A NEW CRATER IN THE MOON 



419 



forms of volcanic activity which still exist upon 

 our own earth, and may, perchance, exist to 

 some degree in the moon. At any rate, if the 

 supposed changes in the moon's surface are to be 

 attributed to vulcanian activity, such activity can 

 only be regarded as belonging to the latest era 

 of the third and last stage of planetary cooling. 

 The question, in fine, which we have to determine, 

 in considering these changes, is simply this : " Are 

 they vulcanian (using the word as Mallet does), 

 or are they to be otherwise explained '? " Or we 

 may put the question thus : " Is the moon's frame 

 dead, or does it still retain the last sparks of 

 planetary life ? " 



Now, there is one question underlying all our 

 inquiries into the moon's actual condition as re- 

 vealed under telescopic scrutiny, which appears 

 to me very difficult indeed to answer. It has 

 been answered, by some, as though it were very 

 readily dealt with. Unfortunately, it has been 

 answered in several different ways. The question 

 is this : " Has the moon once had an atmosphere 

 and oceans like the earth ? " It will be manifest 

 that if we suppose the moon to have resembled 

 the earth in this respect, at a remote epoch, and 

 during a long period of time, we should give to 

 observed appearances on the moon's surface an 

 interpretation very different from that which we 

 should give if we supposed that the moon's globe 

 never had much water upon it, and was never 

 enveloped by an atmosphere of considerable den- 

 sity. In one case we should have to take into 

 account the influence of long-contiuued denuda- 

 tion ; in the other, the only effects we should 

 have to consider would be those resulting from 

 vulcanian energy and those depending on the al- 

 ternation of intense heat and intense cold during 

 the long lunar day of twenty -nine and a half ter- 

 restrial days. 



Assuming that the moon was ever clothed 

 with an atmosphere, and partially covered by 

 oceans, we could form, as I have elsewhere shown, 

 but one opinion as to to the way in which the 

 oceans, at any rate, have been caused to disap- 

 pear. We should be compelled to believe that 

 the water had been withdrawn into the moon's 

 interior as the moon cooled. The disappearance 

 of the atmosphere could hardly be explained in 

 the same way, however. For it must be remem- 

 bered that a lunar atmosphere resembling our 

 own in density, at the former lunar sea-level, or 

 even an atmosphere of only one-tenth or one- 

 hundredth the density of ours, would extend much 

 farther above the moon's surface than our at- 

 mosphere extends above the surface of the earth. 



At a height of about three and a half miles our 

 atmosphere is only half as dense as at the sea- 

 level, at seven miles one-fourth, at fourteen miles 

 one-sixteenth, at twenty-eight miles one-256th, 

 at forty-two miles one-4096th. But whatever 

 the density of the lunar atmosphere at the sur- 

 face of the moon, it would only be reduced to 

 one-half at a height of twenty-two miles, to one- 

 fourth at a height of forty-four miles. So that, 

 if at the surface its density were only one-thou- 

 sandth that of the air we breathe, the density at 

 a height of forty-four miles would be somewhat 

 greater than the density of our own air forty-two 

 miles above the sea-level ; and at all greater 

 heights the density of the lunar air would be 

 enormously greater than that of our own air at 

 corresponding heights. (Indeed, it is worthy of 

 notice that, even assuming the lunar air as rare 

 as it is commonly supposed to be, the density at 

 a great height is greater than that of our own 

 air at the same height.) Seeing, then, that the 

 lunar air is so much less closely packed, so to 

 speak, by the action of gravity than our own, we 

 cannot suppose that an atmosphere at all resem- 

 bling ours in density has been withdrawn into 

 the moon's interior. On the other hand, it is 

 difficult to believe that such an atmosphere has 

 entered into chemical combination with the va- 

 rious substances forming the moon's crust, and 

 has thus disappeared as an atmospheric en- 

 velope. 



On the whole, it seems to me that the balance 

 of probability is strongly against the former ex- 

 istence of a lunar atmosphere resembling our own 

 in density. If this be assumed, then we should 

 no longer have to account for the effects of what 

 geologists call sub-aerial denudation upon the 

 moon. The moon's surface would show only the 

 effects of past forms of vulcanian activity. It 

 must be admitted, I think, by all who have ever 

 studied the moon with the telescope, that the as- 

 pect of the lunar mountains, craters, plains, etc., 

 accords better with this view than with the other. 

 It is almost impossible to believe that the earth 

 will ever present a scene at all resembling that 

 now presented by the moon ; simply because we 

 see that even if the waters of the sea were with- 

 drawn from the earth, and all forms of life, ani- 

 mal and vegetable, disappeared, the effects of the 

 long ages when the earth had air and water 

 would remain, and would perceptibly modify the 

 earth's aspect. There is very little on the moon's 

 surface, as at present seen, which can possibly be 

 attributed to such a cause. 1 Thus the probabil- 

 1 1 am not sure but that even those few features 



