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CHAP. XVII. 



Of Comets. 



HERE is no extravagance of fancy, how wild 

 soever, but what hath been hazarded in different ages, 

 to account for the nature of comets, and the irregula- 

 rity of their course. Even in the last age, Kepler arid 

 He veli us advanced conjectures entirely extravagant re. 

 s peeling the cause of these phenomena. Mr. Cassini, 

 and after him Sir Isaac New too, have at length given 

 certainty to the opinions of the philosophers in this 

 respect, by observations and calculations most just 

 and accurate; or, to speak with more propriety, by 

 recalling and fixing our attention upou what had tor. 

 merly been advanced by the Chaldeans, Egyptians, 

 Auaxagoras, Democritus, Pythagoras, Hippocrates of 

 Chois, Seneca, Apollonius Myndius, and Arteimdo- 

 rus. For in treating of the nature of these stars, their 

 definitions of them, tho. reasons they assign for the 

 rareness of their appearance, and the apologies they 

 make for not having yet formed a more exact theory, 

 are all in the very terms that Seneca had already used. 

 \Vithrespectto the time of that philosopher, we have 

 formerly taken notice, that the collecting together the 

 observations anciently made of the returns of comets, 

 was not sutticien; to establish the theory of them ; be- 

 cause th-ir appearances were so very rare, that there 

 had not been an opportunity of making a proper mini* 

 bcr of observations, to determine whether their course 

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