120 PHOTOGRAPHY OF THE LUNAR SURFACE. 



The hypothesis that the entire surface of the moon is covered with 

 ice appears to us inadmissible. It is natural to inquire whether its 

 accumulation might not take place exclusively in the neighborhood of 

 the poles as in the case of the earth and probably that of Mars. No 

 doubt these regions nearly deprived of the action of solar heat ought 

 to cool down first. 



The vapors formed in the low latitudes would condense at the poles 

 in the form of snow, and would thus be withdrawn from circulation, 

 since the temperature at the poles must have become too low to melt 

 or vaporize it. Starting from the moment this temperature is reached, 

 the ice would not cease to accumulate at the poles until the equatorial 

 zone itself had permanently cooled to the freezing point. Moreover, 

 since this general cooling could only have been accomplished extremely 

 slowly, the poles would have, time to accumulate a great quantity of 

 ice. Their aspect would consequently be more united than that of the 

 equatorial regions on account of the effacement of the smaller depres- 

 sions. The lunar photographs, however, reveal just the contrary. The 

 southern region of the moon is particularly mountainous, bristling with 

 irregular formations. The craters are very numerous in this locality, 

 many touching the limit of visibility, and there are, without doubt, 

 many more which escape our means of observation. On the hypothesis 

 of two polar ice caps it seems impossible for the intermediate zones not 

 to be distinguished by too easily distinguished lines of demarcation 

 undergoing periodic oscillations. Indeed, no known rock taken as a 

 whole possesses a power equal to that of snow or of ice for reflecting 

 and diffusing light. The distinction could perhaps be made without 

 difficulty on terrestrial mountains 200 kilometers off, through the low- 

 est and most highly refracting layers of the atmosphere, conditions 

 under which these mountains would appear infinitely less sharply 

 defined than the craters of the moon. It seems that all we can reason- 

 ably admit is the possible existence of deposits of ice on the floor of the 

 craters in the polar regions. There they would be nearly removed 

 from our sight and sheltered from the radiation of the sun. Eeduced 

 to these terms the question can only be answered by an absolute nega- 

 tive. It may be pointed out, however, that the bottoms of the craters 

 appear relatively white in the neighborhood of the poles but never 

 more so than the neighboring crests and plateaus. There can exist, 

 therefore, only glacial masses of slight thickness and of small extent, 

 probably covered with cinders-. Notwithstanding this, it is to be hoped 

 that the changes which should accompany their temporary liquefac- 

 tion ought to be revealed some day by the careful comparison of 

 photographs. 



XI. — CONCLUSIONS. 



The result of this inquiry, as may be seen, is unfavorable to the 

 actual existence of water, air, and ice in any considerable quantity 

 on the moon. The climatic conditions must therefore be very severe, 



