A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 69 



of ruptured bits of ice. These ridges of ice-fields retain their shape during the 

 contractions of the sheet in which they lie, as the blocks of stone in the moon may 

 do when they have found an adjustment. These lunar features deserve careful 

 study, though the conditions make an inquiry into their nature very difficult. I 

 have rarely been able to discern them clearly, and then for only a brief time. 



ON THE POSSIBILITIES OF A LUNAR ATMOSPHERE. 



The apparent arguments in favor of the existence of an atmosphere on the 

 moon, if not now, then in some former age of that sphere, are so strong that 

 selenologists are hardly to be undeceived by the evident facts that militate 

 against this view. These facts are, in brief, as before noted, as follows : There 

 is no trace of clouds on the moon ; there is no difference in the clearness 

 of the seeing- as between the lowest ground and that which is about six miles 

 higher ; there is not the faintest sign of diffusion of light on the line between day 

 and night ; the effect is that which would take place in what we term a vacuum, but 

 not in the most attenuated part of the atmosphere that lies about our earth. More- 

 over, the course of the licjht of a star which goes behind the moon's disc shows 

 clearly that at a mile above the lowest part of the lunar surface the air, if such there 

 be, has less than the thousandth part of the density of that belonging to the earth 

 at the same height. So, if there be any atmosphere at all on the moon, it is in 

 volume, at least, quite unlike that of our planet, and very like the nearest approach 

 to a vacuum which we can in any way produce. There is, indeed, no other valid 

 reason for supposing that any kind of gas or vapor exists about the moon save 

 that it is deemed necessary to have it in order to explain certain changes of 

 color which are deemed to be evidences of organic life. The value of this 

 evidence I shall consider below. 



There is reason to believe that the moon has had upon its surface ample 

 material derived from the vulcanoids out of which to form an atmosphere. Re- 

 garding the lunar sphere as the offspring of the terrestrial, we may fairly suppose 

 that it received its share of the lighter elements of the original common mass 

 when the separation took place. If we regard the atmosphere of a celestial body 

 as the gaseous remnant remaining on its surface after the more readily solidified 

 elements have consolidated, then the moon should have had an original covering 

 of this kind on a scale proportionate to its total mass, /. c, it should have had an 

 atmosphere equivalent in weight to some inches of mercury. Throughout its 

 recorded history there has evidently been a great efflux of vaporous or gaseous 

 materials from below the crust, in total amount probably enough to have provided 

 an envelope in quantity as great as now lies upon the surface of this planet, yet 

 no trace of it remains. We cannot believe that the materials which should have 

 formed as air on the moon have been largely taken into the crust by chemical 

 action, as is the case on this planet, for there are good reasons to suppose that 

 there is no such action going on there, nor can we accept the suggestion that the 

 air-making gases have been frozen, for while the temperature is at times very low 



