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an orbit with a period of only seven years. As it was quite bright, the question arose why it had never been observed before. This question was soon answered by the discovery that in the year 1886 the comet had passed close to Jupiter. The attraction of the planet had so changed its course as to throw the comet into the orbit which it now describes. Several other periodic comets pass so near to Jupiter that there is little doubt that they were brought into the system in this way.

The question therefore arises whether this may not be true of all periodic comets. This question must be answered in the negative, because Halley's comet does not pass near any planet. The same is true of Encke's comet, which does not come near enough to the orbit of Jupiter to have been drawn into its present orbit. Without the action of that planet, so far as we know, these comets always have been members of the system.

It was supposed, until a recent time, that comets might come into the solar system from the vast spaces between the stars. This view, however, seems to be set aside by the fact that no comet has been proved to move with a much higher speed than it would get by falling to the sun from a distance, which, though far outside the solar system, is much less than the distance of the stars. We shall see hereafter that the sun itself is in motion through space. Even if we grant that comets come from space far outside the solar sj^stem, the fact that we have just cited still shows that they partook of the motion of the