OTHER ELLIPTIC COMETS. 



the more active and intelligent zeal of modern mathematicians and 

 computers, has led to the discovery of the great probability of 

 several among them having revolved in elliptic orbits, with periods 

 not differing considerably from those of the comets above men- 

 tioned. The fact that these comets have not been re-observed on 

 their successive returns through perihelion, may be explained, either 

 by the difficulty of observing them, owing to their unfavourable 

 positions, and the circumstance of observers not expecting their 

 re-appearance, their periodic character not being then suspected ; 

 or because they may have been thrown by the disturbing action 

 of the larger planets into orbits such as to keep them continually 

 out of the range of view of terrestrial observers. 



Among those may be mentioned a comet which appeared in 1743, 

 and was observed by Zanetti at Bologna ; the observations indicate 

 an elliptic orbit, with a period of about 5^ years. 



36. This comet, which was observed by Messier, at Paris, and 

 by La Xux, at the Isle of Bourbon, revolved, according to the cal- 

 culations of Burckhardt, in an ellipse with a period of 5 years. 



37. The history of astronomy has recorded one singular example 

 of a comet which appeared in the system, made two revolutions 

 round the sun in an elliptic orbit, and k then disappeared, never 

 having been seen either before or since. 



This comet was discovered by Messier, in June 1770, in the 

 constellation of Sagittarius, between the head and the northern 

 extremity of the bow, and was observed during that month. It 

 disappeared in July, being lost in the sun's rays. After passing 

 through its perihelion, it re-appeared about the 4th of August, 

 and continued to be observed until the first days of October, when 

 it finally disappeared. 



All the attempts of the astronomers of that day failed to deduce 

 the path of this comet from the observations, until six years later, 

 in 1776, Lexell showed that the observations were explained, not, 

 as had been assumed previously, by a parabolic path, but by an 

 ellipse, and one, moreover, without any example at that epoch, 

 which indicated the short period of 5^ years. 



It was immediately objected to such a solution, that its admis- 

 sion would involve the consequence that the comet, with a period 

 so short, and a magnitude and splendour such as it exhibited in 

 1770, must have been frequently seen on former returns to peri- 

 helion ; whereas no record of any such appearance was found. 



To this Lexell replied, by showing that the elements of its orbit, 

 derived from the observations made in 1770, were such, that at 

 its previous aphelion, in 1767, the comet must have passed within 

 .a distance of the planet Jupiterfifty-eighttimes less thanits distance 

 from the sun ; and that consequently it must then have sustained 



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