THE EARTH. 91 



and sometimes after that at land. This agitation is only a simi- 

 lar effect produced on the waters with that at land, and mav be 

 called, for the sake of perspicuity, a seaquake ; and Xtii also is 

 produced by volcanoes. 



A spouting up of waters to great heights. It is not easy to 

 describe the manner in which this is performed : but volcanoes 

 also perform the same; Vesuvius being known frequently to 

 eject a vast body of water. 



A rocking of the earth to and fro, and sometimes a perpen- 

 dicular bouncing, if it may be so called, of the same. This dif 

 ference chiefly arises from the situation of the place with respect 

 to the subterranean fire. Directly under, it lifts ; at a farther 

 distance, it rocks. 



Some earthquakes seem to travel onward, and are felt in dif- 

 ferent countries at diiferent hours the same day. This arises 

 from the great shock being given to the earth at one place, and 

 that being communicated onward by an undulatory motion, suc- 

 cessively affects different regions in its progress ; as the blow- 

 given by a stone falling in a lake, is not perceived at the shores 

 till some time after the first concussion. 



The shock is sometimes instantaneous, like the explosion of 

 gunpowder ; and sometimes tremulous, and continuing for sev- 

 eral minutes. The nearer the place where the shock is first 

 given, the more instantaneous and simple it appears. At a 

 greater distance, the eaith redoubles the first blow with a sort 

 of vibratory continuation. 



As waters have generally so great a share in producing earth- 

 quakes, it is not to be wondered that they should generally fol- 

 low those breaches made by the force of fire, and appear in the 

 great chasms which the earthquake has opened. 



These are some of the most remarkable phenomena of earth- 

 quakes, presenting a frightful assemblage of the most terrible 

 effects of air, earth, fire, and water. 



The valley of Solfatara, near Naples, seems to exhibit, in a 

 minuter degree, whatever is seen of this horrible kind on the 

 great theatre of nature. This plain, which is about twelve hun- 

 dred feet long, and a thousand broad, is embosomed in moim- 

 tains, and has in the middle of it a lake of noisome blackish 

 water, covered with a bitumen, that floats upon its surface. In 

 svery part of this plain, caverns appear smoking with sulnhur, 



