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6 its line of transit across the system. The surmise needs corroboration, but it is quite a legitimate one. There is no physical reason why we should not pass through the tail of a comet without ever knowing it. The phosphorescent light, seen by Mr Hind, and noticed by others, may have had no connexion with the matter of the comet; but still, it is possible that we may have been immersed in this cometary medium, without any appreciable effects being produced.

The great advantage of the comet, as a convenient vehicle for an excursion, is, that it gives near, as well as extensive views of the system. The drawback of our own globe is, that it always keeps at the same distance, or nearly so, from all the bodies of the system; so that, although it is constantly moving onwards, we are kept at such a distance, that we see but little change in the celestial scenery. It is like an excursion steamer constantly sailing, in a narrow circuit, round a buoy moored in the middle of a wide lake. The view of the surrounding scenery never changes, and the minute objects of the landscape are never seen. The comet, on the other hand, is like the steamer that sails up the whole length of the lake on one side, and comes down along the other. Every object is seen minutely, and from different points of view. Most comets, too, rise above the plane of the solar system, so that we may have a clear view of the relation of one planet to the other.