THIS second part of M. Humboldt's third volume concludes his varied and comprehensive sketch of a physical description of the heavens. In our number for February we gave a brief outline of the first part of the volume, and we are happy to report the completion of this division of so great a work, and at the same time to lay before our readers an account of the contents of the concluding part. It opens with the following description of "the nebula."

"Besides the visible celestial bodies which shine with sidereal light,-either by their own proper light, or by planetary illumination, either isolated, or variously associated, forming multiple stars, and revolving round a common centre of gravity, we behold also other forms or masses having a milder, fainter, nebulous lustre. These -which are seen in some instances as small, diskshaped luminous clouds, having a well-defined outline, whilst in other instances their forms vary greatly, their boundaries are ill-defined, and they are spread over much wider spaces in the sky

Cosmos; Sketch of a Physical Description of the Universe. By Alexander von Humboldt. Vol. III. Part II. Translated under the superintendence of Colonel Edward Sabine, R. A., V. P., and Treas. R. S. London: Murray. 1852.

VOL. XXVI. NO. IV.

appear at the first glance, to the assisted eye which views them through the telescope, to differ altogether from the heavenly bodies which have been treated of in detail in the four preceding sections. As astronomers have been inclined to infer from

the observed but hitherto unexplained movements of visible stars, the existence of other unseen celestial bodies, so the experience of the resolvability of a considerable number of nebula has led in the present and most recent times to inferences as to the non-existence of any true nebulæ, and even of any cosmical or celestial nebulosity whatsoever. Whether, however, the well-defined nebulæ self-luminous nebulous matter, or whether they of which I have spoken be indeed composed of are merely remote, closely-crowded, and rounded clusters of stars, they must ever continue to be regarded as highly important features in our knowledge of the arrangement of the structure of the universe, and of the contents of celestial space."-pp. 215-16.

Sir William Herschel estimated that these "nebula" occupy of the entire surface of the heavens. The places of between three and four thousand have been determined. Their distance from us is beyond calculation. If they are clusters of stars, they obey some mysterious laws of gravitation. M. Humboldt traces the development

28

of our present knowledge of the nebulæ from | the earliest use of telescopes to the magnificent labors of Sir W. Herschel and his son, and the triumphant discoveries of Lord Rosse. The apparent distribution of the nebulæ is remarkable, being most numerous in the northern hemisphere, but more uniform in the southern. Their diversity of individual form is wonderful.

"This is sometimes regular (spherical, elliptical in various degrees, annular, planetary, or resembling a photosphere surrounding a star), and sometimes irregular or amorphous, and as difficult of classification as are the aqueous nebulæ of our atmosphere, the clouds. The normal form of the celestial nebulæ is considered to be elliptical or spheroidal. With equal telescopic power, such nebulæ are most easily resolvable into star-clusters when they are most globular; and, on the other hand, when the compression in one direction, and elongation in the other, is greatest, they are the most difficult of resolution. We find in the heavens gradually varying forms, from round to elliptic, more or less elongated.-(Phil. Trans.,' 1833, p. 494, Pl. ix., figs. 19-21.) The condensation of the milky nebulosity is always progressive towards a centre, or, as in some cases, even towards several central points or nuclei. It is only in the class of round or oval nebula that double

nebulæ are known, and in these, as there is no perceptible relative motion of the individuals in respect of each other (either because no such motion exists, or that it is exceedingly slow), we are without the criterion which would enable us to demonstrate the reality of a mutual relation, and which, in the case of double stars, we possess for distinguishing those which are physically from those which are merely optically double. (Drawings of double nebulæ are to be found in the 'Philosophical Transactions for 1833,' figs. 68-71; compare also Herschel, 'Outlines of Astronomy,' § 878, and Observations at the Cape of Good Hope,' § 120.)-pp. 233, 234.

The account of the rare "annular nebula," the "planetary nebula," the "nebulous stars," and the larger nebulous masses of irregular form, is followed by minute descriptions of the great nebula round Argûs, and other remarkable nebulæ, including the "Cape-Clouds," and the section concludes with the following remarks on the additions which have been made to our knowledge of this department of science :—

"The consideration of the outermost and remotest strata of self-luminous worlds, the distances of nebulæ, and all the subjects which have been crowded into the last of the seven sidereal, or astrognostic sections of this work, fill our imagination with images of time and space surpassing our powers of conception. Great and ad mirable as have been the advances made in the

improvement of optical instruments within the last sixty years, we have at the same time become familiar with the difficulties of their construction not to give ourselves up to such daring, and, indeed, extravagant hopes as those with which the ingenious Hooke was seriously occupied between 1663 and 1665. There, also, we advance further and more securely towards the goal by moderation in our anticipations. Each of the successive generations of mankind is in its turn enabled to rejoice in the greatest and highest results attainstanding place to which art may then have risen. able by man's intellect, freely exerted from the Without enumerating in determinate numbers the extent of space-penetrating power already achieved in telescopic vision, and without laying much stress upon such number, still our knowledge of the velocity of light teaches us that in the faint glimmer proceeding from the self-luminous surface of the remotest heavenly body, we have the most ancient sensuous evidence of the existence of matter.'"-pp. 257, 258.

From the heaven of fixed stars the author descends to our solar and planetary system. There is no direct evidence of dark bodies revolving round other fixed stars. Whether there be such analogous revolutions or not, it is probably not physically possible that they should be seen from our globe, and there is no unconditional necessity for assuming that they exist; for, as there are planets in our system without satellites-Mercury, Venus, and Mars,-it may be that there are also fixed stars without planets.

In the

stars, incontestable advantages in respect to the evidence and certainty of the results obtained. Much of sidereal astronomy is simply contemplative; it is so in regard to star-clusters and nebulæ, and also the very insecurely grounded photometric classification of the fixed stars. The best assured which in our own time has received such exceeding and most brilliant department in astrognosy, and improvement and enlargement, is that of the determination of positions in Right Ascension and Declination, whether of single fixed stars, or of double stars, star-clusters, and nebulæ. Measurable relations of a more difficult class, but yet susceptible of a greater or less degree of accuracy, are presented by the proper notion of stars; the elements by means of which their parallax may be sought; telescopic star-gazings, throwing light on their distribution in space; and the periods of variable