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orbits of the satellites. The satellites themselves he considered to have been formed from portions of matter left behind during the contraction into a globe of such a plane, which had at first occupied the whole space included within the present orbits of the satellites. This view of the formation of the satellites he based on the fact that the period of diurnal rotation in each of them corresponded with the period of its revolution round its primary, which he showed would be the case with any body whatever, if so left behind or lifted off a planet.

The author then discussed the chemical changes that would ensue on the surface of the earth after it had assumed the globular form. Oxidization of its metallic constituents would absorb a vast proportion of its gaseous matter, and the formation of water would remove a great deal in addition. Hence the absence of atmosphere or water on the moon's surface might be accounted for, as she would carry off with her only th portion of the gaseous elements of the planet, and her surface exposed to the chemical action of those elements would be much more than th that of the earth. Water also might be quite absorbed on her surface in the formation of hydrates of the alkaline and earthy bases.

On the earth, sodium would unite with chlorine, and common salt would result; and to the large amount of salt so formed the author ascribed the saltness of the ocean; rivers could only carry to the sea salt obtained from soil originally deposited by the ocean, and which must therefore have derived its salt from the sea. This process must be still going on, and hence Dr. Ashe inferred that the sea could never have become salt, or be now increasing in saltness, from that cause; hence he dissented from that view, which was the one universally put forward by geologists.

On a Group of Lunar Craters imperfectly represented in Lunar Maps.
By W. R. BIRT, F.R.A.S.

One of the objects of lunar maps should undoubtedly be such a representation of the forms of the irregularities of the moon's surface, that a student may readily, at the suitable epochs, ascertain the general outlines and configurations of the parts which he is studying, so as to be certain that he has not misapprehended either the position or form of any particular portion of the lunar surface.

A map constructed for a given epoch, at the full for instance, that shall give those features by which every crater, mountain-chain, and plain may be instantly recognized, is at the present moment a desideratum. Indeed, on such a map some craters would not find place. A certain angle of illumination is necessary to bring out saliently the distinguishing features of a crater or mountain-chain; and a series of maps that would exhibit each to the best advantage, must include as many distinct epochs of illumination in their construction as there are meridians encircling the lunar globe.

One of the greatest monuments of the skill and industry characterizing astronomical science is undoubtedly Beer and Mädler's large map of the Moon. To the student of selenography it is invaluable; his progress would be slow without it. The writer of this paper cannot, however, agree with Crampton "that every mountain and every valley, every promontory and every defile on the moon's surface, finds its representative on that map." On the contrary, in his examination of the lunar surface, he has met with several instances of features not recorded thereon, a recent instance of which forms the subject of the present paper.

In the neighbourhood of a fine chain of craters that come into sunlight from ten to thirteen days of the moon's age, and are well seen under the evening illumination from twenty-one to twenty-four days of the moon's age, lying in the northern regions of the moon from 57° to 74° N. Lat., and from 25° to 50° E. Long., and designated Philolaus, Anaximenes, and Anaximander, with an unnamed crater between Anaximenes and Anaximander, are three crater-form depressions, of which there are numerous examples on the moon's surface,-the usual characteristics being, 1st, an extensive floor, exhibiting a variety of surface in different specimens, often pierced with small craters and diversified with hills; 2nd, a more or less perfect rampart, here and there pierced with craters, and rising into elevated peaks, so that the entire depression is readily recognized as a distinct formation, completely separated from its surrounding neighbours. Two such depressions, lying nearly in the same meridian, and connected by a table-land or plateau, are very imperfectly, if at all,

represented by the German selenographers. The sketch accompanying this communication, taken at Hartwell, on Sept. 18, 1862, under the evening illumination, exhibits the general characters of the northern depression, viz. a floor pierced by a line of eruption (a common feature in several lunar forms), a nearly continuous rampart on the east and west sides, rising into a considerable mountain mass at the north angle marked B by Beer and Mädler, pierced by the crater Horrebow, and connected by the steep rocks that form the north boundary of the plateau. It is proposed, in accordance with a suggestion by Dr. Lee, to designate this depression "Herschel II.”

Beer and Mädler thus describe the table-land :

"South-easterly of Horrebow is a large plateau, fourteen German miles broad, and from twenty to twenty-five German miles long, appearing less from foreshortening. The western border stretches from the western corner of Horrebow to that of Pythagoras, and is rather steep. An offshoot from the same stretches to Anaximander. The southern boundary is denoted by the crater Horrebow B (+58° 9′ Lat., and -42° 0′ Long.), the northern boundary by two craters e and ƒ Pythagoras. It rises on the east, in three great steep mountains of a very dark colour, straight up to the plateau, and only faint traces extend from thence still further towards the east. The most southerly of these three mountains is 919 toises high, while all three of the mountains appear to be exactly similar to each other in height, form, and colour.

"The surface of the plateau itself has, besides several craters,-among which Horrebow A (+58° 40′ Lat., and -45° 30′ Long.), 2.67 German miles in diameter, is the largest, deepest, and brightest,-only a few scarcely perceptible ridges, and may accordingly be considered as an actual level. But whether this landscape, containing nearly 200 square German miles, is to be distinctly recognized as one connected whole, depends very much upon illumination and libration."

It is proposed to designate this table-land "Robinson," in honour of the Astronomer of Armagh.

The following description of the same table-land is taken from the author's observations, dated London, 1862, March 12, 6h to 10h 30m G. M. T., moon's age 124-13, morning illumination. Instrument employed, the Royal Astronomical Society's Sheepshanks telescope No. 5, aperture 2.75 inch.

"South of the crater or depression Herschel II. is another, well defined, but not so large. Between the two is a table-land, in which at least five craters have been opened up. Two are in a line with Horrebow; both are given by Beer and Mädler; the northern one is marked B [Horrebow B], the southern is undesignated. The principal crater in this table-land is marked A by Beer and Mädler [Horrebow A`; the three form a triangle: the two remaining craters are near together, and nearly east of A; the largest is marked d by Beer and Mädler, the other e. All the craters are shown on the map. [Note.-The crater d is referred to in the foregoing translation as f Pythagoras; Beer and Mädler thus speak of it :-"Through an oversight, the lettering Pythagoras d occurs twice on our map; once for a slightly depressed crater on the edge of the previously-described plateau."]

"The table-land lies nearly in the direction of the meridian: the mountains on the north slope, or rather their rugged and precipitous slopes, dip towards the large crater Herschel II.; while those on the south [the three dark mountains before mentioned] dip towards the other and smaller crater, which it is proposed to designate South.' On the west the table-land abuts on the border of the Mare Frigoris, while on the east it extends to some mountain-ranges beyond Anaximander."

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[The reader will notice a discrepancy in the descriptions as regards the points of the lunar horizon. It was thought better to leave each description as given by the writers, rather than attempt a conversion of them; especially as future observers can decide upon which they will adopt, consistent with the principles of lunar topography.]

The form of the table-land before described is irregular. In the sketch it appears to be confined to the area between Herschel II. and "South," and this is the most conspicuous portion of it; but on the night of the 31st of January, 1863, under the morning illumination, it was seen to extend to the north of a crater then coming into sunlight eastward of "South," which it is proposed to designate "Babbage." A

chain of mountains, connecting "Babbage" with Anaximander, forms the eastern boundary of the table-land. Beer and Mädler have left its boundaries undetermined, and further observations are necessary to mark them out with precision.

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Eye-sketch of a chain of Lunar craters, with three large unnamed and unrepresented craters, taken at Hartwell on the morning of Sept. 18, 1862.

I. Philolaus (Riccioli). A ringed mountain.

II. Anaximenes (Riccioli). A ringed mountain.

III. An unnamed crater on Beer and Mädler's map. It is marked "Sommering" by Le Couturier. Beer and Mädler have another "Sommering" near the centre of the disk.

IV. Anaximander (Riccioli). The ring of this crater is imperfect, and requires further observation to define its outline accurately. Between it and V there is a well-marked mountain, besides other interesting features.

V. Herschel II. (Birt). An extensive depression of the character of a walled plain, with a nearly perfect ring not shown by Beer and Mädler, who describe the region between Horrebow, Anaximander, and Fontenelle as an exceedingly rich crater country; the principal part consisting of the region of Herschel II. The following features are common to the eye-sketch and Map:

B. A high mountain mass marked Anaximander B by Beer and Mädler. It really forms the north angle of the wall of the large depression Herschel II. e. A mountain mass forming the N.W. angle of the ring of Herschel II. f. A crater exterior to Herschel II.

c, d. Two craters in the line of eruption that crosses Herschel II. in a curvilinear direction.

The eye-sketch shows the general direction of this eruptive line from the portion of the ring that is absent to a crater east of Horrebow (X). It is not shown on the German map..

VI. The table-land "Robinson" (Birt).

A, B, and C. Craters on the table-land.

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E. A steep mountain "steppe on the south, not shown in the sketch, dipping to the depression "South." It contains the three dark mountains of Beer and Mädler.

F. A steep mountain "steppe" on the north, dipping to Herschel II.

E and F were observed and figured by Schröter in his 'Selenotopographische Fragmente,' T. xxvi. fig. 1.

VII. A depression south of the table-land "Robinson." Proposed name "South" (Birt). The central crater D is shown by Beer and Mädler.

VIII. Another depression eastward of "South," and between it and Pythagoras. The crater A on Beer and Mädler's map is really nearer the west border than shown in the eye-sketch. Proposed name "Babbage" (Birt).

Schröter observed this walled plain. Figures of it, with the interior crater A close to the western edge, are given in T. xxvi. (figs. 1 and 2) of his 'Selenotopographische Fragmente.' It would appear that he designated it "Pythagoras," the crater now bearing that name being termed Pythagoras borealis. By far the most suitable name for the large crater with the central mountain is that on the large

German map, "Pythagoras;" while to prevent misapprehension as to the western walled plain with the included crater, and to distinguish it from the eastern crater, it is proposed to call it "Babbage."

IX. Pythagoras (Riccioli). The largest and most magnificent crater in this part of the moon, showing itself as a conspicuous object with its central mountain, when nearly the whole of the previously-described craters and walled plains are lost to view.

X. Horrebow (Schröter). This crater, which pierces the S.W. angle of the rim of Herschel, has hitherto been treated as being independent of any other formation. Schröter, who named it, figured it in its proper position at the west of the mountains F, and he gives (see T. xxvi. fig. 1, above referred to) the chain of mountains, omitted by Beer and Mädler, forming the continuation of the rim from the steep mountains F to the rim of Anaximander, where he gives a small crater shown in the eye-sketch. On the other hand, Schröter has omitted the western rim extending northwards from Horrebow, which is given by Beer and Mädler. Horrebow is clearly a part of Herschel II.

Schröter does not appear to have recognized or figured "South."

On the Augmentation of the Apparent Diameter of a Body by its Atmospheric Refraction. By the Rev. Professor CHALLIS, M.A., F.R.S., F.R.A.S. For reasons given in another communication, it was assumed that atmospheres generally have definite boundaries at which their densities have small but finite values. Two cases of refraction were considered: in the one, the curvature of the course of a ray through the atmosphere was assumed to be always less than that of the globe it surrounds; and in the other, the curvature of the globe might be the greater. The former is known to be the case with the earth's atmosphere; and it was supposed that, à fortiori, this must be the case with respect to any atmosphere the moon may be supposed to have. On this supposition it was shown that the apparent diameter of the moon, as ascertained by measurement, would be greater than that inferred from the observation of an occultation of a star, because, by reason of the refraction of its atmosphere, the star would disappear and reappear when the line of vision was within the moon's apparent boundary. The same result would be obtained from a solar eclipse. It was stated that, by actual comparisons of the two kinds of determinations, such an excess to the amount of from 6" to 8" was found. This difference may reasonably be attributed to the existence of a lunar atmosphere of very small magnitude and density. The author also stated that from this result there would be reason to expect, in a solar eclipse, that a slender band of the sun's disk immediately contiguous to the moon's border would be somewhat brighter than the other parts, and advised that especial attention should be directed to this point on the next occurrence of a solar eclipse. The case in which the curvature of the path of the ray is greater than that of the globe was assumed to be that of the sun's atmosphere; and it was shown, on this supposition, that all objects seen by rays which come from the sun's periphery are brought by the refraction to the level of the boundary of the atmosphere, whether they proceeded from objects on the surface of the interior globe, or from clouds supposed to be suspended in the atmosphere. Accordingly, the contour of the sun should appear quite continuous, and the augmentation of apparent semidiameter will be equal to the angle subtended at the earth by the whole height of the atmosphere. The apparent diameters of the planets will, for like reasons, be augmented to a certain amount by their atmospheric refractions; and on account of the great distances of these bodies from the earth, the eclipse of a satellite will take place as soon as the visual ray is bent by the interposition of the atmosphere.

On the Zodiacal Light, and on Shooting-Stars.

By the Rev. Professor CHALLIS, M.A., F.R.S., F.R.A.S.

The phenomena of the zodiacal light, as gathered from observations made both in northern and in southern latitudes, were stated to be as follows. As seen in north latitudes, it appears in the West after the departure of twilight, as a very faint light,

stretching along the ecliptic, about 10° broad at its base in the horizon, and coming to an apex at an altitude of from 40° to 50°. It is most perceptible in the West in the months of February and March, at which time its apex is near the Pleiades. Similar appearances are presented in the morning before sunrise in the East in the months of August and September. The light seen in the autumn lies in the same direction from the sun as that seen in the spring. In the southern hemisphere the appearances are strictly analogous, but the times and positions of maximum visibility are, the evenings in autumn in the West, and the mornings in spring in the East. The portion best seen in the southern hemisphere lies in the opposite direction from the sun to that which is best seen in the northern hemisphere. The portion seen, and the degree of visibility, depend on the inclination to the horizon of the part of the ecliptic along which the light stretches. The greater the inclination the better it is seen. At the December solstice opposite portions have been seen in the northern hemisphere, one in the morning and the other in the evening; and in the southern hemisphere opposite portions have been similarly seen at the June solstice. At these seasons the ecliptic is inclined at large and equal angles to the horizon at equal intervals before sunrise and after sunset. The southern observations, from which these inferences are drawn, are those made by Professor Piazzi Smyth at the Cape of Good Hope in the years 1843, 1844, and 1845, and published in vol. xx. of the Edinburgh Transactions,' and evening observations in the autumn of 1848, communicated by a friend of the author resident in the interior of Brazil. More recently, in vol. iv. of the American Astronomical Journal' were published observations by Mr. Jones, a chaplain of the United States Navy, who makes the following statement :-" When in latitude 23° 28′ N., the sun being in the opposite solstice, I saw the zodiacal light at both east and west horizon simultaneously from eleven to one o'clock for several nights in succession." The ecliptic must at the time have nearly passed through the zenith of the observer at midnight. It is clear, therefore, that to be seen an hour before and after midnight, the zodiacal light must have extended beyond the earth's orbit. Taking this as a necessary inference from the observations, it follows that the earth is either always enveloped by the zodiacal light, or at least when passing through the line of its nodes. Professor Challis considers this to be the explanation in part of the luminosity of the sky which is generally perceptible on clear nights, and at some seasons in greater degree than at others. The American observer also states that he saw when at Quito, "every night, and all through the night, a luminous arch from east to west quite across the sky, 20° wide, and most apparent when the ecliptic is vertical." This light is distinguished from the zodiacal light by its being of uniform width. From the ensemble of the observations, the zodiacal light is of the form of a double convex lens, with the sun in the centre, and the principal plane coinciding nearly with that of the sun's equator. As it may be inferred from the foregoing statements that it envelopes the earth, we may conclude that it is simply luminosity, without accompanying bodies. Professor Challis proposes, therefore, to account for it by the effect which the rotation of the vast body of the sun produces on the luminiferous medium, this effect being rendered visible by the disturbance of the gyratory motion by the motion of translation of the sun in space. In a similar manner, magnetic currents are rendered visible in the form of the aurora by the effect of transverse currents. This explanation he stated to be in accordance with the principles of the undulatory theory of light.

The appearance of shooting-stars in the August and November periods was accounted for on like principles, by the disturbance given to the luminiferous medium by the curvilinear motion of the earth resulting from its proper motion and the motion of the solar system through space. At two epochs depending on the variations of the rate of motion, and of the rate of deviation from rectilinear motion, the disturbances would be at a maximum, and these two epochs were assumed to correspond to Aug. 10 and Nov. 12. The kind of disturbance which the earth impresses by its curvilinear motion was supposed to be such as would produce eddies or whirls. Besides this, there might be a disturbance of terrestrial origin, analogous to that which produces the zodiacal light, which might account for the luminous arch noticed by the American observer.

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