66 



KNOWLEDGE & SCIENTIFIC NEWS 



'April, 



1904. 



the deposit of alluvium at the mouths of the great water- 

 courses. We do indeed find valleys and ra\ines on the 

 western slopes, but these often are so blocked or show so 

 many irregularities of level that they cannot be held to 

 be water channels. If this was their original nature, then 

 the more recent history of the moon must have entirely 

 changed their appearance ; we see nothing to remind us 

 of the characteristic arrangement of a drainage area on 

 the earth. More than that, we find in the neighbourhood 

 of Sulpicius Gallus a dark band parallel to the edge of 

 the Mare Serenitatis, as if the Mare w-as actually deeper 

 here than further out in the plain. Such a channel would 

 have inevitably been filled up by the alluvium washed 

 down by rivers draining the highland district. 



It is very instructive to watch the apparent changes 

 produced in any region of the moon by the progress of 

 the lunar day. The fi\e photographs of the regions of 

 the Apennines shown in figs. 1-5 are reproduced from 

 Professor W. H. Pickering's " Photographic Atlas of the 

 Moon," noticed in the last number of " Knowledge," 

 and will give some idea of the great value of this syste- 

 matic mode of study which Professor Pickering has 



F'g.-5- — Evenin;?. 



carried out. It will be seen at once that the change in 

 the lighting produces an immense change in the general 

 appearance of the region. The five photographs we may 

 describe for purposes of reference as showing the district 

 at morning, forenoon, noon, afternoon, and evening ; 

 descriptions which are only roughly correct, but which 

 will suffice for reference. It will be seen at once that the 

 appearance of relief vanishes almost entirely at noonday ; 

 it increases directly in proportion to the obliqueness of 

 the illumination, and is very marked in the last photo- 

 graph of the series taken almost at sunset. The two 

 great craters, Archimedes and Eratosthenes, are prac- 

 tically lost at noon. At this time the brightest objects 

 are the glittering peaks of the Apennine range, the rampart 

 of Conon, and the white mantle surrounding Aratus. 

 In early morning and late evening the gradual slopes of 

 the highlands towards the west, and their steep declivi- 

 ties towards the east, are the regions which respectively 

 shine out most conspicuously. But it is the latter which 

 are by far the most brilliant ; and, looking at the fifth 

 photograph, there would seem not a little to justify Pro- 

 fessor W. H. Pickering's description of them as snow 

 covered. " Many of the higher summits of the Apen- 

 nines," he writes, " are brilliant with snow, although the 



sun is just setting upon them, whilst the slopes of the 

 intermediate valleys and of the foothills are dark." 



Professor Pickering's interpretation of the brilliancy of 

 the eastern slopes of the Apennines involves several 

 assumptions. He considers that the deposition of snow 

 will vary on the moon according to the elevation of a 

 district and according to its distance from the equator. 

 But it should be borne in mind that elevation on the 

 moon will not be nearly as effective in producing con- 

 densation as on the earth. The action of gravity at the 

 lunar surface is but one-sixth of what it is with us. This 

 would have a two-fold effect. Whilst here w-e reach a 

 region of half the surface pressure at a distance of three 

 and a half miles, on the moon we should have to ascend 

 more than twenty-one miles to obtain the same pro- 

 portional diminution, whilst the feebleness of gravity 

 would make any upward motion of the atmosphere ex- 

 ceedingly slow. The cooling of an ascending current of 

 air by expansion, here the most efficient cause of con- 

 densation, would there be practically inoperative, and the 

 great tenuity of the lunar atmosphere would tend in the 

 same direction. There would scarcely be any perceptible 

 difference in the readiness with which condensation 

 would take place between the plains and the mountain 

 summits. 



The comparison of the five pictures, too, does not sup- 

 port the inference that the bright regions are snow- 

 covered. The western gentle slopes are by no means so 

 bright under their best illumination as the steep eastern 

 escarpments are under theirs. Yet it is on the former that 

 we should expect the snow to lie, whilst as they are best 

 lighted by the morning sun, that is to say, just as they 

 emerge from the long lunar night when the snow should 

 be thickest, we should expect them to be far more fully 

 covered, and therefore more brilliant than the steep 

 eastern slopes could be at sunset, after having undergone 

 the continued action of the sun during the whole length of 

 the lunar day. The changes in illumination are indeed 

 just what w-e might expect from the varying incidence of 

 the solar rays, provided that there was some difference in 

 the reflective power of the different surfaces. And in 

 this case there is no difficulty in pointing out a sufficient 

 cause for the steep slopes being more brilliant than the 

 gentle. Mr. Davison (" Knowledge," December, i8g6, 

 p. 278) pointed out that objects on a slope from, the mere 

 effect of the expansion during the heat of the day and 

 contraction under the cold of night, would steadily creep 

 downwards. There would thus be a very slow but con- 

 tinuous transference of free solid particles from the 

 summits of the mountains towards the plains, uncovering 

 fresh surfaces in the higher regions, and this creeping 

 effect would necessarily be much more rapid on such 

 steep declivities as the eastern face of the Apennines 

 than on the gradual slopes towards the west. If then 

 the very tenuous atmosphere which we may readily 

 believe to exist upon the moon be capable of effecting 

 some slight tarnishing or darkening effect in the course 

 of centuries, or if the deposition of meteoric dust, which 

 must be much the same as upon our earth, slowly coats 

 our satellite with a thin dark veil, we shall find a sufficient 

 explanation for the difference in albedo of the mountain 

 peaks and of the great plains. 



This explanation is emphasised by the consideration 

 of a point which Professor Pickering brings forward in 

 proof of the existence of snow deposit. He points out 



• I use the terms "east" and " west" throughout this paper, 

 from our point of view. An inhabitant of the moon would, of 

 course, regard the slopes facing the sunset as the western slopes. 



