smallest particles, because driven away the quickest, produce the straightest tails. The long rigid-looking rays

which issued from Donati's

comet, were, according to this view, made up of hydrogen particles, while its curved plume was composed of carbon. Other comets have thrown out short strangely-bent tails, possibly due to vapour of iron. These are the three types or classes, into which comets' tails have been divided by Bredichin.

294a. The tail is always turned away from the Sun, whether the comet be ap

proaching or receding from that body.

As the comet

still gets nearer the Sun and therefore the Earth, we

FIG. 29-Donati's Comet (showing begin to see in some in

stances that the coma or head contains a kernel or

nucleus, which is brighter than the coma itself, the jets are distinctly visible, and sometimes the coma consists of a series of envelopes. In the beautiful comet of 1858 we saw what this meant; the nucleus was continually throwing off these envelopes or shells which surrounded it like the layers of an onion, and peeled off, and these layers expanded outwards, giving place to others. Seven distinct envelopes were thus seen as they were driven off, they seemed expelled into the tail. Here we have a reason why the tails of comets should, as a rule, increase so rapidly as they approach the Sun, which gives rise to all this violent action :-the tail of the comet of 1861 was

20,000,000 miles in length, but this length has been exceeded in many cases, notably by the tail of the comet of 1843, which was 112,000,000 long, the diameter of the coma being 112,000 miles, that of the nucleus 400 miles.

We have said as a rule, because Halley's comet, as observed by Sir John Herschel, and Encke's comet, furnish us with exceptions. As these comets approached the Sun, both tail and coma decreased, and the whole comet appeared only like a star. Still for all that, in the majority of instances, comets increase in brilliancy, and their tails lengthen as they near the Sun, so much so that in some instances they have been visible in broad daylight. The enormous effect of a near approach to the Sun may be gathered from the fact that the comet of 1680 at its perihelion passage, while it was travelling at the rate of 1,200,000 miles an hour, in two days shot out a tail 20,000,000 leagues long.

295. Since the tail indicates the waste, so to speak, of the head, each return to the Sun must reduce the mass of the comet. A reduction of speed would, moreover, in time be enough to reduce the most refractory comet into a quiet member of the solar family, as the orbit would become less elliptical, or more circular, at each return to perihelion. This effect has, in fact, been observed in some of the short-period comets. Encke's comet, for instance, now performs its revolution round the Sun in three days less than it did eighty years ago. It has been affirmed that this effect is due to the friction offered by an ethereal medium increasing in density towards the Sun—an effect we do not perceive in the case of the planets, their mass being so much larger—as the resistance of the air stops the flight of a feather sooner than it does that of a stone. Sir Isaac Newton has calculated that a cubic inch of air at the Earth's surface—that is, as much as is contained in a good-sized pill-box-if reduced to the density of the air 4,000 miles above the surface, would be sufficient to fill a sphere the circumference of which would

be as large as the orbit of Neptune. The tail of the largest comet, if it be gas, may therefore weigh but a few

ounces or pounds; and the same argument may be applied to the comet itself, if it be not solid. We can understand, then, that with such a small supply there is not much room for waste, and with such a small mass the resistance offered to it may easily become noticeable. This resistance is in all probability due to the interference caused by meteorites met by the comet during its journey.

296. The spectroscopic study of comets proves that their light is partly original, partly that of the Sun reflected. Allowing for the difference of observing conditions, the changes which take place in the spectrum of a comet as it passes from aphelion to perihelion are exactly similar to those which take place during the condensation of a nebulous swarm of meteorites. When the comet is farthest from the sun, its spectrum is that of a planetary nebula. As it approaches the sun, the radiation of carbon is added, and later, that of manganese, lead, and iron. When near perihelion, absorption flutings and lines appear.

297. Biela's comet, mentioned in Art. 288, divided itself into two portions, each separate portion afterwards pursuing distinct but similar orbits. It was first noticed as double December 29, 1845, and the twin comets returned together to perihelion in 1852, but have never since been seen. Yet they ought to have been easily visible when due back again at the Sun in the end of January 1866, as the practically identical track pursued by both intersects the orbit of the Earth at the place occupied by our planet on the 30th of November. The lost comet was, however, strangely replaced by the two brilliant meteoric showers which broke upon the Earth as it passed the node of the comet's orbit, November 27, 1872, and November 27, 1885. These were products of the disintegration of the vanished comet, and may be regarded as a positive proof of the meteoric constitution of comets.

297a. Biela's is probably not the only comet which has broken up into two or more distinct bodies of the same

kind in the course of its travels round the Sun. A "double comet" was observed in Brazil in 1860; and several examples have lately occurred of comets revolving in the same orbit, yet certainly not simply returns of the same body. Thus "Tebbutt's comet," which passed the Sun June 16, 1881, at a distance of 68,000,000 miles, was found to move in the track of a comet observed in 1807, and not again due at perihelion until the year 3346 A.D. The irresistible conclusion is that one original comet split, like Biela's, into two, the fragments following each other along an identical path, while becoming separated (as must inevitably happen) by an ever-growing distance. Even more surprising was the discovery of a similar relation between three great comets-those which appeared in 1843, 1880, and 1882. All of them passed so exceedingly near the Sun as almost to graze its surface; and reliable calculations show them to retire to so vast a distance that 700 or 800 years must elapse before they again successively revisit the solar neighbourhood.

LESSON XXIV.-Luminous Meteors.

November Showers.

aid.

Shooting Stars.

Radiant Points.

298. There are very few nights in the year in which, if we watch for some time, we shall not see one of those appearances which are called, according to their brilliancy, meteors, bolides, or falling or shooting stars. On some nights we may even see a shower of falling stars, and the shower in certain years is so dense that in some places the number seen at once equals the apparent number of the fixed stars seen at a glance; * indeed, it has been calculated that the average number of meteors which traverse the atmosphere daily, and which are large enough to be visible to the naked eye on a dark clear night, is no less than

* Baxendell.