contrasts and grateful vicissitudes-a red and a green day, for instance, alternating with a white one and with darkness -might arise from the presence or absence of one or the other, or both, above the horizon. Insulated stars of a red colour, almost as deep as that of blood, occur in many parts of the heavens, but no green or blue star (of any decided hue) has, we believe, ever been noticed, unassociated with a companion brighter than itself.

CLUSTERS.-When we cast our eyes over the concave surface of the heavens in a clear night, we do not fail to observe that there are, here and there, groups of stars which seem to be compressed together more densely than those in the neighbouring parts; forming bright patches and clusters.

There is a group called the Pleiades, in which six or seven stars may be noticed, if the eye be directed full upon it; and many more if the eye be turned carelessly aside, while the attention is kept directed* upon the group. Telescopes show fifty or sixty large stars thus crowded together in a very moderate space, and comparatively insulated from the rest of the heavens. Rheita affirms that he counted 200 stars in this small cluster. The constellation, called Coma Berenices, is another group, more diffused, and consisting of much larger

stars.

In the constellation Cancer, there is a nebulous cluster of very minute stars, called Præsepe, or the Beehive, which is sufficiently luminous to be seen by the naked eye, in the absence of the moon, and which any ordinary spyglass will resolve into separate stars. In the sword-handle of Perseus, also, is another such spot, crowded with stars. It requires, however, rather a better telescope to resolve it into individual stars.

These are called Clusters of Stars. Whatever be their nature, it is certain that other laws of aggregation subsist in these spots, than those which have determined the scattering of stars over the general surface of the sky. Many of them, indeed, are of an exactly round figure, and convey the idea of a globular space filled full of stars, and constituting, in itself, a family or society apart, and subject only to its own internal laws.

"It would be a vain task," says the younger Herschel, "to

* "It is a very remarkable fact," says Sir John Herschel, "that the centre of the visual organ is by far less sensible to feeble impressions of light, than the exterior portions of the retina."-Ast. p. 398.

Are in lividual stars of a deep colour ever found separate from others? What are clusters of stars? Mention some instance. Describe it. Mention some other instance. Describe the position and appearance of Præsepe. Describe any other cluster which you may recollect. What are the constitution and figure of such groups? What did the younger Herschel say of the number of stars which compose these clusters?

attempt to count the stars in one of these globular clusters. They are not to be reckoned by hundreds; for it would appear that many clusters of this description must contain, at least, ten or twenty thousand stars, compacted and wedged together in a round space, not more than a tenth part as large as that which is covered by the moon.

4. NEBULE.—The Nebulæ, so called from their dim, cloudy appearance, form another class of objects which furnish matter for curious speculation, and conjecture respecting the formation and structure of the sidereal heavens. When examined with a telescope of moderate powers, the greater part of the nebulæ are distinctly perceived to be composed of little stars, imperceptible to the naked eye, because, on account of their apparent proximity, the rays of light proceeding from each are blended together, in such a manner as to produce only a confused luminous appearance.

In other nebulæ, however, no individual stars can be perceived, even through the best telescopes; and the nebulæ exhibit only the appearance of a self-luminous or phosphorescent patch of gaseous vapour, though it is possible that even in this case, the appearance may be owing to a congeries of stars so minute, or so distant, as not to afford, singly, sufficient light to make an impression on the eye.

In some instances a nebula presents the appearance of a faint luminous atmosphere, of a circular form, and of large extent, surrounding a central star of considerable brilliancy.

One of the most remarkable nebulæ is in the sword-handle of Orion. It is formed of little flocky masses, like wisps of cloud, which seem to adhere to many small stars at its outskirts. It is not very unlike the mottling of the sun's disk, but of a coarser grain, and with darker intervals. These wisps of light, however, present no appearance of being composed of small stars; but in the intervals between them, we fancy that we see stars, or that, could we strain our sight a little more, we should see them. These intervals may be compared to openings in the firmament, through which, as through a window, we seem to get a glimpse of other heavens, and brighter regions beyond.-Page 58.

Another very remarkable nebula is that in the girdle of Andromeda, which, on account of its being visible to the naked eye, has been known since the earliest ages of astronomy. It is often mistaken for a comet, by those unacquainted with the

Why are the nebulæ so called? Describe the usual appearances of nebulæ, as seen through a telescope. What other appearance do nebulæ sometines exhibit? Mention some instances of the most remarkable nebulæ. Describe the one in the swordhandle of Orion. Describe the one which is in the girdle of Andromeda.

heavens. Marius, who noticed it in 1612, describes its appearance as that of a candle shining through horn; and the resemblance is certainly very striking. Its form is a long oval, increasing, by insensible gradations of brightness, from the circumference to a central point, which, though very much brighter than the rest, is not a star, but only a nebula in a high state of condensation. No power of vision hitherto directed to this nebula has been able to resolve it into the least appearance of stars. It occupies an area comparatively large -equal to that of the moon in quadrature. This nebula may be considered as a type, on a large scale, of a very numerous class of nebulæ, of a round or oval figure, increasing more or less in density towards the centre.

Annular nebulæ also exist, but are among the rarest objects in the heavens. The most conspicuous of this class, is to be found exactly halfway between the stars Beta and Gamma Lyræ, and may be seen with a telescope of moderate power. It is small, and particularly well defined; appearing like a flat oval ring. The central opening is not entirely dark, but is filled with a faint, hazy light, uniformly spread over it, like a fine gauze stretched over a hoop.

Planetary nebulae are very extraordinary objects. They have, as their name imports, the appearance of planets, with round or slightly oval disks, somewhat mottled, but approaching, in some instances, to the vividness of actual planets. Some of them, upon the supposition that they are equally distant from us with the stars, must be of enormous magnitude. That one, for instance, which is situated in the left hand of Aquarius, must have a volume vast enough, upon the lowest computation, to fill the whole orbit of Herschel!

The nebulæ furnish an inexhaustible field of speculation ' and conjecture. That by far the larger number of them consists of stars, there can be little doubt; and in the interminable range of system upon system, and firmament upon firmament, which we thus catch a glimpse of, the imagination is bewildered and lost. Sir William Herschel conjectured that the nebulæ might form the materials out of which nature elaborated new suns and systems, or replenished the wasted light of older ones. But the little we know of the physical constitution of these sidereal masses, is altogether insufficient to warrant such a conclusion.

Of what class of nebulæ may this be considered as a type? What other species of nebulæ exist in the heavens? Describe the most conspicuous of this class. What other species of nebulæ are more rarely found? Describe the appearance of planetary nebulæ. What do we know in regard to their magnitude? How large must the one be which is situated in the left hand of Aquarius? What did Sir William Herschel conjecture as to the use of the nebulæ? Have we facts sufficient to warrant such a conjecture?

CHAPTER XIV.

VIA LACTEA.

"Throughout the Galaxy's extended line,
Unnumber'd orbs in gay confusion shine:
Where every star that gilds the gloom of night
With the faint tremblings of a distant light,

Perhaps illumes some system of its own,

With the strong influence of a radiant sun."-Mrs. Carter THERE is a luminous zone or pathway of singular whiteness, varying from 4° to 20° in width, which passes quite round the heavens. The Greeks called it GALAXY, on account of its colour and appearance: the Latins, for the same reason, called it VIA LACTEA, which, in our tongue, is MilkyWay.

Of all the constellations which the heavens exhibit to our view, this fills the mind with the most indescribable grandeur and amazement. When we consider what unnumbered millions of mighty suns compose this cluster, whose distance is so vast that the strongest telescope can hardly separate their mingled twilight into distinct specks, and that the most. contiguous of any two of them may be as far asunder as our sun is from them, we fall as far short of adequate language to express our ideas of such immensity, as we do of instruments to measure its boundaries.

It is one of the recent achievements of astronomy that has resolved the Milky-Way into an infinite number of small stars, whose confused and feeble lustre occasions that peculiar whiteness which we see in a clear evening, when the moon is absent. It is also a recent and well accredited doctrine of astronomy, that all the stars in the universe are arranged into clusters, or groups, which are called NEBULE or STARRY SYSTEMS, each of which consists of many thousands of stars.

The fixed star which we call OUR SUN, belongs, it is said, to that extensive nebula, the Milky-Way; and although apparently at such an immeasurable distance from its fellows, is, doubtless, as near to any one of them, as they are to one another.

Of the number and economy of the stars which compose this group, we have very little exact knowledge. Dr. Herschel informs us that, with his best glasses, he saw and

What do you understand by the Milky-Way? By what different names is it called? Why does the contemplation of this constellation fill the mind with ideas of grandeur and amazement? What causes the whiteness of the Milky Way? Into what are all the stars in the universe arranged? To what nebula does the sun belong, and what is probably its distance from its fellows? What knowledge have we of the number and economy of the stars in this group?

counted 588 stars in a single spot, without moving his telescope; and as the gradual motion of the earth carried these out of view and introduced others successively in their places, while he kept his telescope steadily fixed to one point, "there passed over his field of vision, in the space of one quarter of an hour, no less than one hundred and sixteen thousand stars, and at another time in forty-one minutes, no less than two hundred and fifty-eight thousand."

In all parts of the Milky-Way he found the stars unequally dispersed, and appearing to arrange themselves into separate clusters. In the small space, for example, between Beta and Sad'r, in Cygni, the stars seem to be clustering in two divisions; each division containing upwards of one hundred and sixty-five thousand stars.

At other observations, when examining a section of the Milky-Way, not apparently more than a yard in breadth, and six in length, he discovered fifty thousand stars, large enough to be distinctly counted; and he suspected twice as many more, which, for want of sufficient light in his telescope, he saw only now and then.

It appears from numerous observations, that various changes are taking place among the nebula-that several nebulæ are formed by the dissolution of larger ones, and that many nebule of this kind are at present detaching themselves from the Milky-Way. In that part of it which is in the body of Scorpio, there is a large opening, about 4° broad, almost destitute of stars. These changes seem to indicate that mighty movements and vast operations are continually going on in the distant regions of the universe, upon a scale of magnitude and grandeur which baffles the human understanding.

More than two thousand five hundred nebulæ have already been observed; and, if each of them contains as many stars as the Milky-Way, several hundreds of millions of stars must exist, even within that portion of the heavens which lies open to our observation.

"O what a confluence of ethereal fires,

From urns unnumber'd down the steep of heaven
Streams to a point, and centres on my sight."

Although the Milky-Way is more or less visible at all seasons of the year, yet it is seen to the best advantage during the months of July, August, September, and October. When Lyra is on, or near the meridian, it may be seen

How many did Dr. Herschel count in a single spot during the space of 15 minutes? How did he find the stars dispersed, throughout the Milky-Way? Give an example. Give another instance. What changes are taking place in the Milky Way and other nebule? What do these changes indicate? How many nebulæ have been discovered? If each of these nebula contains as many stars as the Milky Way, how many stars must exist even in that portion of the heavens which lies open to our observation 7 Where and at what period may the Milky-Way be seen to the best advantage?