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 vast bodies. These results demonstrate conclusively that the quantity of matter even in a comet is extremely small when compared with its bulk.

The conclusion thus arrived at is confirmed by the fact that our efforts to obtain the weight of a comet have hitherto proved unsuccessful. We have the means of measuring the weight of a planet, by the disturbances of other bodies which it can affect, and if a comet were massive enough to produce disturbances in the planetary movements there would be no difficulty in discovering within certain limits what the cometary mass might amount to. There have been several instances in which a comet has approached so close to a planet, that the attraction between the two bodies must have had significant influence on the planet, if the cometary mass had been at all comparable with that of the more robust body. The most celebrated instance is presented in the case of Lexell's comet which happened to cross the track of Jupiter. The effect upon this body was so overwhelming that it was wrenched from its original path, and started afresh along a wholly different track. The reaction of the comet upon the planet seems, however, to have been incapable of influencing by any measurable quantity the movements of the giant globe. It is, therefore, obvious that the mass of this comet, and it was a large one of its class, was inappreciable in comparison with the mass of Jupiter.

But the rencontre between the two bodies supplies us with an argument of a still more cogent type. The retinue of moons by which Jupiter is attended forms a delicately organized system. Their movements have been observed for centuries, and any derangement introduced into the group by the approach of a considerable foreign body