63 



lightning was the combustion of the volatile parts of 

 the cloud, set on fire by the shock : and Chrysippus 

 taught, that lightning was the result of clouds being 

 set on fire by winds, which dashed them one against 

 another; and that thunder was the noise produced 

 by that re-encounter : Jie added, that these effects 

 were coincident ; our perception of the lightning be. 

 fore the thunder-clap, being entirely owing to our 

 sight's being quicker than our hearing. 



6. There is but one opinion respecting the cause of 

 earthquakes, which deserves any notice ; and it is 

 that of the Cartesians, Newtonians, and all our other 

 able naturalists. They ascribe it to the earth's being 

 filled with cavities of a vast extent, containing in them 

 an immense quantity of thick exhalations, of a fuligU 

 nous substance, resembling the smoke of an extin- 

 guished candle, which being easily inflammable, and 

 by their agitation catching fire, rarify a;id heat the 

 central and condensed air of the cavern to such a 

 degree, (hat finding no vent to issue it, it bursts its en- 

 closements ; and in doing this, shakes the earth all 

 around with dreadful percussions, producing all .the 

 other effects which naturally follow. 



7. This same reason is givenby Aristotle and Seneca, 

 in assigning the cause of such dreadful events. The 

 former after refuting those who ascribed earthquakes 

 to the earth itself, or the water it contains, subjoins 

 his own opinion, * that they were occasioned by the 

 efforts of the internal air in dislodging itself from ^the 

 bowels of the earth ; and he observes, that * on the 

 approach of an earthquake, the weather is generally 

 serene, because that sort of air which occasions com- 

 motions in the atmosphere, is at that time pent up ia 

 the entrails of the earth. 



8. Seneca is still more precise ; we might take 

 him for a naturalist of the present times. He sup- 

 goses ; that ; the earth hides ia its bosom many sub* 



