especially in combination with such a scale, would be the mode of comparative values, in which the light of any given spot is referred to others above and below it in brightness. Changes are suspected in the reflective power of portions of the lunar surface, and it would be interesting to examine it in that aspect; but we have no good map of the Full Moon, and if we had, it never could well express the various gradations of brightness, which must be the object of topographical study

A few peculiarities of arrangement deserve to be mentioned here. The remarkable tendency to circular forms, even where explosive action seems not to have been concerned, as in the bays of the so-called seas, is very obvious; and so are the horizontal lines of communication already mentioned. The gigantic craters or walled-plains often affect a meridional arrangement: three huge rows of this kind are very conspicu ous, near the centre, and the E. and W. limb. A tendency to parallel direction has often a curious influence on the position of smaller objects; in many regions these chiefly point to the same quarter, usually N. and S., or NE. and SW.; thus in one vicinity (between G, L, and M*) B. and M. speak of 30 objects following a parallel arrangement, for one turned any other way; even small craters entangled in such general pressures (as round L), have been squeezed into an oval form; and the effect is like that of an oblique strain upon the pattern of a loosely-woven fabric: an instance (near 27, 28) of double parallelism, like that of a net, is mentioned, with crossing lines from SSW. and SE. Local repetitions frequently occur; one region (between 290 and 292) is characterised by exaggerated central hills of craters; another (A) is without them; in another (185), the walls themselves fail. Incom

* References to Map of Moon.

plete rings are much more common towards the N. than the S. pole; the defect is usually in the N., seldom in the W. part of the circle; sometimes a cluster of craters are all breached on the same side (near 23, 32).* Two similar craters often lie N. and S. of each other, and near them is frequently a corresponding duplicate. Two large craters occasionally lie N. and S., of greatly resembling character, S. usually of size of N., from 18 to 36 miles apart, and connected by ridges pointing in a SW. direction (20, 19: 78, 77: 83, 84: 102, 103: 208, 207, 204: 239, 242: 261, 260: 262, 263: 340, 345). Several of these arrangements are the more remarkable, as we know of nothing similar on the Earth.

The question as to the continuance of eruptive action on the Moon is one of great interest. It is now generally understood that the volcanos, which Herschel I. and others thought they saw in activity on the dark side, were only the brighter spots reflecting back to us the earth-shine of the lunar night with the same proportional vivacity as the sunshine of the day. No valid reason indeed has been assigned for the fact, witnessed by many observers, especially Sm., that one at least of these spots,-Aristarchus,-varies remarkably in nightly luminosity at different periods,† nor for the specks of light which more

* Compare Elwes's account of the small cones on the floor of the great extinct crater Haleakala, some of them 'broken down at the side, nearly always on the NE.'-Sketcher's Tour, 214.

Elger saw a spot on the dark side, apparently Aristarchus, 1867, April 12, 7h 30m to 8h 30m, nearly as bright as a 7 mag. star, with a 4-in. aperture, and too conspicuous to be overlooked by the most careless observer. It was much fainter during the last 15m, and scarcely perceptible at 9. The Moon was 1a 5h after I. Qu. He had seen something similar on former occasions. Schr.'s conjecture that the variations, which he observed in a minor degree in several parts of the disc,

than once Schr. caught sight of on the dark side for a short time:* but in these cases there has been no subsequent perceptible alteration of surface; and they furnish no reply to the enquiry respecting present changes. Such were abundantly recorded by Schr. in his day, chiefly variations in the visibility or form of minute objects; but a great majority of them he, and subsequently G., who witnessed many such appearances, referred to the lunar atmosphere: and B. and M. are disposed to discard them all as the result of inaccuracy or varying illumination. The extraordinary influence of the latter upon the aspect of distant and unknown objects may be estimated by anyone who will sketch the changed effects of light and shade on any familiar terrestrial object at different times of day; still there seems to be a residuum of minute variations not thus disposed of, and in some cases possibly indicating actual local change. Terrestrial analogy is in favour of the idea that disturbing agency may have greatly diminished without having become extinct: but observation, not assertion, must decide the point. There are no traces of any grand convulsion since the date of the first lunar map; and we are scarcely as yet possessed of the means of detecting smaller changes. Schr.'s drawings are very rough; the much more careful ones of L., and B. and M., would be too recent to warrant great expectations, even were they more reliable—especially the latter-as to minute details: the great map of Schm., 6 Paris f. in diameter, which will be crowded with minutiæ—a wonderful specimen of persevering industry—is not yet published. may be due to atmospheric condensation during the lunar night, is more elegant than probable: it may however deserve consideration.

* Such a phænomenon, more extended but very faint, was seen, or fancied, by G.; and recently witnessed, with great distinctness, by Grover and by Williams.

Topographical studies, however, after the manner of Schr., giving repeated views of the same object under differing angles of illumination and reflection, are the only satisfactory means of detecting possible change. We have as yet no monograph representing any one spot for every successive night of its visibility; but such a connected series would be very instructive. The catalogue of objects and attendant map on a scale of 200 inches, now in preparation by Birt, is laying a foundation, of a very different character from any which have preceded it, for an adequate knowledge of our satellite. In the meanwhile the beautiful and not expensive map of B. and M. may be advantageously employed, though not implicitly trusted to this their explanatory work 'Der Mond' (The Moon) is an important addition for the German 'scholar. Á book with the same title by Schm. (1856) is most interesting and valuable, and ought to appear in an English dress.

Little of a satisfactory nature can be said as to a Lunar Atmosphere. That it must be far rarer than any known gas is demonstrable from theory, and proved by observation, which shews us a sharply defined outline, and detects no refraction in stars over which the Moon passes: * and hence its entire absence has been maintained by great astronomers: Schr. asserted its existence from many changes of aspect in minute objects, and from a very dim twilight which he traced through 9 years beyond the points of the horns; his inferences are supported and in part exceeded by G., who frequently sawor imagined-fogs and clouds on the surface. Schr. explains the defined limb and the absence of refraction by limiting

* It has been recently ascertained that the Moon's observed exceeds by 4" its computed diameter. Airy thinks however that this may be explained by irradiation.

the atmosphere to the inferior regions, and leaving the higher grounds free. B. and M., while explaining away-not very satisfactorily-Schr.'s twilight, which they could never distinctly find, do not deny the possibility of a very rarefied gaseous envelope.* Those traces of twilight, which G. confirms, and which I imagined I saw, 1855, June 20, but only doubtfully from want of better optical means, might well engage the student's attention: in order to assist him two figures

are here given from among Schr.'s numerous delineations. They represent the cusps of the Moon, as seen by him, 1792, Feb. 24, with a 7 f. reflector by Herschel I., powers 74, 161.

Schr.) seen a grey border to the

* Some very curious photographic experiments by De La Rue have been thought to tend the same way. It seems hardly probable that oxygen, which forms of the weight of terrestrial earths, should be entirely absent from the Moon; and the whiteness of the small craters in the grey plains is a suggestive fact. Schm. has in a few instances (to which he might have added one from black shadow in craters, but this he ascribes to its being thrown from a very ragged edge. Some modern observers have noted occasional want of definition in certain spots as compared with others. In a few cases M. and Schm. have noticed a blue tint round bright points on the terminator, probably, as Schm. says, due to the 'secondary spectrum' of all achromatics. De La Rue's remark is important, that it is difficult to conceive any chemical formation of matter without an atmosphere.