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of the planet Jupiter, which, by its powerful attraction, started the comet off into some new orbit, so that it never again came within reach of the telescope. This, also, explained why the comet had not been seen before. Three years before Lexell found it, it had come from the neighbourhood of the planet Jupiter, which had thrown it into an orbit different from its former one. Thus the giant planet of our system had, so to speak, given the comet a pull about 1767 so that it should pass into the immediate neighbourhood of the sun, and having allowed it to make two revolutions around the sun, again encountered it in 1779, and gave it a new swing off, no one knows where. Since that time twenty or thirty comets, found to be periodic, have been observed, most, but not all of them, at two or more returns.

The most remarkable fact brought out by the study of these objects has been that they do not appear to be of seemingly infinite duration, like the planets, but are, as a general rule, subject to dissolution and decay, like living beings. The most curious case of a comet being completely disintegrated is that of Biela's comet. This was first observed in 1772, but was not known to be periodic. It was again seen in 1805, and again the astronomer did not notice the identity of the orbit in which it was moving with that of the comet of 1772. In 1826 it was discovered a third time, and now, on computing the orbit by the improved methods which had been invented, its identity with the former comets was brought out. The time of revolution was fixed at six and two thirds years. It should, therefore, appear in 1832 and 1839. But on