There is a notable instance of a comet traveling about the sun in an immense ellipse, but, like the moth, hovering around a flame which finally causes its destruction, this comet returned once too often to the neighborhood of the giant planet Jupiter, and in an encounter between a large and a small body, the latter usually comes to grief. Its path was curtailed at first, and subsequently it was shunted on to another line. Jupiter, acting as pointsman on the cometary railway, is suspected of opening the branch of the ellipse along which the comet had formerly traveled in peace and quiet, with the result that it was ignominiously sidetracked and sought for in vain. The comet was first discovered in June, 1770, by Messier, who described it as a rather insignificant object without a tail, but resembling a nebula with a star-like nucleus. Early in July it had greatly increased in size, the nucleus and surrounding haze extending over a space more than five times the diameter of the moon. At this time it came very near the earth, remaining visible until October, when it grew small and faint, and finally faded away. Meanwhile, astronomers did their best to determine its path, notably Mr. Lexell, of the Academy of Sciences at St. Petersburg. He became so interested in clearing up the past history of this quaint little comet, that it is usually referred to as Lexell’s comet. He came to the conclusion that the comet of 1770 required five years and seven months for its elliptic tour, but he was such a long time in getting at this result, that by the time he obtained it in 1778 the comet was two years overdue. Messier made a careful search for it, but without success. Lexell was of the opinion that at the end of May, 1776, the comet came so close to Jupiter that the attractive pull of that planet was three times greater than that of the sun. When a comet rushes around the sun it has to go full speed ahead, so as to resist being drawn upon its surface, but as it recedes from the danger zone it gradually slackens its pace, with the result that by the time it is crossing the orbit along which Jupiter travels, it is going at reduced speed. Probably Jupiter was not far from the part of its orbit crossed by the comet in 1776, with the result that the unfortunate wanderer was exposed for a longer period to the powerful attraction of the giant planet. This may have caused an important change in the comet’s path, with the result that it escaped from what has been termed the _sphere of activity_ early in October, 1779.