will be produced during a much shorter period of time. In that case the light of the star will not last long. If the onrush of one stream upon another or a more regular swarm is sudden, we shall have a sudden blaze out of light; if the onrushing stream is short, the light will soon die; if it continues for some time, and reduces its quantity, the light will die out gradually. Or again, such a source of supply may fail by the complete passage of one stream through the other. In these ways we shall have various bodies in the heavens, suddenly or gradually increasing or decreasing their light quite irregularly, unlike those other bodies where we get a periodical variation in consequence of the revolution of one round the other. We shall have "new stars" appearing from time to time in the heavens, and they do.
Unfortunately, no photographs of these bodies to which I refer have been taken. Observations have been recorded, however, of their changing light. The changes can be easily explained upon this hypothesis, but, so far as I know, can not be explained upon any other.
In one case we had a known star (in Corona) suddenly blazing out from the ninth magnitude to the second, and almost as suddenly going down again. In another star (Nova Cygni) we had an outburst in a region which observation showed to be without a star, although I do not know whether any special observation of that region had been made for the existence of nebulæ. Suddenly in that part of the heavens a third-magnitude star blazed out; this took a very considerable time to die down, as compared to the first star, in Corona, and ultimately it got down to the tenth magnitude, and now telescopically it appears as a nebula.
As in condensing these swarms get hotter, they will get brighter as their volume decreases, and we shall pass from what we term nebulæ to what we term stars. It can not be too strongly insisted upon that chief among the new ideas introduced by the recent work is that a great many stars are not stars like the sun, but simply collections of meteorites, the particles of which may be probably thirty, forty, or fifty miles apart. Such eddies and systems, which are not simple, will vary in brightness. In the case of double nebulæ condensing we shall get, as I have already stated, a periodic variation in light; and here we have a simple explanation of the facts observed, and hitherto held to be mysterious, in a large number of variable stars. The "new" stars I have already referred to are also easily accounted for on the hypothesis of meteoric streams.
It may be asked, Why, considering the millions of bodies in motion capable by this hypothesis of producing them, are not "new stars" seen more frequently? The reply is simple: We, as a rule, deal with the clashing of small streams; the temperature does