form of tail, some account of which will be given in a later chapter.

Occasionally a comet exhibits besides its principal tail a secondary one usually less bright and shorter than the main tail. For instance, Pons's long-period comet of 1812 at its apparition in 1886 had on Dec. 29 a primary tail 8° long and a secondary one very faint and only 3° long. But the secondary tail is not always the shorter of the two. Swift noted a secondary tail in the case of the comet ii. of 1881, which was some 55° long, the longest secondary tail on record ".

The trains of some great comets have been seen to vibrate in a manner somewhat similar to the Aurora Borealis. The tails of the comets of 1618 (ii.) and 1769 may be cited as instances: the observer in the latter case was Pingré, whose great knowledge of comets adds weight to his testimony. The vibrations commenced at the head, and appeared to traverse the whole length of the comet in a few seconds. It was long supposed that the cause was connected with the nature of the comet itself, but Olbers has pointed out that such appearances could only be fairly attributed to the effects of our own atmosphere, for this reason :— "The various portions of the tail of a large comet must often be situated at widely different distances from the Earth; so that it will frequently happen that the light would require several minutes longer to reach us from the extremity of the tail than from the end near the nucleus. Hence, if the coruscations were caused by some electrical emanation from the head of the comet, even if it occupied but one second in passing over the whole surface, several minutes must necessarily elapse before we could see it reach the tail. This is contrary to observation, the pulsations being almost instantaneous." Instances of this phenomenon are not very common. The most recent case is that of Coggia's comet of 1874. An English observer at Hereford named With noticed an oscillatory motion of the fan-shaped jet upon the nucleus as a centre which occurred at intervals of from

u Work of Warner Observatory, vol. i. p. 22.

× Mém. Acad. des Sciences, 1775, p. 302.

3 to 8 secs. The fan seemed to 'tilt over' from the preceding to the following side, and then appeared sharply defined and fibrous in structure, then it became nebulous, and all appearance of structure vanished." A flickering of the tail of this comet was observed also by Newall".

Respecting the physical constitution of the tails of comets it may be said that probably in many cases they are hollow cones. This theory would accord with the observed fact that single tails usually increase in width towards their extremities and are divided in the middle by a dark band, the brilliancy of the margins exceeding that of the more central portions. Similarly, comets with tails of tolerably uniform width throughout may be regarded as hollow cylinders ".

The following is an excellent instance of the ever-changing appearance of comets; it relates to that of 1769. On Aug. 8, Messier, whilst exploring with a 2-foot telescope, perceived a round nebulous body, which turned out to be a comet. On the 15th the tail became visible to the naked eye, and appeared to be about 6° in length; on the 28th it measured 15°; on Sept. 2, 36°; on the 6th, 49°; and on the 10th, 60°. The comet having now plunged into the Sun's rays, ceased to be visible. On Oct. 8, the perihelion passage took place; on the 24th of the same month it reappeared, but with a tail only 2° long; on Nov. 1 the tail measured 6°; on the 8th it was only 2; on the 30th it was 14°: the comet then disappeared.

Transits of comets across the Sun no doubt occasionally happen, but only one such spectacle has ever been witnessed, and even then the nature of the sight was not understood till afterwards. The German Sun-spot observer, Pastorff, noticed on June 26, 1819, a round dark nebulous spot on the Sun; it had a bright

Ast. Reg., vol. xiv. p. 13. Jan. 1876. 2 Month. Not., vol. xxxvi. p. 279. March 1876.

a This work is a record of facts rather than of theories, and is too bulky already. Otherwise I might have given it a great expansion by embarking on a review of some of the chief theories which have

been broached respecting Comets. For some particulars as to these see a paper by Huggins, Proc. Roy. Inst., vol. x. p. 8, 1882; a paper by Bredichin, Remarques générales sur les queues des comètes; also an article by Ranyard in Ast. Reg., vol. xxi. p. 58, March 1883.

point in its centre. Subsequently when the orbit of comet ii., 1819 came to be investigated, Olbers pointed out that the comet must have been projected on the Sun's disc between 5 and 9h A.M. Bremen M.T. Pastorff asserted that his "round nebulous spot" was the comet. Olbers, and with him Schumacher, disputed the claim, and the matter seems not free from doubt". Comet v. of 1826 was calculated to cross the Sun on Nov. 18, 1826, but owing to the general prevalence of bad weather in Europe, only 2 observers were fortunate enough to be able to see the Sun on that day, and neither of them could obtain a glimpse of the comet.

Sir J. Herschel once watched Biela's comet pass in front of a cluster of stars, but no obliterating effect was noticed, the several stars being all clearly visible through the comet's ethereal body.

b For some further particulars as to this controversy see Webb's Celest. Obj., 4th ed., p. 40, where there is also a facsimile of Pastorff's original sketch. See also an important paper by Hind in

Month. Not., vol. xxxvi. p. 309, May 1876.
Hind seems to have the idea of there
being either error or fraud involved in
Pastorff's narrative.

CHAPTER II.

PERIODIC COMETS a.

Periodic Comets conveniently divided into three classes.—Comets in Class I.— Encke's Comet. The resisting medium.-Table of periods of revolution.—Tempel's second Comet.-Winnecke's Comet.-Brorsen's Comet.-Tempel's First Comet.— Swift's Comet.-Barnard's Comet.-D'Arrest's Comet.-Finlay's Comet.-Wolf'8 Comet.-Faye's Comet.-Denning's Comet.-Mechain's Comet of 1790.—Now known as Tuttle's Comet.-Biela's Comet.-Di Vico's Comet of 1844.-List of Comets presumed to be of short periods but only once observed.—Comets in Class II.-Westphal's Comet.-Pons's Comet of 1812.-Di Vico's Comet of 1846.-Olbers's Comet of 1815.—Brorsen's Comet of 1847.—Halley's Comet.— Of special interest.—Résumé of Halley's labours.—Its return in 1759.—Its return in 1835.—Its history prior to 1531 traced by Hind.-Comets in Class III not requiring detailed notice.

THE

HE comets which I propose to treat of in the present chapter may be conveniently divided into 3 classes:

1. Comets of short periods.

2. Comets revolving in about 70 years.

3. Comets of long periods.

The following are the comets belonging to Class I, with which

ENCKE'S COMET.

No. 1 is by far the most interesting comet in the list, and I shall therefore review its history somewhat in detail.

On Jan. 17, 1786, Méchain, at Paris, discovered a small telescopic comet near the star ẞ in the constellation Aquarius. On the following day he announced his discovery to Messier, who, owing to unfavourable weather, did not see it till the 19th, on which night it was also observed by J. D. Cassini, Jun., and the original discoverer. It was tolerably large and well-defined, and had a bright nucleus, but no tail.

On Nov. 7, 1795, Miss Caroline Herschel, sister of Sir W. Herschel, discovered a small comet, about 5' in diameter, without a nucleus, but yet having a slight central condensation of light. Olbers observed it on Nov. 21, when it was too faint to allow of the field being illuminated, and he was obliged to compare it with stars in the same parallel by noting the times of transit across the field of view. It was round, badly defined, and about 3' in diameter. The orbit greatly perplexed the calculator, and Prosperin declared that no parabola would satisfy the observations.

On Oct. 19, 1805, Thulis, at Marseilles, discovered a small comet, which was faintly visible to the naked eye. Huth stated that on the 20th it was very bright in the centre, though without a nucleus, and 4 or 5′ in diameter. On Nov. 1 the same observer saw a tail 3 long. Several parabolic orbits were calculated, and one elliptic one by Encke, to which a period of 12.127 years was assigned.

On Nov. 26, 1818, the indefatigable Pons, of Marseilles, discovered a telescopic comet in Pegasus, which was very small and ill-defined. As it remained visible for nearly 7 weeks, or till Jan. 12, 1819, a rather long series of observations was obtained; and Encke, finding that under no circumstances whatever would

and physically, very interesting objects; that scarcely a year ever passes that some of them do not return to the Sun and therefore to visibility as regards the

Earth; and that, consequently, they are objects which furnish many instructive chances to the class of students for whom this work is mainly intended.