sun; as well as suggest to us, through all the points between, how it might appear more or less lengthened, or more or less circular.

But this leads us to the understanding of other names of comets. Sometimes, they are neither called bearded stars, nor yet said to have trains, or trails, or tails, but are spoken of as blazing stars, without more particular description; and this is more convenient when their spreading hair or light neither follows them, nor comes before; but appears to surround them like a man's hair his head, or like his mane the head of a lion, or like its rays the sun itself.

Comets have also been seen without any tail, beard, hair, or coma, whatever.

It has long been known that the tails of comets consist but in their vapoury and transparent substance, through which the stars behind them may be seen.

But the heads or nucleuses of comets appear,

at least in some instances, to be equally transparent, showing the stars through them.

It also appears that the light of comets, however effulgent in itself, shows dimly when brought into contrast with the light shining from a star of even the ninth magnitude.

According to M. Arago, more than seven millions of comets are continually passing and repassing across the planetary orbits of our system.

CHAP. LII.

ABOUT HALLEY'S comet, or the comeT OF 1835 AND 1836.

THE Comet which, in conformity with the calculations of Dr. Halley, appeared in 1835 and 1836, was of high importance to the cometic branch of astronomy; but, to the naked eye, it had little either of that splendour, or that singularity of figure, which have so much contributed to make the history of comets generally striking.

Its importance consisted in the verification, by its appearance at that particular time, of the calculation of Dr. Halley, from whose name it is called Halley's Comet.

Its appearance, in conformity with the calculation, supported the doctrine, that comets are permanent bodies, and move in fixed though eccentric orbits, and are not of the transitory nature of meteors, as many had come to suppose. On that account, the year 1835 had been

looked forward to by astronomers with extraordinary interest; and, whether the calculation was to be successful or to fail, had been called "an epoch in astronomy."

Dr. Halley was a man of the most brilliant and diversified capacity and acquirements, who lived in the time of Newton. He died in the year 1742; but, upon astronomical grounds which were offered by Newton, his study of the cometic phenomena had led him to believe and publish, that the comets of the years 1456, 1531, 1607, and 1682, were returns of one same comet, thus periodically seen at intervals of either seventy-five or seventy-six years; or, in other words, thus performing its revolution round the sun in one or other of those periods.

It followed, that he predicted its returning again in the year 1758, or 1759; and it actually did return and become visible at the end of December, 1758; and reach its perihelion on the 12th of March, 1759.

Its reappearance in the year 1835 was exactly at the time, and in the very place, assigned to it by the astronomical prediction; and its actual arrival at its perihelion* a little before noon on the 16th of November in that year, differed from the computed time only by a small number of days.

"The fulfilment of the astronomical prediction," says a writer whom I have before quoted, "is truly wonderful, if it be considered that the comet is seen only a very few weeks during its passage through our system, and that it wanders from the sun for seventy-five years, to twice the distance of Uranus. This enormous orbit is four times longer than it is broad; its length is about three thousand four hundred and twenty millions of miles, or about thirty-six times the mean distance of the earth from the sun.

"At its perihelion, Halley's comet comes

For the meanings of the words perihelion, aphelion, and nucleus, see the notes a little further on.