meteors really so close to the solar orb. We look through a range of meteors two or three hundred millions of miles deep (taking into account space beyond the earth's orbit), and it is the combined effect of the light coming from the whole of this enormous range that we really recognise,— not in the corona, but in that proportion of the coronal light which is due to sun-illuminated meteors. The part of the range nearest to the sun may be the part most densely crowded and most brilliantly illuminated; but its extent is limited compared with that of the whole range; moreover, the meteors there situated turn but half-discs (of reflected light) towards the earth, those beyond showing a much larger proportion of their illuminated halves. It is worthy of notice, indeed, that the farther half of the range supplies much the larger. proportion of the light, on account of the greater fulness of illumination,-for, in such a case as this, distance per se is an element which may be absolutely neglected.1

It need scarcely be pointed out that the spectroscope affords the best means for testing this question. If any considerable proportion of the corona's light is reflected from meteors, this portion of the light should exhibit a solar spectrum, though of great faintness; or, unless great light-gathering power were employed, a faint continuous spectrum would be seen. The zodiacal light, also, should exhibit a continuous spectrum if it represents the outer portion of the sun-surrounding meteor families. Until within the last few months the coronal light had been known to give a continuous spectrum as well as certain bright lines (or one bright line); and it had been stated that the zodiacal

'This at first sight may sound paradoxical; but it is strictly true nevertheless. The question is one of the apparent brightness of certain regions of the heavens, not of the total quantity of light received from given groups of meteors. A group of bright objects so far off as to appear like a cloud would preserve its brightness absolutely unchanged, however far off the observer might remove. Its extent alone would diminish.

light gives a bright line spectrum. The first evidence was questionable; the second seemed opposed to the meteoric theory. But Janssen has examined the coronal light with the most powerful light-gathering means yet employed, and he recognises the solar dark lines in its spectrum. This evidence is unquestionable. And Liais, in the clear skies of tropical South America, has examined the zodiacal light, and gets an infinitely faint continuous spectrum, so that what seemed a strong objection to the meteoric theory is removed. Let us pause, however. Liais has been charged with drawing an ideal picture of the corona during total eclipse (his drawing, by the way, singularly countenancing the meteoric theory). But it was ideal : how, then, shall Liais's evidence be trusted on any other subject? What, however, if it was not ideal at all, but simply characteristic, because Liais observed the eclipsed sun under singularly favourable conditions at his southern stations in 1858? This is precisely the inference fairly deducible from (or rather the conclusion forced upon us by) the evidence of the observers of the recent eclipse. Janssen speaks of special forms resembling those seen by Liais; observer after observer speaks of complicated structures within the corona; the photographs tell the same tale.

In conclusion, I believe little question can exist that a large proportion of these phenomena which have seemed most perplexing, as well in the solar corona as in the zodiacal light, admit of being very readily explained when studied in the light of the now accepted theories of meteoric astronomy.

153

COMETS.

THE year (1881) which was to have seen the end of the world, because of planetary conjunctions and perihelion passages, because Mother Shipton had said so (or was said to have said so), and because the ascending gallery in the Great Pyramid is 1,882 inches long (so that the year 1882 is to introduce a new era), has been remarkable in astronomical annals for the number of comets which have been seen. No less than six were numbered, two of which were of great brightness. Something still remaining-more, indeed, than we are always ready to admit-of old superstitions respecting comets has led many to regard the coincidence as full of meaning. Others, not quite so credulous, have supposed that though comets may not come in flights of half a dozen together to portend the end of the world, they may yet affect our weather in some way; perhaps directly, as the moon is supposed to do (with very little reason); perhaps indirectly, by acting on the sun. To the astronomer the appearance of so many comets -some of them large ones-has been full of interest, because he hopes by the application of the new methods of research discovered within the last quarter of a century to solve some of the mysteries with which the whole subject is still fraught, despite a number of interesting discoveries which have recently been made.

A brief inquiry into some of the facts which have been discovered respecting comets, and a discussion of some of those peculiarities which still remain among the greatest

mysteries of science, will probably prove acceptable at the present time, when comets attract so much interest and attention.

Elsewhere in the solar system we meet with relations not differing greatly in kind from those presented by our own earth. We see a set of globular bodies revolving around the sun in nearly circular orbits, nearly in one plane, and all in the same direction; we find that these globes rotate upon their axis-still in the same direction; they have, apparently, atmospheres proportioned to their dimensions; and many of them are attended upon by bodies resembling our own moon. And therefore, without entering upon the vexed question of the plurality of worlds, we are able to pronounce that, if these globes are inhabited, dwellers upon them have, like us, their years, their days, their seasons; a sun-rising in the east and setting in the west; twilight and moonlight; air and vapour; winds and rain; all things, in fact, as it would seem, necessary to their comfort and convenience. Here and there-as in the zone of asteroids and the rings of Saturn-we meet with novelty of arrangement or configuration; but even then we find a stability, either of figure or motion, which renders such objects comparable, so to speak, with those we are accustomed to.

But with comets the case is wholly different. When we have said that these objects obey the law of gravity, we have mentioned the only circumstance-as it would appear-in which they conform to the relations observed in terrestrial and planetary arrangements. And even this law-the widest yet revealed to man-they seem to obey half-unwillingly. We see the head of a comet tracing out systematically enough its proper orbit, while the comet's tail is all unruly and disobedient.

The paths followed by comets show no resemblance either to the planetary orbits or to each other. Here we see a comet travelling in a path of moderate extent and not very eccentric; there another which rushes from a

distance of two or three thousand millions of miles, approaches the sun with ever-increasing velocity until nearer to him than parts of his own corona (as seen in eclipses), sweeps around him with inconceivable rapidity, and makes off again to where the aphelion of its orbit lies far out in space beyond the most distant known planet, Neptune. Some comets travel in a direct, others in a retrograde, path; a few near the plane of the earth's orbit, many in planes showing every variety of inclination. Some comets regularly return after intervals of a few years; some after hundreds of years; others are only seen once or twice, and then unaccountably vanish; and not a few show by the paths they follow that they have come from interstellar space to pay our system but a single visit, passing out again to traverse we know not what other systems or regions.

The ancients believed comets to be of the same nature as meteors, or shooting stars-either in the earth's atmosphere not far above the clouds, or, at all events, much lower than the moon. These views are, however, much less ancient than the more correct views maintained by the Pythagoreans. Their doctrine was that comets are planetary objects, having long periods of revolution. From whom this opinion was derived is uncertain. Like other opinions attributed to Pythagoras, it was doubtless obtained from Eastern philosophers; but of what country— whether Egyptian, Persian, Indian, or Chaldæan-we have no means of learning. Apollonius, the Myndian, ascribes the opinion to the Chaldæans. He says they spoke of comets as of travellers penetrating far into the upper (or most distant) celestial spaces. Seneca and Pliny held similar views, exhibiting in this respect, says Humboldt, the imitative faculty of the Romans. But the Greek philosopher preferred to look for a theory of the universe in the conceptions of his own brilliant and imaginative mind. As if to show future ages how little was likely to be achieved by the highest mental powers without the habit of patient