return of which has been confidently expected about this time? After various approximations, Mr Hind has finally fixed the limits of 1857 and 1861 within which we may reasonably expect this return. In establishing the identity of a comet, little reliance can be placed on the physical aspect. Comets that, at one period, shone with great splendour, have, at another, come back again, shorn of all their glory. They have not the stable constitution of the planets, which possess an identity altogether independent of their orbit. They may be more fitly compared to a cloud floating in the atmosphere. At sunset we see, in the western horizon, well-defined clouds gleaming with golden glory, and when we look again, the glory is gone, and we fail to recognise the object, its shape being now entirely altered. A comet fluctuates in like manner—its substance being of a much rarer consistency, and subject to influences which are constantly inducing changes in shape and lustre. The comet differs from a cloud in this, that it has a well-defined path by which its identity is established. The periodic revolution of comets cannot in general, however, be fixed with such accuracy as that of the planets. We can observe only one end of the long ovals in which comets move, but the ends of different ovals may be very much alike, and yet they may difier very much in length; consequently, an error made in determining the shape of the ellipse may entail a great error in its length, and the comet's period of
