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movement as a vicarious circulatory function in animals having no true circulatory system." "The circulation of Annelids is carefully described by Quatrefages, rendering full justice to the labours of Milne Edwards. It is to be regretted he has not shown the same favour to Rudolph Wagner, and Rathke. The distinction which he has established between arterial and venous currents appears to me very just in its leading features. Other authors have had similar opinions-witness the name nervarteria, given by Della Chiaje to the ventral vessel, that is, to the aorta, in the sense of M. Quaterfages. The existence of blood corpuscles in the vessels of certain Annelids is now indisputable. M. Quatrefages admits three examples-the Glycerians, Phoronis, and Syllidians. In fact, among the first, the red corpuscles belong to the perivisceral cavity, and Phoronis scarcely preserves its place among the Annelids. But without speaking of the old observations of Rud. Wagner on a Terebella, confirmed by Kölliker, other examples might be cited. In this memoir blood corpuscles properly so called will be described in the Ophelians, Cirratuliaus, and Staurocephalians. (To be continued.)

THE LUNAR ERATOSTHENES AND COPERNICUS.JUPITER'S SATELLITES.-OCCULTATIONS.

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

BEFORE quitting the neighbourhood of the "rampart-work" of Gruithuisen, we shall briefly advert to the region lying W. of it. Here we shall find a deep crater Bode (28), nearly 94m. in diameter, whose wall of 8° of luminosity makes it a conspicuous object. A smaller crater, Bode A, lying at a little distance N.W., is equally reflective. Close to Bode on the S. is an irregular ring, called Pallas, and at some distance W., and in the First Quadrant, another, Ukert, which the extreme influence of S.W. parallelism in its neighbourhood has squeezed almost into a square form, and rendered its aspect quite different at the first glance from that of Bode. At the foot of its wall on the S.E. is a straight ravine, wider than the generality of clefts, running in a S.W. direction. This I have seen interrupted in the middle by a broad, shallow valley, so as to make it appear like a cutting through the wide bank on either side.

The Sinus Estuum (our H), as limited by B. and M. (who, unlike Lohrmann, have excluded from it the hilly region around Schröter) is a depressed, but considerably reflective surface, unique in its way, according to them, from the absence of the

VOL. XII.-NO. IV.

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slightest trace of a crater, as well as from its peculiarly level character; its smoothness being only broken by a few long but very low ridges issuing from Eratosthenes (29). The comparison of this district with one swarming with minute craters a little E. is extraordinary; and these forms, which would have been here most readily distinguishable, appear to be actually wanting. It must, however, be observed that their predecessor, Lohrmann, has delineated one very distinctly, of which, though his work was before them, they have taken no notice; and Mädler subsequently (1841), when in charge of the great Dorpat achromatic, perceived several not laid down in the map. L.'s crater I found very conspicuous with 5 inches, 1861, April 18 and 20, May 17, as well as a second, and, perhaps, a third. These inconsistencies, but too frequently to be met with, introduce so much perplexity and obscurity into the question of lunar change, that we look forward with interest and pleasure to the survey now in progress under the auspices of the British Association, the results of which will certainly not be encumbered with similar ambiguities, at least on so considerable a scale.

Eratosthenes (29), a prominent crater, upwards of thirtyseven miles in diameter, and, therefore, equal in area to some of the smaller English counties, is, as our guides express it, the mighty keystone of the Apennines, and probably (as they think) the site of the outburst of that unknown violent agency which raised the summits of Bradley and Huygens. But for the nearness of the still more imposing Copernicus (30), it would certainly be the most remarkable feature of the neighbourhood. It is, however, very inconspicuous in full illumination. A great mountain occupies its centre, and the interior of the ring is supported by strong, broad terraces; the former, according to Schr., divides into two branches, and the latter, on the E. side, are in part separated by distinct ravines; a fact which deserves the attention of selenologists, as possibly throwing some light on the mode of their formation. The ring is brightest on E. Here its summit lies 15,800ft. above the gulf, while on the opposite side it only attains 10,900ft., that is, is only a little overtopped by our Etna! Schr. had given these measures considerably less, and probably too small. Lohrmann mentions two peaks, N.W. and S.W., the former the point of junction of the narrow prolongation of the Apennines; E. of this point he has placed a small deep crater on the top of the wall, which B. and M. have lowered to the inner terrace; he also speaks of several broad and generally parallel terraces descending as by steps on the exterior, and extending furthest S. The height of the wall from the outside is given by B. and M. as 3200ft. W., 7400ft. E., showing, with as fair a degree of accordance as

may be expected among such irregular surfaces, an interior depression of about 8000ft. This is something entirely unparalleled on the earth, and, at first sight, in connection with the vast extent and height of the ring, would seem to remove the phenomenon out of the reach of terrestrial analogy; when, however, the very different amount of gravity on the Moon, and possibly a very different degree of resistance from cohesion, are taken into consideration, it does not seem necessary to abandon the idea of volcanic action.

On one occasion (1789, Sept. 12), when Eratosthenes was a little less than its own diameter removed from the terminator, Schr. observed that a zone forming the extremity of the shadow which then nearly filled the cavity, being the portion which lay on the interior slope of the ring, and, according to his figure, amounting to nearly of its whole extent, was noticeably less dark than the rest. This he supposed to be the effect of the commingling of the true shadow with the penumbra, or partial darkness, which arises from the apparent breadth of the Sun, and, therefore, borders every shadow in the solar system, where the Sun subtends a sufficient angle to make it perceptible. This is, of course, the reason why the shadows of all objects in terrestrial sunshine are ill-defined in proportion to their distance from the body which casts them; and this hazy-looking edge, which would be of great breadth on Mercury, where all the shadows would be extremely woolly, and imperceptible on Uranus, where they would be almost critically sharp, would, on the Moon, be sensibly equal to what we see on the Earth. But it is most improbable that such could have been the cause of what Schr. observed. The lunar penumbra is, indeed, rendered visible as a narrow border* of diminished brightness where the terminator passes through surfaces making a very small angle with the rays of the rising or setting sun, while they are fully exposed to the direction of our sight; such as the grey levels of the M. Serenitatis, Imbrium, Vaporum, and others; or flat-topped elevations of any height; but where the surface is inclined towards the incident ray, as is evidently the case of the inner slope of a crater-ring after sunrise or (as in this case) before sunset, the penumbra could not possibly attain the projected magnitude described by Schr.t-Schmidt (who

*Schmidt gives its theoretical breadth 8" on the terminator; but practically it will be much less, as the diminished illumination would not be perceptible till a considerable portion of the solar disk was concealed.

On a former occasion the same observer has recorded a somewhat similar but much fainter grey border along the edge of the shadow, then become very narrow, within the crater Eudoxus (17). But in that case, being on the side of the cavity next the Sun, instead of the opposite, the appearance might be readily explained as a true penumbra, or illumination by a portion only of the solar disk, apparently enlarged and rendered more visible by its falling on the gentle slope of the foot of the wall.

does not, however, refer to this) was not unacquainted with such appearances, having noticed them several times in craters (he especially mentions Copernicus, Theophilus, Zach, and Sacrobosco), where an ill-defined edge of brownish grey rendered the length of the shadow difficult of measurement, while other perfectly similar craters in the same neighbourhood were free from any such peculiarity. For an explanation he sees no need of having recourse to penumbra or atmosphere; it might be sufficiently accounted for by a multitude of colossal blocks on the crest of the ring, whose narrow lines of shadow cast upon the opposite wall, with their intervening streaks of light, being separately undistinguishable by us, would produce the confused general impression of a diluted border. In this case, he refers to some terrestrial correspondences, such as the shadow of a row of close-set iron spikes on the top of a door or wall, seen from a suitable position and distance; or the shadow of a fringe of ice-ruins and rock-pinnacles falling on a snowy slope, which he once remarked in great beauty from the Wengern Alp; the edge of a mountain shadow, elsewhere sharp, becoming very indistinct when projected on the very obliquely slanting snows of the Silverhorn and Guggi glacier, the employment of a common eye-glass showed the cause to lie in the confused impression of many long, narrow, separate streaks of shade. (We may observe, by the way, that the eye for terrestrial scenery, evident in this great observer, qualifies him in a high degree for the analogical interpretation of the varied aspects of the Moon.)

But though this is a plausible elucidation, it may not be the true key to the mystery. To the objection that it would be difficult to account in this way for so broad a zone of duskiness as Schr. has represented, falling, too, upon a slope inclined in the wrong direction, it might be answered that his drawing was too rough to be trusted in minute details, and that he seems to have satisfied himself too easily upon the subject. But it is evident that more has yet to be explained, and that the point deserves study. Though there is no improbability in the idea that the summit of a ring should be crested with a row of natural battlements or pinnacles; yet these, if close enough to produce a confused half-tone in the part of the shadow cast by the ridge where it lies facing the sun, would in every other position overlap one another so much in perspective as to intercept too much light, and produce a full and defined shade. Here, therefore, a very powerful instrument would so far decide the question that an equal intensity of halftone along the whole border of the shadow would negative Schmidt's solution. And so would any want of periodical recurrence in the phænomenon-a point which seems to have

escaped attention. Of course, in any comparison of observations, a very close similarity of conditions would be required; but this being fulfilled, the non-appearance of the border would so distinctly point to some unexplained and possibly unsuspected cause, as to invest the inquiry with peculiar interest. Any observer making an especial study of the edges of the interior shadows of great craters might not regret the loss of time in the end.

The position of Eratosthenes is in the midst of landscapes of very contrasted characters-the level Sinus Estuum, the towering Apennines, the vast extent of the Mare Imbrium, and a most remarkable honey-combed district which we shall find to the E. The line of the Apennines may be considered as continued through it in that direction by a broken range of hills, of which the extremity, of great steepness on every side, especially N,-the 7 of B. and M., attains according to them 4000ft. Schr. had given it 250ft. more. Running S. from the E. side of the wall is a more considerable range, reaching near its beginning, according to Schr., 9500ft., and leading down to a large ring named Stadius by B. and M. when they failed in identifying, as has been mentioned, Riccioli's spot of that name. This, 43 miles in diameter, and therefore surpassing in that one respect its overpowering neighbour Eratosthenes, is a strange contrast to it in other ways, the embankment, on which they have figured two or three minute craters, being as a whole scarcely 130ft. high, the mere outline of a wall, so as to have escaped the attention of B. and M. for three years. Its surface is not depressed, and the question may possibly suggest itself, Have we here all that remains visible of a great ring, whose height without and depth within have been subsequently reduced to these trifling proportions by a circumfusion and penetration of matter, once fluid or plastic, but now consolidated? The inquiry is thrown out as a mere suggestion for examination and thought, with the sole addition that there are very many other parts of the lunar surface where such a suspicion might as naturally arise. B. and M. remark in its interior only some ridges and one small crater, probably less elevated even than the ring. Of this more hereafter. We next cross the curious district already alluded to, and to be described at a future time, to the magnificent Copernicus (30), one of the most imposing and best-developed specimens of its class. There are many its equals or superiors in size and depth in other parts of the Moon, but few more remarkable at once in themselves and their situation; its structure is very perfect, and its insulated position, and the absence of any material foreshortening, exhibit it to especial advantage. The diameter of its colossal wall is about 56 miles. This wonderful rampart, which does

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