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 moderate, so that it is not at all unlikely that a system at least 100 times as massive as the system we have supposed would be required if this star was to be recalled.

The result of this inquiry is to be stated as an alternative: either our sidereal system is not an entirely isolated object, or its bodies must be vastly more numerous or more massive than a liberal interpretation of observations would seem to warrant. If we adopt the first alternative, then we see that 1830 Groombridge, having travelled from an indefinitely great distance on one side of the heavens, is now passing through our system for the first and the only time. After leaving our neighbourhood it will retreat again into the depths of space, to a distance which, for anything we can tell, may be practically regarded as infinite. Although we have only used this one star as an illustration, yet it is not to be supposed that the peculiarities which it presents are absolutely unique. It seems more likely that there may be many other stars which are at present passing through our system. In fact, considering that most or all of the stars are actually in motion, it can be shown that in the course of ages, the whole face of the heavens is gradually changing. We are thus led to the conclusion that our system may not be an absolutely isolated group of bodies in the abyss of space, but that we are visited by other bodies coming from the remotest regions of space. The whole range of astronomy presents no speculations which have attracted more attention than the celebrated nebular hypotheses of Herschel and of Laplace. We shall first enunciate these speculations, and then we shall attempt to indicate how far they seem to be warranted by the actual state of scientific knowledge. In one of his