218 



HARDWICKE'S SCIENCE-GOSSIP. 



[Oct. 1, 1SG8. 



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 im- 

 possible 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 Biobamba a muddy and inflam- 

 mable 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 Novem- 

 ber, 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 equinoc- 

 tial 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. 



The hypothesis according to which, in the earth- 

 quakes of Cumana, elastic fluids tend to escape 

 from the surface of the soil, seems confirmed by the 

 great noise which is beard during the shocks at the 

 borders of the wells in the plain of Charas. Water 

 and sand are sometimes thrown out twenty feet 

 high. Similar phenomena were observed in ancient 

 times by the inhabitants of those parts of Greece 

 and Asia Minor abounding with caverns, crevices, 

 and subterraneous rivers. Nature, in her uniform 

 progress, everywhere suggests the same ideas of 

 the causes of earthquakes, and the means by which 

 man, forgetting the measure of his strength, pre- 



tends to diminish the effect of the subterraneous 

 explosions. What a great Roman naturalist has 

 said of the utility of wells and caverns is repeated 

 in the New World by the most ignorant Indians of 

 Quito, when they show travellers the guaicos, or 

 crevices of Pichincha. 



The subterranean noise, so frequent during earth- 

 quakes, is generally not in the ratio of the force of 

 the shocks. At Cumana it constantly precedes 

 them, while at Quito, and recently at Caracas, and 

 in the West India islands, a noise like the discharge 

 of a battery was heard a long time after the shocks 

 had ceased. A third kind of phenomenon, the most 

 remarkable of the whole, is the rolling of those 

 subterranean thunders, which last several months, 

 without being accompanied by the least oscillatory 

 motion of the ground. 



In every country subject to earthquakes, the point 

 at which, probably owing to a particular disposition 

 of the stony strata, the effects are most sensibly felt, 

 is considered as the cause and the focus of the 

 shocks. Thus, at Cumana, the hill of the castle of 

 San Antonio, and particularly the eminence on 

 which stands the convent of St. Francis, are believed 

 to contain an enormous quantity of sulphur and 

 other inflammable matter. We forget that the 

 rapidity with which the undulations are propagated 

 to great distances, even across the basin of the 

 ocean, proves that the centre of action is very 

 remote from the surface of the globe. From this 

 same cause no doubt eai'thquakes are not confined 

 to certain species of rocks, as some naturalists sup- 

 pose, but all are fitted to propagate the movement. 

 Keeping within the limits of my own experience, I 

 may here cite the granites of Lima and Acapulco ; 

 the gneiss of Caracas ; the mica-slate of the penin- 

 sula of Araya ; the primitive thonschiefer of Tepe- 

 cuacuilco, in Mexico ; the secondary limestones of 

 the Apennines, Spain, and New Andalusia ; and, 

 finally, the trappean porphyries of the provinces of 

 Quito and Popayan. In these different places the 

 ground is frequently agitated by the most violent 

 shocks ; but sometimes, in the same rock, the 

 superior strata form invincible obstacles to the 

 propagation of the motion. Thus, in the mines of 

 Saxony, we have seen workmen hasten up alarmed 

 by oscillations which were not felt at the surface of 

 the ground. 



If, in regions the most remote from each other, 

 primitive, secondary, and volcanic rocks share 

 equally in the convulsive movements of the globe, 

 we cannot but admit also that within a space of 

 little extent certain classes of rocks oppose them- 

 selves to the propagation of the shocks. At 

 Cumana, for instance, before the great catastrophe 

 of 1797, the earthquakes were felt only along the 

 southern and calcareous coast of the Gulf of Cariaco, 

 as far as the town of that name; while in the penin- 

 sula of Araya, and at the village of Maniquarez, the 



