in this island, the name of the Tyrant's Port.)
The great earthquake of 1797 produced some changes in the configuration of the shoal of Morro Roxo, towards the mouth of the Rio Bordones. Similar swellings were observed at the time of the total destruction of Cumana, in 1766. At that period, the Punta Delgado, on the southern coast of the gulf of Cariaco, became perceptibly enlarged; and in the Rio Guarapiche, near the village of Maturin, a shoal was formed, no doubt by the action of the elastic fluids, which displaced and raised up the bed of the river.
In order to follow a plan conformable to the end we proposed in this work, we shall endeavour to generalize our ideas, and to comprehend in one point of view everything that relates to these phenomena, so terrific, and so difficult to explain. If it be the duty of the men of science who visit the Alps of Switzerland, or the coasts of Lapland, to extend our knowledge respecting the glaciers and the aurora borealis, it may be expected that a traveller who has journeyed through Spanish America, should have chiefly fixed his attention on volcanoes and earthquakes. Each part of the globe is an object of particular study; and when we cannot hope to penetrate the causes of natural phenomena, we ought at least to endeavour to discover their laws, and distinguish, by the comparison of numerous facts, that which is permanent and uniform from that which is variable and accidental.
The great earthquakes, which interrupt the long series of slight shocks, appear to have no regular periods at Cumana. They have taken place at intervals of eighty, a hundred, and sometimes less than thirty years; while on the coasts of Peru, for instance at Lima, a certain regularity has marked the periods of the total destruction of the city. The belief of the inhabitants in the existence of this uniformity has a happy influence on public tranquillity, and the encouragement of industry. It is generally admitted, that it requires a sufficiently long space of time for the same causes to act with the same energy; but this reasoning is just only inasmuch as the shocks are considered as a local phenomenon; and a particular focus, under each point of the globe exposed to those great catastrophes, is admitted. Whenever new edifices are raised on the ruins of the old, we hear from those who refuse to build, that the destruction of Lisbon on the first day of November, 1755, was soon followed by a second, and not less fatal convulsion, on the 31st of March, 1761.
It is a very ancient opinion,* (* Aristotle de Meteor. lib. 2 (ed. Duval, tome 1 page 798). Seneca Nat. Quaest. lib. 6 c. 12.) and one that is commonly received at Cumana, Acapulco, and Lima, that a perceptible connection exists between earthquakes and the state of the atmosphere that precedes those phenomena. But from the great number of earthquakes which I have witnessed to the north and south of the equator; on the continent, and on the seas; on the coasts, and at 2500 toises height; it appears to me that the oscillations are generally very independent of the previous state of the atmosphere. This opinion is entertained by a number of intelligent residents of the Spanish colonies, whose experience extends, if not over a greater space of the globe, at least over a greater number of years, than mine. On the contrary, in parts of Europe where earthquakes are rare compared to America, scientific observers are inclined to admit an intimate connection between the undulations of the ground, and certain meteors, which appear simultaneously with them. In Italy for instance, the sirocco and earthquakes are suspected to have some connection; and in London, the frequency of falling-stars, and those southern lights which have since been often observed by Mr. Dalton, were considered as the forerunners of those shocks which were felt from 1748 to 1756.
On days when the earth is shaken by violent shocks, the regularity of the horary variations of the barometer is not disturbed within the tropics. I had opportunities of verifying this observation at Cumana, at Lima, and at Riobamba; and it is the more worthy of attention, as at St. Domingo, (in the town of Cape Francois,) it is asserted, that a water-barometer sank two inches and a half immediately before the earthquake of 1770. It is also related, that, at the time of the destruction of Oran, a druggist fled with his family, because, observing accidentally, a few minutes before the earthquake, the height of the mercury in his barometer, he perceived that the column sank in an extraordinary manner. I know not whether we can give credit to this story; but as it is nearly impossible to examine the variations of the weight of the atmosphere during the shocks, we must be satisfied with observing the barometer before or after these phenomena have taken place.
We can scarcely doubt, that the earth, when opened and agitated by shocks, spreads occasionally gaseous emanations through the atmosphere, in places remote from the mouths of volcanoes not extinct. At Cumana, it has already been observed that flames and vapours mixed with sulphurous acid spring up from the most arid soil. In other parts of the same province, the earth ejects water and petroleum. At Riobamba, a muddy and inflammable mass, called moya, issues from crevices that close again, and accumulates into elevated hills. At about seven leagues from Lisbon, near Colares, during the terrible earthquake of the 1st of November, 1755, flames and a column of thick smoke were seen to issue from the flanks of the rocks of Alvidras, and, according to some witnesses, from the bosom of the sea.
Elastic fluids thrown into the atmosphere may act locally on the barometer, not by their mass, which is very small, compared to the mass of the atmosphere, but because, at the moment of great explosions, an ascending current is probably formed, which diminishes the pressure of the air. I am inclined to think that in the majority of earthquakes nothing escapes from the agitated earth; and that, when gaseous emanations and vapours are observed, they oftener accompany or follow, than precede the shocks. This circumstance would seem to explain the mysterious influence of earthquakes in equinoctial America, on the climate, and on the order of the dry and rainy seasons. If the earth generally act on the air only at the moment of the shocks, we can conceive why a sensible meteorological change so rarely precedes those great revolutions of nature.