Page:Popular Science Monthly Volume 28.djvu/841

Rh retumbos. I estimate the average number of shocks felt annually in Central America at two hundred and fifty. Several conclusions may be drawn from the study of the twenty-three hundred and thirty-two earthquakes that have been registered since the conquest. First, contrary to the opinion generally prevailing from Chili to Mexico, the tremors occur about alike through the whole year, and not principally at the transitions between the rainy and dry seasons. But, to perceive this clearly, it is necessary to leave out of the account some series of earthquakes that mask the truth, such as that of December, 1879, at San Salvador, in which more than seven hundred shocks occurred in ten days, and which was the prelude to the appearance of a new volcano in the center of Lake Ilopango. With this precaution, a tendency to equality may be observed between the several months, and I am satisfied that a term of four years will be sufficient to make this equality plain. The same may be said of the retumbos. The maximum of eruptions appears to occur in July. Kluge puts it in August for the whole globe. The coincidence which the same author has predicated between the maxima of auroræ boreales and sun-spots and of volcanic and seismic manifestations has not been historically verified in Central America. The minute study of twenty years of observations at the Institute of Guatemala and my own observations at San Salvador have proved to me that, if the movements of the crust of the earth are connected with those of the barometer, the law of the relation is deeply hidden. I do not deny it, but I have observed nothing analogous to what Scrope believes he has established for Stromboli, and Waltershausen for Etna. Earthquakes and retumbos are apparently more frequent at night than in the daytime. I say apparently, because it may be that manifestations, quite perceptible in the stillness of the night, pass unobserved amid the bustle of the day. From what I have seen, I think I can affirm that the signs of terror given by domestic animals are more marked the longer the shock lasts, and that without reference to its intensity.

While I do not think that it is possible in the present state of knowledge to predict earthquakes, I believe that the phenomena are frequently connected with an indefinable aggregation of atmospheric conditions which, subjected to many years of study, might lead to the discovery of some law. This is so true that persons who have lived long in the country often say when they meet, without knowing why, "There will be an earthquake to-day"; and they are seldom mistaken. Towns in Central America, situated near active volcanoes, have much less to fear than those which, being in the dangerous zone, are more distant from them. This may be proved by the local history. Guatemala was destroyed seven times, between 1541 and 1773, while it was near the extinct volcano of Agua; but it has not suffered since 1775, when it was removed to its present position near the active volcano of Fuego, of which forty-four eruptions have taken place. Izalco, built