tains, these latter are more nearly identical in position with the range formerly described. The question is, however, chiefly one of name, for there is little doubt that the really lofty mountains are grouped near Kilmanjoro, and are not far from the coast; while the high ground north of Tanganyika, though a water-shed, is by no means important in any other sense. That the existence of a chain separating the basin of the Nyanza from the basin of the Nile is absolutely disproved by Captain Speke, is a result of extreme interest, and also settles points that had been long disputed. It still remains to decide whether, according to Captain Burton, "an elevated mass of granite and sandstone crosses from the shores of the Indian Ocean to the centre of tropical Africa;" for this is of course possible, although it does not, as Captain Burton supposed, exclude Nyanza from the drainage basin of the Nile.

We now await the last stage in the discovery of the sources of the Nile. It is still possible that, far away towards the centre of the continent, there are waters that run eastwards to supply the great river, but it is certain that so far as the east and south are concerned the limits are determined. Whether Consul Petherick's party, or Madame Tinne and her suite, will be the next to give us information it is not easy to say. Both parties are busily engaged in the same work, and the result, whatever it may be, will be looked for with extreme interest by the civilized world. Meanwhile Captain Speke has worthily and well carried out and completed the task he had so boldly set himself.

CLUSTERS OF STARS AND NEBULE.-THE
SURFACE OF THE MOON.

BY THE REV. T. W. WEBB, M.A., F.R.A.S.

OUR recent survey, in these pages, of the more conspicuous and interesting of the double and binary stars will, it may be hoped, have led the astronomical inquirer to wish for a still more extended knowledge of that glorious host of suns which surrounds him in every direction. It will be our object to give him such further assistance as we can in this noble pursuit, and therefore, quitting for the present the branch of study which has hitherto occupied us, we will choose a fresh point of view, and proceed to the consideration of other and more complicated arrangements.

It is impossible to look upon the open heavens without an instinctive conviction of the unequal distribution of the stars in space; some regions are left comparatively vacant; in others, there is a crowding together of stars so nearly corresponding in magnitude as to leave no doubt as to their actual combination in groups; the individuals of which may possibly be separated by distances baffling all human conception, but which yet show mutual connection as compared with the void space surrounding them. Of course a similar appearance would be the result, if they had only an optical and not a real proximity, being merely arranged nearly in the same direction with regard to our sight. But in order to produce this effect, there would necessarily be so strange a counterbalancing of distance and brightness, that we perceive at once the improbability of such an explanation. There are, no doubt, occasional instances of apparent equality resulting from such compensations of remoteness by superior brilliancy, because amid the profusion of the heavens there is room for every alternative; and, as it has been truly observed, an event, against the probability of which there are 10,000 chances to one, will yet actually occur, if it has 10,000 chances given to it. But still we shall be quite safe in concluding that such instances of apparent proximity are not the rule but the exception, and that in the case of large aggregations such an exception will approach impossibility. It may be very possible with regard to any two equal and adjacent stars, that their apparent neighbourhood may be a deception, from the compensation of brightness by distance; but the probability of such an arrangement will be much less where three are concerned, and will very rapidly diminish with increasing numbers. The evidence of sense is fully in accordance with this unquestionable deduction. We cannot look upon the three well-known stars in the belt of Orion without a dis

tinct impression that they are all nearly at an equal distance from us; the same is the case with the seven of the Great Bear, and especially with the little constellation Delphinus, easily recognized during the evenings of the present season at some height in the southern sky, to the left of the brilliant Al Tair. No principle of perspective could be reasonably called in to account for the aggregation of so many stars of nearly similar magnitude within so small an area. Other instances might be given of groups, as such associations of stars may be properly called, visible to the naked eye; but it is in the expanded range of the telescope that they become more universally apparent. There we shall frequently meet with fields characterized by the prevalence of stars of a certain size, and where the evidence of actual vicinity, and oftentimes of very remarkable arrangement, is too obvious to be overlooked. These " groups, however,-adopting the accurate classification of Sir W. Herschel, who thus designates compressed collections of stars without central condensation, but forming insulated systems, -present only the simplest form of combination. In the continuance of our researches, and especially if we progressively increase the light of our telescopes, so as to penetrate further into the depth of the heavens, we shall discover a regular series of these aggregations. Groups will come into view not only more rich in numbers, but more thronged by the real or apparent nearness of their individual components; we shall not only notice a tendency to condensation towards the centre of the mass, which may be, though to a very limited extent, the effect of perspective from the greater length of the central visual line, but we shall find, in many instances, a degree of internal compression which perspective is inadequate to explain; and thus we shall be obliged to infer a degree of mutual proximity in that region which we have no means of estimating, but which, upon any supposition, must lead to thoughts of wonder as to the general structure of the mass. Such an assemblage, if its general outline were circular, as is frequently the case, would no longer be denominated a group," but a "globular cluster," "clusters" being, according to Sir W. Herschel's definition, groups so arranged as to indicate the existence of a central force. To many of these most interesting objects we hope to direct our readers.

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But we shall soon find that we are, practically at least, dealing with infinitude. Some of these clusters will exhibit their component stars with little difficulty in an ordinary telescope; in others, from the minuteness or the compression of their members-a natural result in either case of increased distance we shall obtain a less perfect picture, and the general mass will assume a hazy aspect, though its real character will

be fully indicated by the sparkling points of light which mark the places of the brighter individuals, or, perhaps, the spots where two or three fall into the same line of sight. Such an object would be called an "imperfectly resolved nebula," a Latin word signifying fog, and well expressing the distinctive aspect of these bodies. Others, refusing to give up separately their component stars, present a mottled or granulated appearance, which the experienced observer well understands, and which enables him to class them as "resolvable nebulæ," that is, nebula which would, under other circumstances of distance or optical power, be resolved into stars. While lastly there are those, whose light is so milky, to use an epithet introduced by the elder Herschel, so uniform in its diffusion, as to mask completely their starry nature, and to render them "irresolvable." This remarkable and instructive progression cannot be adequately exemplified to the naked eye, because it so happens that we have no good specimen of a globular cluster sufficiently near to us to form the first step in the scale by its sensible resolution; but every telescope of sufficient power to resolve fairly the nearer clusters exhibits a series of this kind, passing from the group or cluster, which is a fully resolved nebula, to the irresolvable nebulosity which from analogy is, or may be, a group or cluster at an unapproachable distance; and each telescope has a series of its own, corresponding with its optical capacity; every increase of aperture, by its greater resolving power, increasing the number of groups and clusters at the expense of nebulæ, and yet so adding to the ranks of the latter by the addition of still fainter and more evanescent objects, that the sum total still goes on progressing, as though it were actually illimitable. Many an aggregation which appears but as a dim and hazy speck in our smaller telescopes, is wholly disentangled into separate stars by the reflectors of the Earl of Rosse, or Lassell, or Chacornac, or by achromatics such as those of Clark, or Bond, or Struve, or Secchi; while these again convert the "milky" into the "resolvable " nebulæ, and draw out of the unknown depths of space yet further, and obscurer, and previously imperceptible evidences of the incomprehensible magnitude of this great and wonderful universe.

Such is a broad and simple view of this grand subject. Whether this apparent simplicity is founded in the truth of nature, or whether it may prove one of those premature generalizations which only obstruct the path while they seem to clear it, need not be discussed at the present time, though it will ultimately come before us. But it is desirable, before proceeding further, to obviate any misconception which might arise from this mode of statement. We must bear in mind that our two assumptions, of sphericity of form in the mass, and

similarity of magnitude in the individuals, are only partially applicable. The forms of clusters and nebulæ are indefinitely varied; many being elliptical, a few spiral, others perforated, others again extended in length, branching, or contorted in various figures, while their components are by no means always all equally bright, the reverse frequently opposing a difficulty in the way of complete resolution, when the telescope can master the more resolvable portion, but leaves the smaller individuals merged in a general haze. In another respect, too, our statement requires qualification. It has been asserted that the apparent minuteness of the individual stars in certain clusters, as compared with others, may be the result of increased distance from the spectator, and the series which we have imagined is based upon this idea. It was that of Sir W. Herschel, who thus assigned the probable distances of nebulæ, in progressive remoteness, till they passed even beyond the space-penetrating power of his forty-feet reflector, which he estimated one hundred and ninety-two times greater than that of the eye -a value corresponding to three hundred thousand times the distance of Sirius! But this magnificent conception has been entirely overthrown by more recent discoveries. It rested upon the assumption that the distances of the stars are, on an average, in the inverse ratio of their apparent magnitudes-an assumption not merely recommended by its simplicity and facility, but by its being probably the only one whose adoption could lead to any positive or numerical result. Of late years, however, its inadequacy has become more and more evident, upon grounds so lately stated in the present publication (INTELLECTUAL OBSERVER, Xviii. 454), that they need not be repeated here; and now it must be admitted that after so many years we find ourselves without a guide in the interminable wilderness of nebulæ. It is very possible that Sir W. Herschel's hypothesis may yet be correct as regards the majority of these objects, and it is unquestionably supported by the evidence of sense; but how deceptive this is has been demonstrated in the case of insulated stars and systems, and analogy has begun to lead us in an opposite direction. Analogy, with all its deficiencies, is like the one-eyed man, who has been said to be a king in the country of the blind; and reasoning from its indications where no others are attainable, we should have to conclude that the brightest and most resolvable clusters may be possibly as distant from us as some of those little heaps of 'star-dust," whose aspect would bespeak for them a far more inconceivable remoteness, and that certain, again, of these latter may be even nearer to our system than some of those great and conspicuous stars, beyond which they were formerly placed at an immeasurable distance. In fact, whatever may appear pro