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VII. bright comet with a stellar nucleus. On July 1 it had greatly increased in apparent size, and though no tail was visible the nebulosity surrounding the nucleus had swollen to a diameter 2° or more than five times the diameter of the Moon—dimensions still remaining wholly unprecedented. The comet remained visible altogether for nearly 4 months, and disappeared from view owing to the increase of its distance from the Earth, it having become when last seen very small and faint. Various attempts were made by different astronomers, but unsuccessfully, to reconcile the observations with a parabolic orbit. Some years later Lexell, a member of the Academy of Sciences at St. Petersburg, investigated anew the orbit, which he found for a certainty to be elliptic; and that the comet's period was about 5 years. Supposing this had been correct the comet should have returned to perihelion in 1776, but it was not seen, though Messier and others were constantly on the watch for new comets generally.

Lexell's researches disclosed to him the fact that in May, 1767, the comet had passed very close to Jupiter, and had remained for a considerable time exposed to the influence of this planet. Lexell thought that this fact had exercised such a material effect on what had been the previous orbit of the comet as to transform that orbit into the short-period ellipse which he found represented the comet's movements in the year 1770. With the materials before him, Lexell put forth the suggestion that the comet ought to be seen again in the Summer of 1781, after again passing under the powerful influence of Jupiter in the Summer of 1779. Diligent, but unsuccessful, search was made for it at the time of its expected reappearance; and the conclusion drawn by Lexell was that as Jupiter in 1767 had driven the comet into its small elliptic orbit, so in 1779 the same planet had driven the comet out of its small elliptic orbit into a new one which could not be, and never has been, traced. It was because of the prolonged and comprehensive labours of Lexell on this comet that astronomers have always agreed to attach his name to it.

Nothing more was done in the matter until 1806 when Burckhardt, an eminent French Mathematician, traversed