only to its own internal laws. It would be a vain task to attempt to count the stars in one of these globular clusters. They are not to be reckoned by hundreds ; and on a rough calculation, grounded on the apparent intervals between them at the borders (where they are seen not projected on each other), and the angular diameter of the whole group, 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, whose angular diameter does not exceed eight or ten minutes; that is to say, in an area not more than a tenth part of that covered by the moon.

(615.) Perhaps it may be thought to savour of the gigantesque to look upon the individuals of such a group as suns like our own, and their mutual distances as equal to those which separate our sun from the nearest fixed star: yet, when we consider that their united lustre affects the eye with a less impression of light than a star of the fifth or sixth magnitude, (for the largest of these clusters is barely visible to the naked eye,) the idea we are thus compelled to form of their distance from us may render even such an estimate of their dimensions familiar to our imagination; at all events, we can hardly look upon a group thus insulated, thus in seipso totus, teres, atque rotundus, as not forming a system of a peculiar and definite character. Their round figure clearly indicates the existence of some general bond of union in the nature of an attractive force; and, in many of them, there is an evident acceleration in the rate of condensation as we approach the center, which is not referable to a merely uniform distribution of equidistant stars through a globular space, but marks an intrinsic density in their state of aggregation, greater at the center than at the surface of the mass. It is difficult to form any conception of the dynamical state of such a system. On the one hand, without a rotatory motion and a centrifugal force, it is hardly possible not to regard them as in a state of progressive collapse. On the other, granting such a motion and such a force, we

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find it no less difficult to reconcile the apparent spheri city of their form with a rotation of the whole system round any single axis, without which internal collisions would appear to be inevitable. The following are the places, for 1830, of a few of the principal of these remarkable objects, as specimens of their class:

*

(616.) It is to Sir William Herschel that we owe the most complete analysis of the great variety of those objects which are generally classed under the common head of Nebulæ, but which have been separated by him into-1st, Clusters of stars, in which the stars are clearly distinguishable; and these, again, into globular and irregular clusters. 2d, Resolvable nebulæ, or such as excite a suspicion that they consist of stars, and which any increase of the optical power of the telescope may be expected to resolve into distinct stars; 3d, Nebulæ, properly so called, in which there is no appearance whatever of stars; which, again, have been subdivided into subordinate classes, according to their brightness and size; 4th, Planetary nebulæ; 5th, Stellar nebulæ ; and, 6th, Nebulous stars. The great

power of his telescopes has disclosed to us the existence of an immense number of these objects, and shown them to be distributed over the heavens, not by any means uniformly, but, generally speaking, with a marked preference to a broad zone crossing the milky way nearly at right angles, and whose general direction is not very remote from that of the hour circle of Oh and 12h. In some parts of this zone, indeed, -especially where it

* See a note on this subject at the end of the work, p. 415.

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crosses the constellations Virgo, Coma Berenices, and the Great Bear, they are assembled in great numbers; being, however, for the most part telescopic, and beyond the reach of any but the most powerful instruments.

(617.) Clusters of stars are either globular, such as we have already described, or of irregular figure. These latter are, generally speaking, less rich in stars, and especially less condensed towards the center. They are also less definite in point of outline; so that it is often not easy to say where they terminate, or whether they are to be regarded otherwise than as merely richer parts of the heavens than those around them. In some of them the stars are nearly all of a size, in others extremely different; and it is no uncommon thing to find a very red star much brighter than the rest, occupying a conspicuous situation in them. Sir William Herschel regards these as globular clusters in a less advanced state of condensation, conceiving all such groups as approaching, by their mutual attraction, to the globular figure, and assembling themselves together from all the surrounding region, under laws of which we have, it is true, no other proof than the observance of a gradation by which their characters shade into one another, so that is impossible to say where one species ends and the other begins.

(618.) Resolvable nebulæ can, of course, only be considered as clusters either too remote, or consisting of stars intrinsically too faint to affect us by their individual light, unless where two or three happen to be close enough to make a joint impression, and give the idea of a point brighter than the rest. They are almost universally round or oval-their loose appendages, and irregularities of form, being as it were extinguished by the distance, and only the general figure of the more condensed parts being discernible. It is under the appearance of objects of this character that all the greater globular clusters exhibit themselves in telescopes of insufficient optical power to show them well; and the conclusion is obvious, that those which the

most powerful can barely render resolvable, would be completely resolved by a further increase of instrumental force.

(619.) Of nebulæ, properly so called, the variety is again very great. By far the most remarkable are those represented in figs. 2. and 3. Plate II., the former of which represents the nebulæ surrounding the quadruple (or rather sextuple) star 0, in the constellation Orion; the latter, that about, in the southern constellation Robur Caroli: the one discovered by Huygens, in 1656, and figured as seen in the twenty feet reflector at Slough; the other by Lacaille, from a figure by Mr. Dunlop, Phil. Trans. 1827. The nebulous character of these objects, at least of the former, is very different from what might be supposed to arise from the congregation of an immense collection of small stars. It is formed of little flocky masses, like wisps of cloud; and such wisps seem to adhere to many small stars at its outskirts, and especially to one considerable star (represented, in the figure, below the nebula), which it envelopes with a nebulous atmosphere of considerable extent and singular figure. Several astronomers, on comparing this nebula with the figures of it handed down to us by its discoverer, Huygens, have concluded that its form has undergone a perceptible change. But when it is considered how difficult it is to represent such an object duly, and how entirely its appearance will differ, even in the same telescope, according to the clearness of the air, or other temporary causes, we shall readily admit that we have no evidence of change that can be relied on.

(620.) Plate II. fig. 3. represents a nebula of a quite different character. The original of this figure is in the constellation Andromeda near the star v. It is visible to the naked eye, and is continually mistaken for a comet, by those unacquainted with the heavens. Simon Marius, who noticed it in 1612, describes its appearance as that of a candle shining through horn, and the resemblance is not inapt. Its form is a pretty

long oval, increasing by insensible gradations of brightness, at first very gradually, but at last more rapidly, up to a central point, which, though very much brighter than the rest, is yet evidently not stellar, but only nebula in a high state of condensation. It has in it a few small stars; but they are obviously casual, and the nebula itself offers not the slightest appearance to give ground for a suspicion of its consisting of stars. It is very large, being nearly half a degree long, and 15 or 20 minutes broad.

(621.) This 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 central point: they differ extremely, however, in this respect. In some, the condensation is slight and gradual; in others great and sudden: so sudden, indeed, that they present the appearance of a dull and blotted star, or of a star with a slight burr round it, in which case they are called stellar nebulæ ; while others, again, offer the singularly beautiful and striking phænomenon of a sharp and brilliant star surrounded by a perfectly circular disc, or atmosphere, of faint light in some cases, dying away on all sides by insensible gradations; in others, almost suddenly terminated. These are nebulous stars. A very fine example of such a star is 55 Andromeda R. A. 1h 43m, N. P. D. 50° 7′.

Orionis and of the same constellation are also nebulous; but the nebula is not to be seen without a yery powerful telescope. In the extent of deviation, too, from the spherical form, which oval nebulæ affect, a great diversity is observed: some are only slightly elliptic; others much extended in length; and in some, the extension so great, as to give the nebula the character of a long narrow, spindle-shaped ray, tapering away at both ends to points. One of the most remarkable specimens of this kind is in R. A. 12h 28m; N. P. D. 63° 4′.

(622.) Annular nebulæ also exist, but are among the rarest objects in the heavens. The most conspicuous of this class is to be found exactly half way between the