EARTHQUAKES AND THEIR CAUSES. 735 



Such being some of the more prominent peculiarities attending 

 earthquakes, let us now apply them to the theories above referred to, 

 and endeavor to ascertain the causes of these distui'bances or the agen- 

 cies employed in producing them. 



They do not support the theory of the foiling in of caverns being 

 the cause of these phenomena ; for they ai-e invariably attended by an 

 upheaval of the ground, and often with a horizontal or a circular 

 motion. This theory, therefore, cannot be maintained, and more es- 

 pecially as it does not explain the electric and magnetic accompani- 

 ments. 



The hypothesis that they are caused by steam or the explosion of 

 confined gases has scarcely a better foundation. These agents might 

 produce vertical motion and subterranean noises, but it is difficult to 

 conceive how they could bring about circular motion at the surface ; 

 and it is quite impossible that the explosion of gases or the escape of 

 steam could, immediately preceding a shock, attract the clouds float- 

 ing in the atmosphere, so that they should remain fixed over particu- 

 lar spots. Other characteristics also cannot' be explained on this 

 theory, as the lightning and disturbance of the compass. 



The volcanic and igneous theory is not so easily to be disposed 

 of; for it appears very clear that volcanic eruptions do produce earth- 

 quake. A remarkable instance is that of Santorini in 1650. Earth- 

 quake is also very comnion where volcanic action is extensively 

 developed, as in South America and the neighborhood of Etna and 

 Vesuvius. 



Volcanoes produce these disturbances in two ways: 1. By their 

 own direct motion ; 2. By disturbing the electric equilibrium in their 

 neighborhood. This electric disturbance was noticed by Pliny, who 

 records that an officer, one of the Decuriones Municipales of Pompeii, 

 was struck by lightning in 79, although the sky was perfectly un- 

 clouded ; and these indications have been put to practical use. The 

 presence of lightning is also a prominent feature during volcanic erup- 

 tions. When Kattleguia, in Iceland, now extinct, was last in a state 

 of eruption, lightning proceeded from it and killed a farmer and his 

 servant, togetlier with some horses and cows. We cannot, therefore, 

 exclude the consideration of volcanoes as producers of earthquake, 

 sometimes by direct action, at others through the medium of electric 

 disturbance. 



But by far the most prominent agent seems to be electricity, and 

 the Italians, who sufier so much from these calamities, consider it to 

 be the only cause. The evidences of the activity of the electric fluid 

 in this respect are so palpable that they cannot be controverted. As 

 some may be skeptical on this point, it will not be amiss to examine 

 a few cases in which the operation of this agent is quite apparent. 



When considering this part of the subject, we must not omit to 

 notice the frequency with which the greatest weight of these calami- 



