408 LECTURE XLII. 



might interfere in such a manner with a planet, as to deflect it a little from i 

 its course, and retire again without coming actually into contact with it. 



Nearly 500 comets are recorded to have been seen at different times, and 

 the orbits of about a hundred have been correctly ascertained : but we 

 have no opportunity of observing a sufficient portion of the orbit of Sny v 

 comet, to determine with accuracy the whole of its form as an ellipsis^ 7 

 since the part which is within the limits of our observation does not sensibly N 

 differ from the parabola, which would be the result of an ellipsis prolonged 

 without end. 



Two comets at least, or perhaps three, have been recognized in t^ir 

 return. A comet appeared in 1770, which Prosperin* suspected to iipvT? 

 in an orbit materially different from a parabola : Mr. Lexellf determjUed 

 its period to be 5 years and 7 months, and its extreme distances i/o be 

 between the orbits of Jupiter and of Mercury ; but it does not appear that 

 any subsequent observations have confirmed his theory. It has, however, 

 been calculated, that supposing the theory correct, it must afterwards have 

 approached so near to Jupiter as to have the form of its orbit entirely 

 changed. 



Dr. Halley J foretold the return of a comet about 1758, which had 

 appeared in 1531, in 1607, and in 1682, at intervals of about 75 years ; 

 and with Clairaut's further correction for the perturbations of Jupiter 

 and Saturn, the time agreed within about a month. The mean distance 

 of this comet from the sun must be less than that of the Georgian planet ; 

 so that by improving our telescopes still more highly, we may, perhaps, 

 hereafter be able to convert some of the comets into planets, so far as 

 their remaining always visible would entitle them to the appellation. 

 Dr. Halley also supposed the comet of 1680 to have been seen in 1106, in 

 531, and in the year 44 before Christ, having a period of 575 years ; and 

 it has been suspected that the comets of 1556 and 1264 were the same, the 

 interval being 292 years ; a conjecture which will either be confirmed or 

 confuted in the year 1848. Some persons have even doubted of the perfect 

 coincidence of the orbits of any comets, seen at different times, with each 

 other, and have been disposed to consider them as messengers forming a 

 communication between the neighbouring systems of the sidereal world, 

 and visiting a variety of stars in succession, so as to have their courses 

 altered continually, by the attraction towards many different centres ; but 

 considering the coincidence of the calculation of Halley and Clairaut with 

 the subsequent appearance of the comet of 1759, this opinion can scarcely 

 be admitted to be in any degree probable with respect to the comets in 

 general, however possible the supposition may be in some particular cases. 

 (Plate XXXII. Fig. 472... 475. Plate XXXIII. Fig. 476... 485.) 



* On Com. 1770, 4to, Upsal. 1776. 



t Lexell, Mem. de 1'Acad. Par. 1776, p. 638 ; and Disquisitio de Temp. per. Co- 

 metse An. 1770, Ph. Tr. 1779, p. 68. 



t Ph. Tr. 1705, p. 1882 ; and Gregory's Elements of Ast. 1726. See History of 

 Halley's Comet, with an Account of its Return in 1835, as predicted by MM. Damoi- 

 seau and De Pontecoulant, translated from the French of M. De Pontecoulant, by 

 Gold, 1835. Moseley's Lect. on Ast. 1839. 



Journal des Savans, 1759. See his Theoriedu Mouvement des Cornetes, Paris, 



