Page:The story of the comets.djvu/47

III. be a natural result of things, independently of the question what is the form of the tail, whether cylindrical, or flat, or solid, or hollow.

As regards the actual formation of comet tails, probably in all cases they are hollow, but whether hollow cones or hollow cylinders depends on circumstances. In either case this theory accords, as it naturally should do, with the observed fact that single tails usually are divided in the middle by a dark band, the brilliancy of the margins exceeding that of the more central portions; but it must be confessed that this theory breaks down where the outer edges of a tail are fainter



than the centre, which seems to have a luminous spine of light running down it. Where the tail increases in width towards its extremity it is permissible to suppose that its general form is that of a hollow cone; where the width is fairly uniform from end to end the tail may be regarded as a hollow cylinder.

The trains of some great comets are said to have been seen to vibrate in a manner somewhat similar to the Aurora Borealis. The tails of the Comets of 1618 (ii.) and 1769 may be cited as instances; the observer in the latter case was