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mathematicians now made computations of the effect of the planets in changing its period. So exact was their work that two of them hit the time within five days: Professor Rosenberger assigned November eleventh as the date of return, and Pontécoulant predicted it for November thirteenth. It actually passed perihelion on November sixteenth. After being observed for several months it disappeared from view and has not since been seen. But so exact is astronomical science that an astronomer could, at any time during the intervening interval, have pointed his telescope exactly at the object, after making the necessary calculations to determine its position.

Its next return is now approaching, but the exact date has not yet been computed. It will probably be some time between 1910 and 1912.

The most striking discovery of a comet after Halley announced the one which bears his name, was made by the French astronomer Lexell, in June, 1770. The object soon became visible to the naked eye. On laying down the orbit in which it moved, it was found, to the surprise of astronomers, that the orbit was an ellipse, with a period of only about six years. Its return was, therefore, confidently predicted, but it never reappeared. The cause was, however, speedily discovered. When it returned at the end of six years, it was on the opposite side of the sun, and therefore could not be seen. Passing out to complete its revolution, it was found by calculation that it must have gone into the immediate neighbourhood