The Jupiter group of comets deserves a few additional words. There are certainly nine, and perhaps twelve comets revolving round the Sun in orbits of such dimensions that they either reach up to or slightly overreach the orbit of Jupiter. The effect of this condition of things is that on occasions Jupiter and each of the comets may come into such proximity that the superior mass of Jupiter may exercise a very seriously disturbing influence over a flimsy and ethereal body like a comet. There is reason to suppose that some of the comets now belonging to the Jupiter group have not done so for any great length of time, but having been wandering about, either in elliptic orbits of great extent, or even in parabolic orbits, have accidentally come within reach of Jupiter, and so have been, as it were, captured by him. Hence, the origin of the term, the “capture theory,” as applied to these family groups of comets which I have just stated to exist, each presided over, as it were, by a great planet. It may be that at some future time this theory will help us to a clue to the fact that besides the comets of Lexell of 1770, Blainpain of 1819, and Di Vico of 1844, short period comets unaccountably missing, there are several others presumed to have been revolving in short period orbits when discovered, and as to which it is very strange that they should not have been seen before their only recorded visit to us, and equally strange that they should never have been seen since. Is there any reason to fear the results of a collision between a comet and the Earth? None whatever. However vague may be, and in a certain sense must be, our answer to the question, “What is a comet?” certain is it that every comet is a very imponderable body—a sort of airy nothing, a mass of gas or vapour.[6] At the same time it always has been and perhaps still is difficult to persuade the public that whatever might be the effect on a comet if it were to strike the Earth, the effect on the Earth, were it to be struck by a comet, would be _nil_. This is not altogether a matter of speculation, for according to a calculation by Hind, on June 30, 1861, the Earth passed into and through the tail of the great comet of that year at about two-thirds of its distance from the nucleus. Assuredly there was no dynamical result; but it seems, however, not unlikely that there was an optical result; at any rate, traces of something of this sort were noted. Hind himself, in Middlesex, observed a peculiar phosphorescence or illumination of the sky which he attributed at the time to an auroral glare. Lowe, in Nottinghamshire confirmed Hind’s statement of the appearance of the heavens on the same day. The sky had a yellow auroral glare-like look, and the Sun, though shining, gave but feeble light. The comet was plainly visible at 7.45 p. m. (during sunshine), and had a much more hazy appearance than on any subsequent evening. Lowe adds that his Vicar had the pulpit candles