round the sun, and it would not move with increasing velocity as it neared the mighty mass until it had gathered the energy for its own escape in the enhanced and quickened momentum. In the first instance the ready obedience to the attraction, and then the overshooting of the spot from which it is exerted, combine to establish the comet's right to stand ranked at least amongst the ponderable bodies of space.

But comets are, beyond this, distinguished members of the ponderable confraternity on the ground of size. Anyone who has approached towards a really effective notion of what the enormity of the distance is that extends between the earth and the sun-a vast chasm that it would take the ordinary travelling speed of the railway more than three centuries to cross-and who has connected with this notion the statement already made in a preceding page that comets have been seen with tails trailed out into space nearly as far again as the earth is from the sun-will be quite prepared to admit that such must be the case. The head of the comet of 1811 measured 127,000 miles across, and the coma or external investment 643,000 miles. A body with such ample dimensions has, therefore, a good claim to be ranged among the giants, at least, of the system. It is not necessarily, however, as potent or influential as it is big. The distinction is one of size, rather than of power, as will presently be further shown.

The illustrious author of the Theory of Universal Gravitation conceived the idea, as a part of his grand system, that these giant luminaries of the sky were really ponderable bodies, moving in elliptical orbits about the sun, and remarked that they should be seen returning at regular periodical intervals to pay their obeisance to the great ruling orb of the system. It remained, however, for Edmund Halley, the second in the series of the distinguished men who have acted as the Astronomers Royal of England, and who happily was intimately associated with Newton in his labour of publishing the Principia,' to prove the truth of this sagacious conception. Halley undertook to examine the circumstances under which some of the most remarkable of the earlier comets had been observed, and in doing so dealt first with the records of twenty-four well-known and well-authenticated visitants, and he soon came to the conclusion that three of the records referred to only one body; that, namely, which described a comet seen by Appian in 1531; that which related to a comet seen by Kepler in 1607; and that which gave the elements of a comet watched by himself with much interest in 1682.

He

unhesitatingly announced that these were all instances of the return of one and the same luminary which was revolving in a very elongated ellipse about the sun, and which presented itself at these particular times within the range of human observation. By a subsequent extension of his investigation he found that there were conspicuous comets also seen in the years 1305, 1386, and 1456, which most probably were only earlier returns of the same visitant. There were slight irregularities in the periodic returns of these comets, but such only as seemed to Halley to prove, rather than to invalidate, his conclusion, because under the circumstance of the universality of gravitation it would only be in accordance with proper rule that there should be incidental augmentation and retardation of pace caused by the perturbing influence of planetary or other bodies that chanced to be near the course of the comet's movement. If the period of the planet Saturn were disturbed to the extent of several days by the influence of the neighbouring planet Jupiter, how much more liable to ' derangement,' to use Halley's own words, 'must a comet be 'whose excursion into space was four times greater than that of Saturn, and whose orbit was so eccentrically drawn out that if the velocity of the traveller were increased by the 120th 'part of its value, the elliptic course would be changed into a parabola!' Upon a final review of the whole argument, and taking into consideration the fact that the influence of Jupiter would of necessity be exerted in retarding the return of the comet, Halley ventured to prophesy that the same luminary should return into the range of human vision after another revolution of between seventy-five and seventy-six years, and that it should therefore be again seen from the earth at the end of 1758, or at the beginning of 1759.

Halley assumed the labours of Astronomer Royal at Greenwich at the advanced age of sixty-four years, in order that he might himself carry through a task which he had entered upon in the interests of astronomical science; namely, the uninterrupted observation of the movements of the moon through an entire cycle of its revolutions, which must occupy about nineteen years; and it is a notable fact in the annals of astronomy that the veteran observer accomplished this task notwithstanding the strength of the chances against his doing so. He ruled over the instrumental work of the Royal Observatory for a period of twenty-two years, and therefore saw the moon well through its round of recurring phases. He was less fortunate, however, in regard to the movements of his comet. He died in the beginning of the year 1742, when

the comet was still sixteen years' travel away from its predicted return. The anticipated visit was, nevertheless, not lost sight of. Astronomers looked anxiously forward to it at the appointed time. In 1757, as the anticipated visit approached, the French mathematicians Clairaut and Lalande undertook the more exact investigation of the probable influences of Jupiter and Saturn upon the movements of the comet, and they arrived at the conclusion that these planets should retard the return not less than 518 and 100 days respectively, and that therefore the period of the comet should be seventy-six years and 211 days, with the 13th of April, 1759, for the date of its next perihelion passage. The comet was observed approaching the earth by George Palitsch, an amateur Saxon astronomer, on the 25th of December, 1758, and it actually made its perihelion passage before the sun on the 13th of March, 1759; and in this way the discovery that comets do move in courses that are amenable to the great laws of material attraction and orderly elliptical movement was happily accomplished, and a remarkable experimental proof of the sufficiency of the Newtonian theory was secured. The comet of Halley, now of such surpassing interest, was again seen at its next return in the beginning of 1836, and it was then, fortunately, an object of close study to Sir John Herschel, who watched its passage, night after night, with the mirror of his large telescope. Mr. Hind, one of the best living authorities in matters of this class, believes that this interesting comet can be tracked back in the records of human chronology through not less than twenty-five

returns.

Halley's comet, however, is not now alone in this peculiarity of orderly and predictable return. It has already a somewhat large band of associates in this particular. But most of the periodic comets are faint objects requiring powerful telescopes and technical skill for their observation. Of comets of this character there are several that are known to have short periods, reaching from three to sixteen years, and moving in elliptical paths that stretch scarcely as far from the sun as the orbit of Saturn. These periodic comets are, for the most part, distinguished in the annals of astronomical science by the names of the observers who picked them up with their telescopes during their passage amongst the stars. Prominent among these godfathers of comets stand the well-known astronomers Encke, Biela, Faye, D'Arrest, and Lexel. Five other comets besides the notorious one of Halley make wider excursions like it, and have periodic returns ranging between sixtyseven and seventy-four years. Some others stand narrowly

suspected of having periods of still longer measure. Coggia's Comet of the present year takes prominent rank among these, having had an elliptical orbit assigned to it by Schulhof, of Vienna, with an elongation from the sun 430 times larger than the earth's mean distance, and with a period of revolution of not less than 12,184 years. This comet is only surpassed in the length of its periodic excursion into space by the great comet of 1844, which is believed to have a period of 102,000 years!

Halley's method of fixing the identity of a comet depends upon observing that the successive returns occur after approximately equal intervals, and that on each return the luminary moves along nearly the same track among the stars. There is another plan, however, of accomplishing the same end of forecasting a comet's reappearance which proceeds upon an essentially different method, inasmuch as it works forwards instead of backwards. By this plan three distinct points of the comet's path among the stars require to be definitely ascertained, and then the curve in which the luminary is moving in the sky is traced out in continuation of the line connecting these points. If it appears that this trace lies along the course of an elliptical curve, which gives a closed figure in the extension returning into itself, it is then inferred that the comet is a periodic one, and will assuredly come back into sight again. But if, on the other hand, the track runs into a more open curve that passes on into a line that diverges for ever from the direction of approach, it is then inferred that the luminary which moves in this path must be an accidental and passing visitor coming once into the range of human vision and then vanishing for ever into the remote depths of space.

A catalogue of the bright comets that have been seen, however, by no means gives an adequate idea of the number of these luminaries that come under human observation. Three or four telescopic comets are now entered upon astronomical records every year. Lalande had a list of 700 comets that had been observed in his time. Arago's estimate of the number that must pass within the influence and range of the planetary system, at some time or other, amounted to 7,000,000! And this estimate of Arago's is well worthy of a passing thought, if it be only for the grand idea it affords of the enormity of space that finds room for 7,000,000 of bodies whirling about in all directions, although not more than three or four of the vast series can be caught by the observer in any single year. This result is obviously a natural consequence of the rapid way in which the bulk of a spherical space is augmented with the progressive enlargement of its radius.

VOL. CXL. NO. CCLXXXVI.

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The sun is 104 times as wide again as the earth; but it has a bulk many hundred thousand times as large again as the bulk of the earth. If the spherical dimensions of the sun be imagined to be expanded to anything like the available space of the surrounding universe, it will be comprehended how there may be many millions of comets circling in the spacious field, although not more than three or four of them can be seen at any one instant from the earth.

Whenever it happens that a large comet comes within convenient distance for observation the opportunity is eagerly seized to examine with unintermitting assiduity the appearances it presents as it passes through the ordeal of near approach to the sun. Newton was quite aware of the curious fact that the nucleus and head of a comet grow smaller as well as brighter on approach to the sun, and that they grow larger as well as fainter as they move into space. He explained this peculiarity by conceiving that the comet veiled itself in smoke as it passed before the glory of the sun's face, and then scattered the veil as it withdrew from the bright presence. Sir John Herschel probably advanced a step further in the line of reasonable explanation when he connected the condensation and brightening of the comet on its advance towards perihelion with the conversion of visible mist into transparent invisible air, and its subsequent enlargement and paling away with the re-condensation of transparent air into visible mist and cloud.

That this luminous mist, of which the chief bulk of the comet is composed, is a something of almost inconceivable fineness and rarity, is beyond all question. This is abundantly and incontrovertibly manifested by the slight physical influence which the filmy substance exerts when it comes into close relation with other ponderable bodies. In the year 1779, Lexell's comet approached so near to the earth that it would have increased the length of the sidereal year by three hours if its mass had been equal to the earth's. It would have increased the length of the sidereal year by one second if its mass had been equal to a five thousandth part of the earth's. It had, indeed, its own journey very materially retarded by the earth's influence; yet not only did it not change the length of the sidereal year even to the extent of a single second, but it did not even raise the tidal swell of the ocean in any perceptible degree. Both in the year 1767 and 1779 this comet passed through the midst of Jupiter's satellites, and became entangled temporarily among them. But not one of the satellites altered its movements to the extent of a hair's breadth, or of a tenth of an instant. This extreme tenuity of the