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234 of the double stars. If the number of double stars warranted the inference of a physical connexion, a like inference is equally warrantable in the case of the nebulæ. It has been clearly ascertained, that our sun has a proper motion in space, carrying all its system of planets and satellites with it; and it has even been attempted to assign its probable velocity. According to the most reliable calculations, it advances on its path daily, by a space equal to its own semidiameter. This, combined with the detected motion of the double stars, warrants the presumption that revolution is not confined to the lower steps of grouping among the celestial bodies. There is a strong probability that every orb in the universe is in motion, and that all are related by one great bond of union. That gravitation is the great bond of connexion in the stellar spaces, as well as in the solar system, has been clearly ascertained. This is settled by the revolution of one star round another in an ellipse. The same power that determines the shape and the fall of a rain-drop, also determines the shape and the motions of the remotest galaxy.

The apparent magnitudes of the nebulæ are very various. The nebula in Andromeda, visible to the naked eye, has, when its minutest boundaries are brought into view, a length equal to five, and a breadth equal to two diameters of the moon. Again, there are nebulæ so minute, that a hundred might be packed within the circumference of the moon's disc.