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232 gaze of the astronomer, hitherto altogether unknown in celestial mechanics. It is impossible to view the strange figures of the nebulae without being persuaded that they indicate action, vitality, progress. We cannot survey the many forms which the clouds in our atmosphere assume, without being persuaded that they are the result of forces determining their peculiar configuration. The nebulæ, in like manner, speak of mighty forces moulding them into their particular form and constitution. The nebular hypothesis contemplates particles of matter sweeping round into a vortex and forming a solid body; but the astronomer, on the other hand, has now to contemplate streams of bright suns hastening on along their spiral course to some unknown destiny. The idea of effort is written upon these strange forms—effort to reach some more perfect ultimate form. Sir John Herschel threw out a conjecture as to the manner in which the great condensation observable in some nebula may have been produced. He imagines that the motions of the stars might be destroyed, to some extent, by collisions, which would have the effect of making them revolve round the common centre of gravity in a narrower circle; just as the earth would contract its orbit, should part of its velocity be destroyed by a collision with a comet. The effect of such collision would be, to produce great apparent condensation in the group of stars. This great condensation, with a faint surrounding nebulosity, was one of the strongholds of