Supplcincnl to " Xalure," May 5, 1904 



Tiia>ked by the varying conditions of illumination. 

 This difficulty does not, of course, disappear even when 

 ■ series of photographs are under examination ; in the 

 words of the author (p. gi) : — 



." It was soon found that for certain regions, notably 

 those in the northern half of the disc, the change in 

 appearance produced by the difference of lighting 

 rendered it absolutely impossible to identify the same 

 formation upon the plates taken at (lunar) sunrise and 

 sunset and those taken at noon." 



Photographs at intermediate phases were accord- 

 ingly taken, and by aid of these the connection can be 

 traced. 



Photographs, indeed, introduce another difficulty. 

 Slight changes in exposure and development were 

 found sometimes to produce very misleading results, 

 and it is pointed out that the only safe procedure is to 

 confirm all suspected changes by extended observations 

 under different conditions with the telescope. There 

 is, however, no reason to suppose that the author is 

 unfamiliar with the many pitfalls, and the interesting 

 ■results of his labours may therefore be' received with 

 some confidence, or at least as demanding careful in- 

 'vestigation by other observers. 



.Attention was directed by the author ten years ago 

 to the variability of many of the dark spots which 

 are dotted over the lunar surface, the three in 

 Alphonsus being probably familiar to most observers. 

 The view then expressed that these are patches of 

 organic growth resembling vegetation, which spring 

 up and die during the long lunar day, still seems to 

 ■give the only simple explanation of the appearances 

 ■■observed. The spots are said to be darkest near full 

 moon, when shadows are geometrically impossible, and 

 a real change in the reflecting surface therefore seems 

 to be highly probable. 



On the question of active lunar craters, the chief 

 facts relating to Plato and the much discussed'case of 

 Linne are summarised, and an account is given of 

 phenomena observed in the crater forming the source 

 ■of Schroter's valley which bear a striking resemblance 

 to those accompanying the active eruption of a 

 terrestrial volcano. Part of the description reads : — 



Dense clouds of white vapour were apparently 

 rising from its bottom and pouring over its south- 

 western crater wall in the direction of Herodotus " 

 ■(p 4")- 



The changes in this "vapour column " .-ire s.-iid to 

 be visible with a 6-inch telescope under ordinary atmo- 

 spheric conditions, so that the reality of the pheno- 

 menon need not long remain in doubt, whatever 

 explanation may be adopted. The author evidently 

 'believes that there is an actual emission of vapour, and 

 he points out that as water cannot exist as a liquid 

 on account of the rarity of the lunar atmosphere, it 

 would take the form of snow or hoar frost. 



Many of the changing appearances of lunar details 

 are, in fact, attributed to deposits of snow and hoar 

 fro.st which melt under the influence of the sun's rays, 

 and are re-deposited when those rays are withdrawn. 

 Among other evidence that there is snow on the moon, 

 two photographs of the full moon arc reproduced, one 

 NO. 1801, VOL. 70] 



representing it as ordinarily seen, while the other is 

 intended to exhibit the principal snow-covered areas ; 

 as these are differently printed copies from the same 

 negative, the illustration is anything but convincing in 

 the absence of details as to the printing processes. 

 Other examples are more satisfactory. Linne, for 

 instance, is surrounded by a white halo, which is stated 

 to be not only now permanently smaller than it was 

 thirty years ago, but to change with the altitude of 

 the sun in a manner analogous to the seasonal vari- 

 ations of the polar caps of Mars. In this case the 

 author had the happy thought to inquire if there were 

 any variation during a lunar eclipse, the idea being 

 that the withdrawal of sunshine for a couple of hours 

 or so might produce an appreciable increase in size. 

 .Such an enlargement appears to have been established 

 at the Lowell Observatory in i8g8, and by the author 

 himself in 1899, 1902, and 1903; another observer, Mr. 

 Saunder, however, seems to have been somewhat 

 doubtful as to the reality of the slight increase which 

 his measures indicated in the eclipse of 1903, and as 

 his observations would make the halo twice as great 

 as those which the author made on the same occasion, 

 further observations of this kind are evidently desir- 

 able. 



It is also considered probable that manv of the re- 

 markable changes which have long been recognised 

 in the craters Messier and Messier .\ are to be 

 accounted for by varying depositions of snow. 



Permanent deposits of snow in the craters themselves 

 are believed to furnish an adequate explanation of the 

 striking brightness of such craters as .\ristarclius, and 

 even thi' long bright streaks, such as those which 

 radiate from Tycho, are attributed to the same sub- 

 stance. The long streaks are considered to be com- 

 posed of a multitude of smaller snow streaks issuing 

 from small white craterlets, usually less than a mile 

 in diameter, many of which show a tendency to occur 

 along lines which are probably cracks or lines of weak- 

 ness in the lunar surface. 



The "riverbeds" and lunar "canals," which the 

 author has detected, present many features of interest, 

 and the latter may be of special importance in view of 

 the light which they may throw on the nature of the 

 corresponding features of the planet Mars. 



While some of his researches tend to modifv the 

 prevalent idea that the moon is a dead world, the 

 author has no revolutionary views to put forward as 

 t.) the general! character of the lunar fiinnations. He 



" There seems, indeed, to be no feature found upon 

 the moon which is not presented by the Hawaiian 

 volcanoes, and there is no feature of the volcanoes that 

 does not also have its counterpart uuon the moon. 

 Even the cause of the bright streaks upon the moon 

 . . .is partly illustrated by Hawaii " (p. 25). 



Sufficient has been said to indicate the interesting 

 character of this work, but its value as a contribution 

 to science can scarcely be gauged until independent 

 observations of the unexpected phenomena have been 

 made. It is fortunate that some of the .investigations 

 suggested are within range of very modest instru- 

 ments, even as low as 4 inches aperture. 



