Chap. 48.] Caufe of Earthquakes. 363 



The great earthquake at Lifbon, in 1755, was felt 

 as far as Scotland, and from the phenomena which 

 attended it, it was evident that the ground had a wavr- 

 ing motion from fouth to north. All the oblong 

 lakes, that lay from north to fouth, were much agi- 

 tated, the wave commencing at the fouth end, whilft 

 all other lakes which lay acrofs, from eaft to weft, were 

 much lefs affected. 



The great diftance to which earthquakes extend 

 depends on the compreflibility and elafticity of the 

 earth, which may be underftood from the vibration of 

 the walls of houfes, occafioned by the patting of car- 

 riages in the adjacent flreets. Another inftance is 

 the vibration of fteeples by the ringing of bells or 

 gufts of wind. The Eddiftone lighthoufe often vi- 

 brates from the force of the waves which beat againft 

 its foundation. 



Previous to an eruption of Vefuvius the earth al- 

 ways trembles, and fubterraneous explofions are heard. 

 On. the ift of November, 1755, tne era fo fatal to 

 Lifbon, the ifland of Madeira was violently maken by 

 an earthquake, accompanied with fubterraneous ex- 

 plofions. So thoroughly convinced, indeed, are the 

 inhabitants of volcanic countries of the connection 

 between earthquakes and volcanoes, that when a great 

 eruption takes place from a volcano, they congratulate 

 themfelves on having efcaped an earthquake. 



Earthquakes as well as volcanic eruptions are al- 

 ways preceded by a violent agitation of the fea. Pre- 

 vious to the breaking out of Vefuvius, the fea retires 

 from the adjacent mores till the mountain is burn: 

 open, and then it returns with fuch impetuofity as 

 to overflow its ufual boundary. About an hour after 

 the firft mocks, which alarmed the city of Lifbon 

 in 1755, the fea was obferved to come rufhing to- 

 wards 



