of short period (like that which our correspondent Mr. F. Denning, of Bristol, discovered this year) gives the astronomer more perfect mastery of the comet's motions. The return could be predicted with sufficient accuracy in 1832 to cause the comet to be easily redetected. The next visible return might have involved a difficulty, because the comet had in the interval made two circuits. But that return was

successfully predicted. The return in 1845-46 was still more accurately calculated. Nor did the breaking up of the comet into two on that occasion interfere with the suc cessful calculation of the return in 1852. The case may be compared to the rating of a clock, which is more satisfactorily effected in a week than in a day, for the simple reason that any error of observation is spread in one case over seven times as long a period as in the other, and therefore affects

the estimate of any given circuit of the hands by an error only one-seventh as large. Just so, whatever error an astronomer might make in observing Biela's comet in, say, 1843, was distributed over all the revolutions of the comet which had taken place since 1826 (one might almost say since 1772), and in a correspondingly small degree affected the astronomer's estimates of the comet's motion during any single revolution. This being so, astronomers had good reason for believing that in 1866 Biela's comet would return. When the time came that it should have been visible, telescopes were turned towards the spot where it should have been seen. Night after night from that time its calculated track was swept with the finest telescopes in Europe and America. But no trace of the comet could be seen. It is now,' wrote Sir John Herschel, in February, 1866, 'overdue. Its orbit has been recomputed, and an ephemeris calculated. Astronomers have been eagerly looking out for its reappearance for the last two months, when, according to all former experience, it ought to have been conspicuously visible, but without success! giving rise to the strangest surmises. At all events, it seems to have fairly disappeared, and that without any such excuse as in the case of Lexell's-the preponderant attraction of some great planet. Can it have come into contact, or exceedingly close approach to some asteroid as yet undiscovered; or, peradventure, plunged into and got bewildered among the ring of meteorolites, which astronomers more than suspect?'

Be the cause what it might, the comet was not seen in 1866. In 1872 it was looked for even more carefully. Every possible contingency depending on planetary perturbations was considered; and the telescopes of astronomers swept, not only the calculated path, but to a considerable distance on either side of it. No trace of the comet was seen, however, in 1872 any more than in 1869. So far as telescopic observation is concerned, Biela's comet seems to have come to the end of its career as a comet.

Yet the observations of 1852 were not the last which

were made on this interesting object. It has been seen again, though not as a comet. Nay, the occasion on which it was seen in the way referred to was predicted, and the prediction fulfilled, even in details. For a full account of its reappearance as a meteor stream-I refer the reader to my essay on Biela's Comet in 'Familiar Science Studies.'

COMETIC MYSTERIES.

DURING the last two years several comets—some telescopic, others visible to the naked eye, and even conspicuous objects in the heavens-have been observed, not only by the older methods, but by some which have only been available within recent years. It is naturally expected, therefore, by the general public that some new light should be thrown on these mysterious objects, whose phenomena still remain among the unexplained, seemingly the inexplicable, problems of the celestial depths.

We propose to consider here what has thus been learned, and what also (unfortunately it is much more) remains still to be learned, respecting comets. But first it will be well to show what are the special phenomena which present themselves for explanation.

A comet apparently comes out from the remote depths of space in a condition of comparative calm. It appears as a small round nebulous object, looking like a tiny cloud of extreme tenuity-the idea of tenuity being suggested by the exceeding faintness of the comet's light. This cloud appears somewhat condensed towards the middle. As the comet draws nearer to the sun, it usually grows somewhat long in the direction of the sun; and before long a portion within the part nearest the sun is seen to be brighter than the rest, and to have a more or less defined outline. This is the nucleus-sometimes seen as a larger dull disc of nearly uniform brightness, at others as a mere bright point, not unlike a star. The fainter light around this is the coma, or hair,

which resembles a luminous fog round the nucleus, usually brighter on the side towards the sun, and on the other side growing fainter and fainter till it can no longer be seen. Later, this lengthening of the comet in directions towards and from the sun becomes more marked, until at length the comet may fairly be said to have a head directed towards the sun and a tail directed from him. Nucleus, coma, and tail may be very different in appearance in different comets, and in particular the tail may be more or less complicated in structure, being sometimes a mere straight streak, at others twofold, multiple, curved, with thwart streaks, and so forth-no two comets, in fine, having tails resembling each other except in general details.

Dr. Huggins, in a rather disappointing article on comets, recently communicated to the Nineteenth Century, remarks that the nucleus, though an apparently insignificant speck, 'is truly the heart and kernel of the whole thing-potentially it is the comet.' This has scarcely yet been proved, though it appears exceedingly probable. It is true, however, as he adds, that this part only of the comet conforms rigorously to the law of gravitation, and moves strictly in its orbit. 'If we could see a great comet,' he proceeds, 'during its distant wanderings, when it has put off the gala trappings of perihelion excitement, it would appear as a very sober object, and consist of little more than nucleus alone.' This again seems probable, though it has never yet been proved, and the division of some comets into two or more parts, each having coma, nucleus, and tail of its own, shows that the nucleus cannot be, in every case, what Dr. Huggins seems here to suggest. Dr. Huggins has done well in saying (though scarcely with sufficient emphasis, considering how often the mistake is repeated) that though many telescopic comets are of extremely small mass, nucleus included-so small, indeed, that they are unable to perturb such small bodies as Jupiter's satellites—yet we should mistake greatly if we were to suppose that all comets are "airy nothings." In some large comets the nucleus may be a few hundred