January 5, 1905J 



NA TURE 



229 



in the liquid form reaches them only by capillary 

 absorption from the soil. 



Turning now to our observations, as early as 1837 it 

 was pointed out by Madler that there were two small 

 spots in the crater Alphonsus which always became 

 very dark at about the time of full moon, while earlier 

 and later they were much lighter. A similar observ- 

 ation had been made by him regarding a region just to 

 the south of the Mare Crisium. Little else was known 

 regarding the matter until 1892. Since that date 

 spots presenting these characteristics have been found 

 all over the moon's surface, except in the vicinity of 

 the poles. The most northern spot known is in lati- 

 tude + 55°, the most southern in latitude —60°. It is 

 possible that -some of the maria, notably Tranqullli- 

 tatis, and part of the borders of Serenitatis and 

 Vaporum, are covered with these spots, but in any 

 case they do not cover more than 5 per cent, of the 

 moon's visible surface, and" possibly it is very much 

 less. 



It should be mentioned here that the western spot 

 shown by Madler in Alphonsus is now comparatively 



maintained by the south-western quadrant of the floor 

 throughout the lunation. About three days after sun- 

 rise a dark spot appears on the north-western slopes 

 of the central mountain range. The regions at its 

 immediate base darken at about the same time, and 

 an irregularly mottled dark sector appears as the result. 

 On the fourth day the centre of the sector lightens, 

 leaving two canal-like forms radiating from the central 

 peaks. Although in a small telescope these canals 

 appear straight, yet when well seen with a large glass 

 they are found to present considerable irregularity of 

 structure. On the eleventh day the southern one fades 

 out, and just before sunset the northern one also dis- 

 appears. 



A faint X-shaped marking distinguishes the north- 

 eastern quadrant of the floor at sunrise. The centre 

 rapidly darkens as the sun rises upon it, and soon 

 becomes intensely black. Three branches of the X 

 successively fade away, leaving only the south-eastern 

 one, which on the seventh day becoines very pro- 

 nounced. A new branch or canal forms by gradual 

 darkening on the east, while the canal on the north- 



F.G. 3, — rgoi, April 2, 5"6days, 79". 



Fig. 4. — 1901, M.i'-ch 5, 7*0 days, 97°. 



inconspicuous, but that north and south of it lie two 

 others, which with Madler's eastern spot form a very 

 striking isosceles triangle at full moon. 



We will now direct our attention to the crater 

 Eratosthenes, which has been more carefully studied 

 than any other region presenting these phenomena, 

 and which exhibits the changes on a sufficiently large 

 scale to enable us to make use of photographv. The 

 four photographs here shown were taken in the Island 

 of Jamaica in igoi, and are enlargements from some 

 o' the negatives used in printing the Harvard " Atlas 

 of the Moon." Beneath each figure is given the date 

 on which it was taken, the number of terrestrial days 

 that had elapsed since the sun rose upon it, and the 

 colongitude of the sun, taken from Crommelin's 

 ephemeris. The photographs are all on the same scale 

 of 1/2,000,000, or about thirty-two miles to the inch. 

 Upon this scale the moon would be 685 inches in 

 diameter. 



When the sun rises upon this formation the whole 

 of the floor is at first of a light grey tint, whatever 

 detail there is being but faintly marked. This tint is 

 NO. 1836, VOL. 71] 



east, which had disappeared, forms anew by a pro- 

 gressive growth downwards from the crater rim. 

 This growth progresses for five days at a nearly uni- 

 form rate of 250 feet per hour, or about 4 feet a minute. 



The south-eastern quadrant, while very light at 

 first, soon surpasses all the others in darkness. The 

 dark area on the outer wall, which in the first figure 

 is undoubtedlv in part due to shadow, must very soon 

 be partly due also to something else, for it still shows 

 upon the third figure, which was taken but o-8 day 

 before full moon, when shadows are geometrically 

 impossible. The last figure was taken 0-8 day after 

 full moon, and the darker portion of the spot is seen 

 to have rapidly increased in size and to have grown 

 downwards with considerable velocity towards the 

 central peaks. 



Since this dark "area cannot be shadow, our only 

 alternative seems to be that we have here a real change 

 in the character and brightness of the lunar surface. 

 Since we do not know of any mineral which gradually 

 darkens as the sun shines upon it, and later fades out 

 again, our only alternative seems to be to call in the 



