ASTRONOMY. 197 



The latest, and in practice one of the best, is 

 that of DE LAMBRE, Abrege d'Astron. legon 21. 

 24. 



205. The only Comet which is known with 

 absolute certainty to have returned, is that of 

 1682, which, conformably to the prediction of 

 Dr HALLEY, appeared in 1759- 



Dr HALLEY was led to this prediction by observing, 

 that a Comet had appeared in 1 607, and another 

 in 1531, and that the elements of their orbits, 

 when calculated from the observations made on 

 them, agreed nearly with those of the Comet of 

 1682, the period being between seventy-five and 

 seventy-six years. 



Though there can be little doubt that it was the same 

 Comet which was seen at all the four dates just 

 mentioned, the appearances were considerably 

 different. In 1531 the Comet was of a bright 

 gold colour; in 1607 it was dark and livid; it 

 was bright again in 1682, and obscure" in 1759. 

 PJNGRE', Cometographie y torn. n. p. 189. 



The return of some of the other Comets is probable, 

 though not certain. 



The great Comet of 1680, was supposed by Dr HAL- 

 LEY to have a period of 575 years, and to be the 

 same which had appeared a little before the death 

 of JULIUS G/ESAR, in the year 44 A. C. j again, in 

 reign of JUSTINIAN, in the year 531 P. C. ; 



and 



