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when those that are higher and more condensed, fall 

 upon those that arc lower, with so muph force as sud- 

 denly to expel the intermediate air, which vigorously 

 expanding itself, in order to occupy its former space, 

 puts all the exterior air in commotion, producing 

 those reiterated claps which we call thunder. I stop 

 not to examine a third theory, which makes the mat- 

 ter productive of thunder, the same with that which 

 is the cause of electricity ; for though it be the most 

 probable of any, yet the truth of it is stUl contested. 



3. Of those two sentiments of the ancients, which 

 have been adopted by our modems, the latter bejongs 

 to Aristotle, who says, that * thunder is caused by a 

 dry exhalation, which falling upon a humid cloud, 

 and violently endeavouring to force a passage for it- 

 self, produces the peals which we hear.' And Anax- 

 agoras refers it to the same cause. All the other 

 passages, which occur in such abundance among the 

 ancients, respecting the formation of thunder, evident- 

 ly contain the reasonings of the Newtonians, and 

 sometimes join together the two sentiments which du 

 v.de the moderns. 



4. Leucippus held, that * thunder proceeded from 

 a fiery exhalation, which enclosed in a cloud 9 burst 

 it asunder, and forced its way through.' Democri- 

 tus asserts^ that it is the effect of a mingled collection 

 of various volatile particles, which impel downwards 

 the cloud which contains them, till by the rapidity of 

 their motion, they set themselves and it on fire. 

 Seneca ascribes it to a dry sulphureous exhalation 

 arising out of the earth, which he calls the aliment 

 of lightning ; and which becoming more and more 

 subtilised in its ascent, ai last takes fire in the air, 

 and produces a violent eruption, 



5. The Stoics distinguished two things in thunder, 

 the lightning and the noise. < According to them, 

 ; thunder was occasioned by the shock of clouds ,. and 



