Bredichin wishes it to be inferred that the tails of the 1st type are probably composed of hydrogen; those of the 2nd type of some hydro-carbon gas; and those of the 3rd of the vapour of iron, probably with some admixture of sodium and other substances. Bredichin, as a reason for these conclusions, supposes that the force which generates the tails of comets is a repulsive force, with a surface action the same for equal surfaces of any kind of matter; the effective accelerating force therefore measured by the velocity which it would produce would depend upon the ratio of surface to mass in the particles acted upon, and so, in his view, should be inversely proportional to their molecular weights. Now it happens that the molecular weights of hydrogen, of hydro-carbon gases, and of the vapour of iron bear to each other just about the required proportion. I am here stating the views and opinions of others without definitely professing to be satisfied with them, but as they have met with some acceptance, it is proper to chronicle them, though we know nothing of the nature of the repulsive force here talked about. It might be electric, it might be anything. The spectroscope certainly lends some countenance to Bredichin’s views, but we need far more knowledge and study of comets before we shall be justly entitled to dogmatise on the subject. [Illustration: Fig. 22.—Holmes’s Comet, Nov. 9, 1892 (Denning).] [Illustration: Fig. 23.—Holmes’s Comet, Nov. 16, 1892 (Denning).] This has been rather a digression. I go back now to prosaic matters of fact, of which a vast and interesting array present themselves for consideration in connection with comets. Let us consider a little in detail what they are, to look at. We have seen that a well-developed comet of the normal type usually comprises a nucleus, a head or coma, and a tail. Comets which have no tails generally exhibit heads of very simple structure; and if there is a nucleus, the nucleus is little else than a stellar point of light. But in the case of the larger comets, which are almost or quite visible to the naked eye, the head often exhibits a very complex structure, which in not a few cases seems to convey very definite indications of the operations going on at the time. Figs. 22 and 23 may be taken as samples of a complex cometary head, though no two comets resemble one another exactly in details. Fig. 24 forcibly conveys the idea that we are looking at a process of development analogous to an uprush of water from a fountain, or perhaps I might better say, from a burst waterpipe. There is a distinct idea of a jet. This self-same idea, in another form, presents itself in the case of those comets which exhibit what astronomers are in the habit of calling “luminous envelopes.” The jet in this case is not strictly a jet because it is not a continuous outflow, or overflow, of matter; the idea rather suggests itself of an intermittent overflow resulting in