228 



NA TURE 



[January 5, 1905 



summer lies sixty miles due south of Conon. 

 Although difficult objects, the former has been seen 

 in Cambridge, Mass. A sketch of it is given in the 

 Harvard Annals, xxxii., plate vii. 



Turning now from theory to fact, one of the clearest 

 evidences of hoar frost upon the moon is found in 

 connection with the pair of small craters known as 

 Messier and Messier A. Sometimes one of these 

 craters is the larger and sometimes the other. Some- 

 times they are triangular and sometimes elliptical in 

 shape. When elliptical their major axes are some- 

 times parallel and sometimes nearly perpendicular to 

 one another. When the sun first rises on them they 

 are of about the same brilliancy as the mare upon 

 which they are situated, but three days later they both 

 suddenly turn white, and remain so until the end of 

 the lunation. When first seen the white areas are 

 comparatively large, especially that surrounding 

 Messier itself, but it gradually diminishes in size under 

 the sun's rays. By the eighth day little is left outside 

 the crater itself, while at the end of the lunation 

 only the bottoms and interior western walls remain 



They reach their minimum size five days after sun- 

 rise, when the smaller is about half a mile in diameter. 

 They then begin to. increase, the northern one attain- 

 ing a length of five miles shortly before sunset. If 

 these markings are due to white quartz, or some 

 similar rock, it is difficult to account for their change 

 in size. 



The third class of physical changes with which we 

 shall deal the writer believes to be due to the presence 

 of vegetation. Changes of this class are more con- 

 spicuous than those of either of the other two, and if 

 the explanation of vegetation is admitted, both the 

 other explanations almost necessarily follow. It is 

 therefore important to study these changes with the 

 greatest care. 



Before describing the facts, it may be well first to 

 deal with the principal objection that has been made 

 to the suggested explanation, namely, the lack of 

 water on the moon in the liquid form. The reason 

 that w-e believe liquid water to be lacking is that it 

 is known that as we reduce the atmospheric pressure 

 the boiling point of water is gradually lowered, until 



— 1901, March 31, 3"5 days, 54 . 



brilliant. The general character of these changes can 

 be followed even with a 4-inch telescope working under 

 only moderate atmospheric conditions. Photographs 

 of these craters showing their varying shapes and 

 sizes will be found in the Harvard Anna\s (li., p. 28). 

 Those to whom the Anna\$ are not accessible will find 

 these photographs and most of the other illustrations 

 referred to in this article in my book " The Moon." 



The white area surrounding Linn6 also shows 

 evidence of change in size during the lunation. Soon 

 after sunrise it measures 4" in diameter, at noon 2", 

 and at sunset 3".5. The change is evidently analogous 

 to that shown by the polar caps of the earth and Mars, 

 lunar noon in this case corresponding to midsummer 

 for the planets, and sunrise and sunset to spring and 

 autumn. 



In the crater Eratosthenes there is a brilliant white 

 area on the summit of the central mountain range. 

 When the sun first rises on it it measures five miles 

 in length by two in breadth, ll soon, however, begins 

 to dwindle, and two and a half days Inicr all is gone 

 save two little spots, each about a mile in diameter. 



NO. 1836, VOL. 71] 



when we reach a pressure of 4.(5 millimetres the boil- 

 ing and freezing points coincide. Below this pressure 

 ice changes at once into the gaseous form without 

 passing through the liquid state. While, therefore, 

 there can be no free water upon the surface of the 

 moon, there is yet nothing to prevent it from occurring 

 beneath the surface of the ground, retained by the 

 capillary action of the soil. This action is so strong 

 that, as has recently been shown by Cameron (Science, 

 1903, xviii., p. 758), it is capable of extracting water 

 from a membrane against a calculated osmotic pressure 

 of ^6 atmospheres. 



Since on the earth plants can live on moisture which 

 they have in turn extracted from such a soil, there 

 seems to be no difticully in understanding how they 

 could live on the mo:)n, in a soil which could thus 

 retain considerable moisture in spite of the low atmo- 

 spheric pressure. .Mthough in a state of nature, even 

 in desert regions, all plants are occasionally exposed 

 to water in the form of rain or dew, yet under artificial 

 conditions we kno-v that even such highly organised 

 structures as house plants can flourish on water that 



