William Herschel was, that it was a very common thing for double nebulæ to make their appearance in his gigantic telescope. Now, it is difficult for us to imagine that these double nebulæ, like their allied systems of stars, should not be in motion; and if we imagine a condition of things in which one swarm is going around a larger one in an elliptic orbit, and occasionally approaching it and mingling with it, we shall have at one part of the orbit the centers nearest together; so that a greater number of particles of meteoritic dust will be liable to encounters at this time than at others. Hence we shall get a cause of increased temperature of a periodic kind; there must be variable stars in the heavens—and there are.
As a third possible condition we have the known movement of these swarms of dust through space. If we take note of the known movements of the star which forms the center of our own system, we can learn that these movements may be gigantic. We know that the sun is traveling nearly half a million of miles every twenty-four hours toward a certain region; we know that other stars are moving so quickly that Sir Robert Ball has calculated that one among them would travel from London to Pekin in something like two minutes. We have, therefore, any amount of velocity. Now suppose that without the formation of either a single or a double system, such as we have considered—by the ordinary condensation of an initial single or initial double swarm—we have what we may call a "level crossing" at which two or more streams of meteoritic dust meet. There, of course, we shall have a tremendous cause of collisions. Have we such instances in the heavens? Again I appeal to Mr. Roberts's photographs of the Pleiades; we see in them four nebulæ which have been stated to surround four of the stars. But if we look at the nebulæ more carefully, we find that distinct stream-lines are seen in each in certain directions; we have interlacing, the meeting of these streams at some angle or other, and in each such region we have the locus of one of the chief stars.
This may be considered to be an irregular cause of a production of high temperature; but so long as such an action as that continues, an apparent star will be seen, distinct, of constant light, and not to be discriminated, without such photographs as these, from those stars which have been produced by more ordinary sequences connected with the more ordinary processes of condensation.
If, however, the above explanation be the true one, we should expect to find cases in which we may see such an action beginning or ending suddenly; the action will be less constant and durable—that is to say, the supply of these streams of meteoritic dust may not be continuous; it may be smaller, and then the effect
