Next consider the great comet of 1861. This comet was discovered on May 13, by Mr. John Tebbutt, jun., of New South Wales, and first accurately observed at the Sydney Observatory, on May 26. It passed northwards from the southern skies, and first became visible in Europe in the last week of June, 1861. The first recorded observations were made on the evening of June 30, nineteen days after it had passed its point of nearest approach to the sun. I remember well observing it on the morning of July 2, 1861. For some reason, I found it impossible to sleep that morning, and getting up about three in the morning (the exact hour I do not remember, but it must have been very early), I saw in the east what looked at first like the rays of an aurora borealis. But presently I noticed that these rays proceeded (unlike those of the aurora) from a bright centre, which had been hidden by clouds when my observations began. I used at that time to keep a four-inch' telescope, mounted on a three-legged stand, in my bedroom. This

I had quickly ready for action (noting that the object, owing to the approach of sunrise, was getting fainter every minute), and turning it on the comet, I drew a picture of the nucleus and coma so closely resembling that which appeared a week or two later in the 'Illustrated London News,' that I might have supposed my picture had been surreptitiously sent to the office of the Illustrated,' had I not found it resting just where I had put it in my scientific portfolio.

The comet appeared to the eye as shown in fig. 9. Sir John Herschel, who observed it at Collingwood, in Kent, remarked that it was far more brilliant than any comet he had ever seen, not even excepting those of 1811 and 1858. The Padre Secchi, at Rome, found that in the clear skies of Italy the tail was fully 118° in length, corresponding to nearly one-third more than the distance between the horizon and the point overhead. This comet, by the way, though only favourably visible for a very short time, remained within the range of telescopic vision much longer. Hind remarks

that the number of separate observations for the determination of its orbit exceeded 1,150, and extended over a period of 11 months. It travelled on a course favouring observation, coming from remote distances south of the plane in which

the earth travels to the northern side of that plane-and, as it chanced, crossing the plane (about five-sixths of the way from the sun to the earth's orbit) just when the earth lay in the same direction from the sun, so that for a time she was within the bounds of the comet's tail-like appendage-and then travelling northwards on a path almost at right angles

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FIG. 9.

to the plane of the ecliptic. Thus the comet could be tracked on its retreat until, finally, distance concealed it from our view.

Now, the tail of the comet of 1861, as seen in fig. 9, had something of the fan-like expansion observed in the tail of the comet of 1744; but what was known of the comet's position at the time when this fan-like form was seen, explained the peculiarity, and showed the necessity of taking into account the position of a comet before attaching undue importance to the apparent figure of its tail. For the fan-like form seen on this occasion was a mere effect of perspective. The end of the tail appeared very much wider than the part near the head—not that it really was so, but simply because it was very much nearer to the observer on earth. When we were actually immersed in the tail, the part nearest to us, being all round, had, to all intents and purposes, an infinite extension. But even when the comet was beyond that position, or a few days earlier, before it had reached it, the end of the tail was much nearer to us than the comet's head, and thus appeared far more proportionately widened than was actually the case.

Such considerations must always be taken into account in dealing with cometic phenomena. Comets, more than any other celestial objects (the Milky Way, regarded as a whole, being, perhaps, alone excepted), are affected in shape, and (apparently) even in their very nature, by position, and consequent fore-shortening.

It may be well here to consider a case in which some active force (other than gravity), exerted by the sun, seems to have wrought the destruction of a comet, or at least to have broken up the comet into unrecognisable fragments.

No comet ever observed has exhibited phenomena more remarkable than those displayed by the comet known as Biela's (more properly called Gambart's). I wish I could agree with a modern astronomer, who has said that no comet has thrown more light on the nature of these bodies; but, in point of fact, it is only promise of light, not light itself, that we have obtained.

Discovered in 1826, Biela's comet was presently found to be identical with one seen in 1772 by Montaigne, and again by Pons in 1805. A careful study of the observations showed that the comet travels round the sun in a period of about 6 years, or, roughly, thrice in twenty years. Its path was found to approach very near to the path of our earth. The comet returned in 1832, when the ignorant were scared

much as they have been recently by the threatened influence of the larger planets in perihelion. The comet crossed the earth's track several weeks before she herself came to the place where the two orbits approach nearest, and it is hardly necessary to say that the comet's passage did not injure the earth's roadway in any appreciable degree.

In 1839 the comet returned, but was not seen, travelling across a part of the heavens only. above the horizon in the day-time, so that the comet's light was hidden by the sun's.

It was at the next return in 1845-46 that the comet first attracted special attention. On that occasion, instead of behaving as comets usually do, Biela's, which in the first days of 1846 had presented the appearance shown in fig. 10, was found to have divided into two. There is some little doubt as to the time when the comet underwent division. Lieut. Maury reported on January 15 that he had seen the comet double on January 13; but Wichmann observed it as a single comet on the 16th. But Professor Challis, in his account of his own observations on the comets, states that even on January 15 the second comet might easily have been overlooked. M. Valz saw nothing unusual on the 18th and 20th; but on the 27th: 'I was struck with amazement,' he says, 'to find two nebulosities, separated by an interval of two minutes of arc, instead of one nebulosity alone. Each head was followed by a short tail, whose direction was perpendicular to the line joining the two nebulosities.' Earlier, only the larger comet had had a tail, the appearance presented by the double comet being that shown in fig. II.

The two comets travelled along, side by side, until at last both passed out of view, at which time the distance between them amounted to about 157,000 miles.

In 1852 both comets returned. Sir John Herschel says, in his‘Familiar Lectures on Scientific Subjects,' that when they returned, the distance between them was unchanged. This, however, was a mistake. The distance now amounted to about 1 millions of miles. Again they passed before the interested gaze of astronomers, travelling side by side, though rather far apart, until finally they disappeared from view-I say finally, for neither has ever been seen again.

Whether the two comets returned in 1859 is doubtful. It is certain that, if they did, they would have been invisible, for the same reason that the comet was invisible when it returned in 1839.

But in 1866 the double comet should have been well
It should be remembered that each return of a comet

seen.

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