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appear to oppose some resistance to their transit, and may be stretched in consequence. The utricles, floating freely in the water, become the seat of endosmotic and chemical actions, especially when the surrounding water has a temperature of from 16° to 15° (C). The utricles enclose at the beginning a mucilaginous liquid, in which a gas-bubble soon appears, and increases in size : this is the oxygen evolved under the influence of light and heat. The plant disengages itself from the soil, and mounts towards the surface; the secretion of gas becomes more abundant, and the flower-stems are raised above the water. The oxygen secreted in the air-vesicles seems to exercise a chemical influence in changing the colour of the cell-walls, which become rose, lilac, and blue. The colouration of the envelope affects the internal processes of the cell. We know that in organs not coloured green, like the petals of a flower, there is no evolution of oxygen, but an absorption of oxygen and an evolution of carbonic acid. This gas does not leave the utricle, but is probably assimilated there; the utricle becomes again filled with mucilage, and water which it absorbs augments in weight and causes it to descend. Thus the utricles have a respiratory as well as a hydrostatic function.

GRUITHUISEN’S CITY IN THE MOON.-JUPITER'S

SATELLITES.-OCCULTATIONS.

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

We will now, in continuation of our subject, direct our attention to the region lying S. and S.W. of the great clefts recently described.Dionysius (25), a small crater (14 miles across, Lohrm.), lying on the shore of the M. Tranquillitatis, and having perhaps 3800 f. of depth, or more than enough to hide the peak of Snowdon, is chiefly remarkable on account of its brilliancy, amounting to 7o for its interior, and go for its wall. Yet it is not to be seen in the earth-shine, probably, as B. and M. remark, on account of its small dimensions. It would be an interesting and not unpromising investigation, to ascertain whether this is the true cause, and attended with no great trouble, as it would not be difficult to select other small craters which are perceptible on the dark side, and whose magnitude would be comparable with that of Dionysius. I have never made the attempt, but trust some of my readers may have at once sufficient instrumental power and leisure to take it in hand. We have seen that changes of colour, of which little explanation can be given, frequently come on with the advancing lunar day; and there is no antecedent improbability in the idea that similar periodical alternations may be in progress during the lunar night. Dionysius has been selected by Birt as a standard of magnitude among craters of a similar character.–Ariadæus, and Ariadeus a, are a pair of craters at the W. end of the great cleft which bears that name.* Silberschlag, a crater 9 miles across, and of 8.0 of brightness, appears, with those already mentioned, in the diagram of this region in our August number. There also we shall find Agrippa (26), a fine ring, somewhat elliptical, 27 miles broad, carrying a peak S., and interrupted by a little crater N. ; in the steepest parts its interior slope amounts to 60°; and it ranges on the W. 6900 f., and 1000 f. more on the E., above the interior. It has several terraces, and a central hill.Godin (27) has a narrow but very steep ring of 8° light, equally deep with Agrippa (Schr. makes it deeper), and of a somewhat quadrilateral form : its breadth is 23 miles. It is connected with Agrippa by ridges running in an oblique S.S. W. direction, a peculiarity of which other instances might be given. Several very defined craters lie in the neighbourhood. E. of Godin we find a small but very brilliant crater, Rhæticus b, which attains go of luminosity. Rhæticus itself, which will be found in our recent diagram, is an irregular ring, chiefly distinguished as being bisected by the lunar equator, and as being one of the few spots to which both Sun and Earth may be vertical; all these being, of course, comprised in an elliptical area, whose centre is that of the Moon, and its boundary the extreme amount of libration (which is greater in longitude than latitude), as referred to the centre of the Earth (not to the observer's position, in which it may be increased by parallax). Very strange, certainly, would be the aspect of the sky to any one of ourselves, if we could conceive ourselves transported there ;-—the Sun describing a slow but cloudless course from rising to setting, through the vertical region of the sky, and often through the zenith itself; and the Earth oscillating around that point for a short distance successively in every direction-an enormous globe, waxing and waning with all the features of the Moon, and turning every part in comparatively rapid rotation to the eye of the spectator. The Rhæticus of

* It should have been stated in our last number that the minuto prolongation of this clest, noticed in p. 97 as having been discovered by Gruithuisen, has been seen on several occasions by Messrs. Birt and Freeman.- Í may be permitted also to take this opportunity of rectifying two former mistakes, which have been obligingly pointed out to me by Messrs. Knott and Proctor. The first occurs in INT. OBS. vii. 134, where the R.A. of the Great Star of 1572 has been given at 4h. 19m. 57:79., instead of 4° 19' 57.7" (= 0h. 17m. 19.8s.): the difference also from Hind should have been, not 3m. 109., but 3' 10".-The other is in Int. OBS. X. 148, where, instead of density of the ring of Saturn, it should have been mass.

objects, that i he date of thos than at presente

Riccioli, it should be observed, lies further E., but its name was transferred to this spot by B. and M., in despair of its identification. This, no doubt, may have been impracticable, so far as it depended upon the relief of the surface, and an apology is thus obtained in this case for a change, generally speaking to be avoided; but the original Rhæticus, which has only recently been recovered by Knott, is a grey opening among the luminous rays issuing from the S.W. side of Copernicus (30), and consequently only to be recognized, where B. and M., no doubt, did not think of looking for it, under high illumination. Three contiguous dark spaces of a circular form were figured here by Hevel, and denominated Lacus Herculei : from Riccioli they received the separate names of Rhæticus, Stadius, and Dominicus Maria, occupying respectively the E., S.W., and N.W. angles of the triangular area in which they are grouped. They are not difficult objects in the Full Moon, Rhæticus, in particular, which is the darkest, and is divided centrally by a more luminous ray; yet still they are now, especially the other two, so unimportant in character, being merely duller patches in a labyrinth of bright streaks, and so much less conspicuous than very many other anonymous objects, that a suspicion may reasonably arise, whether they were not, at the date of those early observations, of a more decidedly contrasted grey hue than at present. Should there be anything in this, it would of course involve a consequence of some interest—that the streak-system of Copernicus is, in this place at least, on the increase; and when we bear in mind the very small amount of our actual knowledge as to the local colouring of the Moon, we shall feel that attention may be suitably directed to this spot, where identification and comparison are proportionally easy. Instances may be given in which variations of brightness in high illumination are probable - Linné, and a bright spot in Werner, may be specified ; and it is time that observers should take this curious point in hand. On the earth, analogous changes, no doubt, may be perceived, but they would result from that cultivation of which we have no suspicion in our satellite.

It is in this ancient Rhæticus that we are to look for one of the curious “rampart-works" discovered by Gruithuisen. His sketch, in the “Astronomisches Jahrbuch” for 1828, represents a comparatively regular white figure in a longish grey area, consisting of one vertical stripe, bent to the left at the top, where it terminates in a small hill casting a shadow; ending in something like a little crater, with internal and external shade, at the bottom; and crossed at an angle of about 60° by four similar bright streaks: the figure might have been worth copying, but that he complains, in the next volume, of its

inaccuracy, there being one transverse line too many on W. side, and all of them being too bright: it had, he says, been very seldom visible, and “selenospherically obscured” for the last year. This astronomer assuredly thought, and published, an uncommon amount of nonsense. If we are to believe him, there is not a trace, ancient or modern, of volcanic action on the Moon, the so-called craters having been formed by the fall of enormous fragments from space (colossal aerolites), whose points still project above the once-plastic surface in the form of central hills. The Moon had been first a comet, then an asteroid, afterwards a satellite, and had been once covered with a primeval sea, deeper than its highest mountains, on whose features strong marks of degradation remain. He considers the clefts as indications of animal existence, and looks upon some of the smaller ones, which show no embanked edges, and may be 30 to 80 feet in depth, as being probably broad, straight clearings through forests, and forming connections of the nature of roads between all the fertile regions of the lunar surface. The “ Selenites” themselves, he owns, we could hardly expect to distinguish individually on their journeys, but does not think it impossible that large bodies of them might be detected in these roads by their difference of colour, especially if meeting and separating again. The regular straight ridges which he describes he does not seem to refer to fortification, but inclines to the idea of their being the roofs of long inhabited halls, and thinks some of the minute crater-chains, of which such wonderful examples are to be found between Eratosthenes (29) and Copernicus (30), are dwelling-places :with other dreamy matter of a similar kind. Yet this man made good use of a keen eye and sharp instrument, and saw much, and if he had spared us his inferences, would have been accepted as an observer of no little weight. I have not been able to ascertain whether his intended work, “ On the Habitability of the Moon, and Traces of its being Inhabited,” which was prepared in 1825, but, from his desire of greater completeness, remained in MS. in 1836, ever saw the light : and I have not had access to his observations in Kästner's “ Archive,” a German scientific periodical of that day ; but it would seem worth the while of some astronomer who has more time at his command than falls to the lot of everybody in these days of speedy progress, to collect and compare his observations, and sift out what may be really worthy of preservation. The occasional verification of some of his assertions sufficiently proves that he was not uniformly mistaken : and we have no reason to suspect him of falsifying the evidence which he turned to such absurd account.

But it is time to return from the ancient Rhæticus, whichi,

in fact, lies beyond our proper bounds, to the spot so called by B. and M., with which we complete our survey of the First Quadrant of the Moon.

We commence the second, at the centre of the disk, with the Sinus Medii, G on our Index-map. This is an undefined tract of level ground, of little comparative importance, excepting from its lying in the visible centre of the hemisphere. It contains only two small craters, and a few short and low ridges; but varied illumination produces much change in its shading. From its position relative to the Sun and Earth, this is the point of the Moon which receives the greatest share of illumination ; here, with the exception of a few polar summits, is the minimum of darkness, and eyes like ours would be able at all times (except during a total eclipse) to read writing of a moderate size. Here, too, the smallest possible margin is left for deception; the changes of libration and phasis have the least effect upon the prospect; and it is accordingly especially adapted for the study of those who make the discovery of physical change their object: it is unfortunate, our authors might have added, that the region is little marked by any interesting peculiarity.

Such, however, is not the case with the district lying E. and N.E. of it, some of the features of which are so curious, that the accompanying diagram is introduced to assist in its

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identification. The Moon's centre is here indica ted by the two crossing lines, of which that from right to left is the equator,

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