pily a French Jesuit, named Gaubil, had the good fortune while a missionary in China to unearth some of their astronomical records. He sent a translated copy to France, and the information thus furnished to Europe enabled Mr. Hind to identify the comet of the Conquest with Halley's.

Passing on to the middle of the fifteenth century-to the year 1456-we again find Halley's comet occupying a conspicuous place in European affairs. At the moment of its first appearance the conquering Turks, under Mahomet the Second, were threatening to lay waste Christendom. Already Constantinople had fallen into their hands, and from the summits of her stately edifices gleamed the Ottoman crescent. The cities of the Danube were rapidly giving way before their irresistible march, and now Belgrade was invested. In the midst of the consternation, faction raged in the Courts of Europe and conflicting counsels distracted the Latin Church.

The comet came into view on the 29th of May, and was seen gliding over the sky towards the Moon. Writers of the eventful period say that it shone with sparkling lustre and spread out a fan-shaped golden train; others describe its form as that of a Turkish scimitar, which, glinting in the moonlight, was regarded as a sign from Heaven of the conflict raging between the Christian and the Pagan hosts. Onward it came with sweeping glories, speeding its way towards the Sun; it reached perihelion on the 9th of June, then, turning its course, it gradually passed away into the depths of space. A circumstantial account of the comet's appearance has been left us by a Bavarian Jesuit named Pontanus. relates his story on the authority of George Phranza, Grand Master of the Wardrobes to the Emperor of Constantinople. He speaks of the comet rising in the west, moving towards the

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east, and approaching the Moon. sequent writers, stirred by a love of the marvellous, have made out that the comet actually eclipsed the Moon. A lunar eclipse certainly took place while the comet was in the sky, caused as always by the Earth's shadow falling on the Moon; nothing Pontanus had said, however, warranted the inference that the comet had occasioned the obscuration. But this lapse has left no trace in astronomical history at all comparable with the myth woven about the Pope's action in regard to the comet. Leading writers one after another declare in slightly varying terms that the reigning Pontiff, Calixtus the Third, was so alarmed by the apparition in the heavens that he issued a Bull exorcising the evil thing, and that to the Ave Maria be added the words, "From the Turk and the Comet, good Lord deliver us." Arago, Babinet, Guillemin, Hind, Draper, and others all repeat the story; and the Angelus de Midi, we are told, owes its origin to the fears of the aged Pope (then in his eightieth year). The Rev. J. Gerard, S.J., has given a careful sifting to all the documents bearing upon the subject, and in his accustomed calm and dispassionate manner he reduces the story to a figment hardly bearing the semblance of a grain of truth. He shows that the myth may possibly have sprung from the historian Platina's reference to the comet given in his Vitæ Pontificum, published in Venice in 1479. He was living in Rome at the time, and could not help but be alive to the events which were then passing before his eyes, and in respect to the comet he writes:

A hairy and fiery star having then made its appearance for several days, the mathematicians declared that there would follow grievous pestilence, dearth and some great calamity. Calixtus, to avert the wrath of God, ordered supplications that if evils were impending for the human race He

would turn all upon the Turks, the enemies of the Christian name. He likewise ordered, to move God by continual entreaty, that notice should be given by the bells to call the faithful at midday to aid by their prayers those engaged in battle with the Turk.

No mention is made of a Bull or an exorcism against the comet and the Turks, either singly or conjointly.

Between the epochs of 1759 and the ensuing return new methods of computation had greatly lessened the labor of mathematical investigation, and the more accurate measures of the planets, the discovery of Uranus and his moons, the wonderfully increased power of telescopes, all contributed to swell the interest with which the comet's next arrival was anticipated. For with these advantages it was justly inferred that the time of the next appearance would be foretold with a degree of accuracy which had been impossible in the previous case.

The Academy of Sciences at Turin called attention to the approaching epoch so early as the year 1817, and stimulated inquiry by offering their prize to competitors of all nations for the best theory of the comet's movements during its outward and inward journey. French astronomers again were first in the field-namely, Count Pontécoulant and Baron Damoiseau. The former fixed upon the 7th of November 1835 for the comet's arrival at perihelion; the latter, the 4th of that month. Some time afterwards Pontécoulant finally assigned the 12th of November. Meanwhile, two German mathematicians-Dr. Lehmann and Professor Rosenberger-took up the task, and in their results came very near to those of the French computists, Rosenberger giving the 11th of November and Lehmann the 26th. Observers were told to direct their telescopes towards the space between the constellations Auriga and Taurus; and diligent

watch was kept over this region throughout the winter months of 1834-5; but this was too early, and nothing came of it. Among the numerous observers who all through the summer nights of 1835 patiently awaited the comet's return, Father Dumouchel was the first to get a glimpse of it. Favored with the pure skies of Rome and a powerful telescope, he, and the astronomers of the Collegio Romano, caught sight of the comet on the 6th of August, close to the place Rosenberger had indicated for that day. It was then a faint, misty object, barely discernible with the aid of the telescope. The presence of moonlight and cloudy evenings delayed the discovery in this country until the 26th. Pursuing the course Rosenberger had foretold, it arrived at perihelion on the 16th of November, five days behind his computed time, and only four days later than that of Pontécoulant. Rosenberger's careful analysis furnished the interesting information that the attraction of our own planet hastened the comet's return by no fewer than 15 2-3 days; and that Venus, Mercury, and Mars together shortened its period by six days more. He also took into account the probable effects of a resisting medium of cosmical ether on its motion, and this he estimated might bring it a week earlier. This at first sight seems paradoxical, until we remember that the effect of such a resistance would be to contract the comet's orbit by reducing its projectile force. So full and complete were Rosenberger's computations that astronomers generally accorded him the merit of having wrought out the best and most elaborate investigation. The Royal Astronomical Society of Great Britain awarded him their gold medal. M. Pontécoulant was adjudged a similar distinction by the Paris Academy of Sciences. But the Turin Academy conferred their prize upon Baron Damoiseau.

In this country almost continuous bad weather obscured the comet from view until the middle of September, and then only transient glimpses could be got of it. But during October it shone out brightly, and put forth a train twenty degrees long, its course taking it over the constellations Ursa Major, Hercules, and Ophiuchus. The records of this return afford abundant evidence of a general agreement among astronomers to subject the nucleus to a rigorous scrutiny, in order to get a better knowledge than they had of the composition and character of these still mysterious bodies. The names of Herschel, Struve, Arago, Bessel, and Maclear are among the foremost of the investigators.

No sooner had the comet become generally visible than phenomena of an interesting description were noticed, such as completely altered its form, and which seemed to be connected with the production of a stream of nebulous matter flowing from the head and repelled with great force away from the Sun, in whatever direction the comet was moving. On the 2nd of October a luminous jet, resembling in shape a partially opened fan, was seen issuing from the nucleus towards the Sun; then, apparently meeting with some extraordinary repulsion, was vehemently swept backward, and, passing down either side of the body, blended into one stream and so formed a prolonged train. Bessel, observing the comet at Königsberg, was perfectly sure that he had now discovered the true source of supply for the tail. He declared that this appendage is a purely electrical phenomenon, produced by a repulsive force residing in the Sun, and about twice as powerful as the Sun's attractive or gravitational force. He noticed that this stream of matter oscillated like a pendulum to and fro, across a line joining the Sun and nucleus, in a period of 4 3-5 days.

On the 6th, the comet having got much nearer to the Sun, the beautiful spray-like ejection disappeared, the efflux having entirely ceased; then the nucleus, as if cleared of much of its grosser matter, shone out brilliantly, exhibiting a sharp, well-defined round body. Two days later the efflux was resumed with an energy likened to a volcanic eruption, and showing signs of excessive agitation running through the whole body of the comet, giving it a quivering motion like that of a compass needle, or the oscillation of a railway train in transit when viewed from an eminence. As it gained the vicinity of the Sun, flame-like jets were seen, and the head of the comet (according to Professor Struve, of Dorpat) glowed "like a red-hot coal of oblong form." Bessel described the appearance as that of a "blazing rocket," the flame from which was driven aside as by a strong wind, or as the stream of fire from the discharge of a cannon when the sparks and smoke are carried backwards by the surrounding air. Three independent ejections burst forth from the body of the comet and streamed off in different directions, changing their course while under the gaze of the ob

server.

ing evidence of the operation of some powerful physical agency which had wrought an entire transformation of its whole structure. In the body of the comet was seen a miniature comet possessed of a vivid nucleus, having a head and tail of its own, considerably more distinct in 'outline than the outer comet; and in the head of the smaller one was a point of a brightness so intense as to resemble a small star shining amidst a dense chevelure. Sir John Herschel, always doubtful as to the solidity of comets, did not see in this case sufficient evidence to warrant positive assurance to the contrary. As the comet receded farther from the Sun it gradually became larger, the nebulosity expanding and becoming denser as it sped on its dark, mysterious way. last, shrouded in a dense coma, it faded from terrestrial view.

At

Sir John Herschel directed his attention chiefly to a consideration of the cause which operated to produce the immense stream of matter we call the tail, though on its outward journey this offshoot precedes the comet. He asks, "What is the secret of its development within view of the observer; of its outward direction, always pointing from the Sun, in its approach, in its perihelion passage and outward flight?" After a careful examination of all the conditions, he finds himself driven to the conclusion that an energy of an entirely different kind from gravity, and far more powerful, must exist in the Sun to produce such effects. In his Results of Astronomical Observations (1847) he says:

Nor let anyone be startled in the assumption of such a repulsive force as is here supposed. Let it be borne in mind that we are dealing with

phenomena utterly incompatible with ordinary notions of gravitating matter. If they be material in that ordinarily received sense which assigns to them only inertia and attractive gravitation,

where, I would ask, is the force which can carry them round in the perihelion passage of the nucleus, in a direction continually pointing from the Sun in the manner of a rigid rod swept round by some strong directive force, and in contravention of all the laws of planetary motion, which would require a slower angular movement of the more remote particles, such as no attraction to the nucleus would give them, supposing it ever so intense?

The mystery grows upon him when he considers the immense stream of matter which issued from the body of the great comet of 1680, and which in five days after passing the perihelion stretched out far past the Earth's orbit; in that brief interval it had shifted its angular direction nearly 150 degrees. Continuing his argument, Sir John asks, "Where are we to look if only gravity be admitted for any reasonable account of its projection outward from the Sun, putting its angular motion out of the question?"

Professor Encke's computation, showing the comet of 1680 to have had a velocity at perihelion of a million miles an hour, is conclusive evidence against an angular motion of the tail. A continuous repulsion of new matter alone can explain the phenomenon.

Astronomers of to-day identify the production of the tail with electrical action exerted by the Sun on the lighter or volatile particles of cometary "matter raised by the solar heat. The German physicist, J. Karl Zöllner, shows that, owing to evaporation and other changes produced by rapid approach to the Sun, electrical processes of considerable intensity must take place in comets; also that their original light is immediately connected with these, and depends upon solar radiation, rather through its electrifying effects than through its seemingly thermal power. Comets are not bodies incandescent through heat, but glowing by electricity; and this is com

patible with a relatively low temperature. It is perfectly well ascertained that the energy of the push backwards produced by electricity depends upon the surface of the body acted upon; that the energy of gravity depends upon its mass. Solar electrical repulsion increases as the size of the body diminishes. Therefore very small cometary bodies-particles of matter indeed-will virtually cease to gravitate, and will be wholly under the repellent solar force. These discoveries in the physical domain of the comet-world afford a complete explanation of the origin and formation of the long stream of almost ethereal matter projected from the body of the comet, and we see how it happens that it is always directed away from the Sun.

A comparison of old recorded appearances of Halley's comet with the appearance it presented in 1835 leads to the conclusion that it is gradually wasting away. In a long, interesting letter to the Times, dated the 30th of September 1881, Mr. Hind, under the heading "The Comet of the Bayeux Tapestry," remarks that its magnitude and intensity of light in 1066 would be half as great again as it exhibited in 1835. It is not difficult to believe that the matter surging from the head of the comet, when in the vicinity of the Sun and transported millions of miles distant, must be permanently separated from the body, and remain dissipated in the planetary spaces, to offer there a resistance to the movements of the planets, or else to form the elements of some new combinations. Considerations such as these awaken a new and profound interest in the comets, particularly now that it is known they are so frequently sweeping through the solar system. They lead directly to Encke's startling doctrine of a Resisting Medium, which inevitably involves the ultimate destruction of the solar system—though

millions of years may elapse, yet still within a definite period. It seems not improbable that in course of time the influence of this ethereal medium upon the comet's rate of motion will be known; and that future astronomers will learn by the accuracy of its returns whether it has met with any unknown cause of disturbance on its

journey outwards of 3,370,000,000 miles. Or it may be the means of revealing to the inward eye of the mathematician the existence of an unknown planet lying beyond the visible boundary of our system, even as the perturbation of Uranus revealed to Adams and Leverrier the existence of Neptune.

Again the veteran computist, M. de Pontécoulant, entered upon an investigation of the comet's movement, with the view of determining the date when it would reach its perihelion. So early as 1864 he announced, in the Comptes Rendus de l'Académie des Sciences, that the next arrival at perihelion would be on the 24th of May 1910. An examination of his work led to doubt as to strict accuracy: it was noticed that he had only considered the disturbing action of Jupiter, Saturn, and Uranus; it was obvious, too, that he had made some numerical errors. Two able computists of the Royal Observatory, Greenwich-Messrs. Cowell and Crommelin-have gone further into the abstruse calculations, taking into account the effect on the comet's movements of all the planets except Mercury and Mars. They do not claim absolute rigor. The task of taking into account every element of computation is simply appalling. At first they reached the conclusion that the comet would arrive at perihelion eleven days earlier than Pontécoulant had assigned. Further investigation has enabled Mr. Crommelin to announce the most probable date to be the 16th of April 1910. It will pass round the Sun with a speed of