134- 



M A T H E 



A T I C A L AND 



Though comets doubtlefs 



f-"' 



one part of which is much farther from the Sun than ano- 

 ther; fome of them are very eccentric; that which appear- 

 ed Anno 1680 v,'as twelve thoufand millions of miles 

 from the Sun in aphelk), it was not half a milHon in 

 perihelio. The period of the comet which appeared 

 Anno 1758 is 75 years. That of 166 1 is 120 years. 

 And that of 1680 is 575 years. 



move in an ellipfis, yet from the extreme length of their 

 path, the fmall part that falls under our obfcrvation, the 

 difficulty in determining the comet^s abfolute diftance or 

 velocity, &c. we have obtained no certainty concernnig 

 the period of any comet except the three I hnve mention 

 ed, nor fhall we ever determine their periods in all proba- 

 bility, except by a feries of obfervations on the return of 

 each particular comet, which may require feveral thou- 

 fands of years. 



Comets receive their light and heat from the Sun, for 



they appear to have no light of their own, and are thence 



invifible, except on their near approach to the Sun. . In 



the year 1723, an aftronomer had the fortune to difcover 

 a comet by means of his telefcope before it was bright 

 enough to become vifible by the naked eye.^ The great 



eoniet which appeared Anno 1743 feemed no larger than 

 a ftar of the fourth magnitude when firft difcovered; as it 

 came down towards the Sun it acquired a tail, and increaf- 



ed gradually in fize and luftre till it obtained that amazing 

 form with which it terrified half the world. As this co- 



/ 



met departed from the Sun, its tail decreafed, it loft its 

 brightnefs, till in a fliort time it became invifible; this has 

 alfo been the fate of every other comet; hence it is plain 

 that their light, like that of other planets, is borrowed 

 from the Sun. 



Having juft mentioned thofe general properties in which 

 comets evidently agree with other planets, I j(hall now try 

 to account for that luminous train which attends them on 

 their approach to the Sun, from which they are generally 

 denominated blazing ftarsj and are fuppofcd to differ ef- 



fentially 



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