mathematical triumph was far greater. Damoiseau, Rosenberger, and Pontecoulant calculated the comet's return to perihelion within two or three days, instead of a month, and the time when it passed this point of its orbit corresponded, within a few hours, to the mean of their several estimates. On the northern heavens, where it was first seen, the comet presented a remarkable appearance, with a long and brilliant tail stretching over an arc of many degrees upon the sky. When it had passed from our northern skies, it was carried (after a short interval, during which it was lost to view in the sun's rays) to the southern heavens. Sir John Herschel, and Maclean (Astronomer Royal at the Cape), were prepared to receive it; but when first observed by them it showed none of the features which made it so remarkable in our skies. It had no tail and scarcely any head. In fact, Sir John Herschel, in one account, says, that as first seen it could only be distinguished from a fixed star by its motion. The study of its gradual change of aspect from that time threw so much light on the nature of comets' tails and other appendages (or at any rate of that particular comet's tail) that Sir John Herschel, not accustomed to be over-confident, said there could be no doubt as to the true interpretation of the observed phenomena.

Most persons know that the name 'comet' is derived from the word coma, or hair, and is applied to celestial objects which appear to have a hairy appendage. Modern astronomers do not, indeed, use the word coma in this sense, but draw a distinction between the coma and the tail. There can be no doubt, however, that the part now called the comet's tail was that from which these objects derived their The word cometa or cometes is not a lately-formed one; but was used by Cicero, Tibullus, and other ancient writers, and it is worthy of notice that all the names applied to comets by the Romans had a reference to hairinessstella comantes, crinite, concinnata, they are called by Ovid, Pliny, and Cicero. The last term-signifying stars which show a curled or crisped hairiness-would not be very

name.

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applicable, by the way, to any comets that have appeared in modern times. The Chinese applied to comets the name sui, or' broom.'

It might be supposed that the hairy, broom-like, or taillike appendage, so commonly seen in comets, is really a distinctive feature of these comets. This, however, is far from being the case. A very large number of comets have no visible tails. We refer, of course, principally to telescopic

comets; for very few comets which have been conspicuous to the naked eye have wanted this appendage.

-is never

The coma-in the modern astronomical sensewanting. This term is applied to a misty, hazy light, surrounding on every side a small bright spot, which is termed the nucleus of the comet.

When first seen in the telescope, a comet usually presents a small round disc of hazy light, somewhat brighter near the centre. As the comet approaches the sun, the disc lengthens, and, if the comet is to be a tailed one, traces begin to be seen of a streakiness in the comet's light. Gradually

a tail is formed, which is turned always from the sun (fig. 2). The tail grows brighter and longer, and the head becomes developed into a coma surrounding a distinctly-marked nucleus. Presently the comet is lost to view through its near approach to the sun. But after a while it is again seen, sometimes wonderfully changed in aspect through the effects of solar heat. Some comets are brighter and more striking after passing their point of nearest approach to the sun (or perihelion) than before; others are quite shorn of their splendour when they reappear. The latter was the case with the comet of 1835-36, as we have already seen. On the other hand, the comet of 1861 burst upon us in its full splendour after perihelion-passage.

Some comets have more than one tail. One appeared in 1744 which had five or six double tails, symmetrically disposed (if one can trust the pictures handed down to us) in the figure of a half-opened fan (fig. 3). Others have presented a yet more peculiar appearance, having, besides a tail in the usual position, a second 'unconformable' tail, at right angles to the first, or inclined to it at some incongruous, out-of-the-way angle-for instance, in one case, one hundred and sixty degrees. Sometimes the peculiarity is presented of a perfectly dark gap separating the tail from the head. More commonly a dark space is seen behind the head, but on each side of this space the light from the head is continued so as to form a bright border on each side of the tail.

As a comet approaches the sun we have seen that a change takes place in the appearance of the coma and nucleus, and that in some instances a tail is generated. The process actually observed is generally this: in the forward part of the nucleus a turbulent action is seen to be in progress, leading to the propulsion towards the sun of jets or streams of misty-looking matter. Sometimes a regular cap or envelope is seen to be projected in this manner towards the sun, or even a set of envelopes one within the other. The matter thus thrown off is not suffered to pass very far

from the nucleus towards the sun, but is swept away, as fast as formed, in the contrary direction. If the funnel of a

steam-engine were directed forwards, instead of upwards, then the appearance presented by the emitted steam, as the

The same sun which attracts emitted matter with incona moment what took place When this comet earth (ninety milBut the comet was Onwards it rushed

engine rushed on (against a hurricane, suppose, to make the illustration more perfect) would exemplify the process which seems to be taking place around the front of the nucleus, and far behind it, as the matter formed is continually swept away from the sun. the nucleus seems to repel the ceivable energy. Consider for with Newton's comet in 1680-81 (fig. 4). was about as far off from the sun as our lion miles) it began to throw out a tail. going far nearer to the sun than this. under the powerful influence of the sun's attraction, until it had crossed the whole space of ninety million miles, making —almost in a straight line-for a point only one hundred and thirty thousand miles from the sun's surface. In four weeks it traversed that vast distance, and then, suddenly (in a few hours) sweeping half round the sun, started on its return journey. But note this as it approached the sun, the comet had thrown out a tail continually increasing in length, and pointing back almost along the orbit; then the comet is lost to sight for a few days, and when it is next seen returning rapidly from the sun, it has a tail pointing forwards (a tail which must be a different one, since—as Herschel says 'we cannot conceive a comet's tail to be brandished round like a stick') and ninety million miles in length. So that, whereas the comet, already moving with a tremendous acquired velocity, had taken four weeks in traversing a distance of ninety millions of miles under the sun's attraction, the matter composing the tail had been thrown to the same enormous distance by the sun's repulsion in scarcely one-tenth part of the time, possibly (for the tail was formed when first seen) in a few hours!

The comet of 1843 (fig. 5), was yet more remarkable for the dimensions of its tail and for its close approach to the sun. The tail of this comet stretched half-way across the sky in March, 1843. Its real length was two hundred million miles at least, for the end of the tail was lost to view