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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



mosphere of our earth. Then the glowing surface 

 of the moon gradually cooled, until at length the 

 moon must have been a fit abode for life. But 

 whether, when thus swept and garnished into fit- 

 ness for habitation, the moon actually became an 

 inhabited world, is a question which will be vari- 

 ously answered according to our views respecting 

 the economy of Nature in this respect. Those 

 who hold that Nature makes nothing in vain will 

 need only to ask whether the support of life is 

 the one sole purpose which a planet can subserve ; 

 if that should appear probable, they would at 

 once decide that the moon must during its habi- 

 table stage have been inhabited. Others who, 

 looking around at the workings of Nature as 

 known to us, perceive, or think they percive, that 

 there is much which resembles waste in Nature, 

 will be less confident on this point. They may 

 reason that, as, of many seeds which fall upon the 

 ground, scarce one subserves the one purpose for 

 which seeds can be supposed to have been pri- 

 marily intended, as many younglings among ani- 

 mals perish untimely, as even many races and 

 types fail of their apparent primary purpose, so 

 our moon, and possibly many such worlds, may 

 never have subserved and never come to subserve 

 that one chief purpose for which the orbs peopling 

 space can be supposed to have been formed, if 

 purpose indeed reigns throughout the universe. 



But we are not here concerned to inquire 

 carefully whether the moon ever was inhabited ; 

 we care only to show the probability, the all but 

 certainty, that the moon during one stage of her 

 existence was a habitable body, leaving the ques- 

 tions whether she ever actually had inhabitants, 

 and what (if she had) their nature may have been, 

 to the imagination of the reader. Most certainly 

 there is little reason for believing that on this 

 point men will ever have any real information for 

 their guidance. 



Combining together several considerations — 

 viz., first that the moon must have been fashioned 

 as a planet many millions of years before the 

 earth, that her original heat must have been 

 greatly less than that of the earth (corresponding 

 to a reduction of many millions of years in the 

 time required for cooling down to the habitable 

 condition), that each stage of the moon's cooling 

 must have lasted less by many millions of years 

 than the corresponding stage for the earth's cool- 

 ing, and that, lunar gravity being so much less 

 than terrestrial gravity, the moon's vulcanian vi- 

 tality must have lasted for a much shorter time 

 than the earth's — we perceive that the mnon 

 must have passed that stage of her history which 



corresponded to that through which our earth is 

 now passing, many, many millions of years ago. 

 It would probably be no exaggeration whatever 

 of the truth to say that more than a thousand 

 millions of years have passed since the moon was 

 a habitable world. But we may quite confidently 

 assert that fully a hundred millions of years have 

 passed since that era of her history. And as the 

 changes which she has undergone since then 

 have occurred at a much more rapid rate than 

 those by which the earth is now passing on, and 

 will continue to pass on for ages yet to come, tow- 

 ard planetary decrepitude, we may assert with 

 equal confidence that the moon is passing through 

 a stage of planetary existence which the earth 

 will not reach for many hundreds of millions of 

 years yet to come. The moon, thus regarded, 

 presents to us a most interesting subject of study, 

 because she illustrates, in general respects if not 

 perhaps in details, the condition which our earth 

 will attain in the remote future. 



Let us, then, examine the principal features 

 of the moon — those which may be regarded as 

 characteristic, which, at any rate, distinguish her 

 from the earth — and consider how far it is prob- 

 able that our earth will one day present similar 

 features. We can also inquire how far the 

 moon's present condition may be regarded as 

 that of a dead world, in this sense that she can 

 neither now be, nor (under any conceivable cir- 

 cumstances) hereafter become, once again a habi- 

 table world as formerly she presumably was. 



There is one very remarkable feature of the 

 moon's motions which is commonly not explained 

 as we are about to explain it, but in a way which 

 would correspond better with the general views 

 indicated in this article than the interpretation 

 which seems to us preferable. We refer to the 

 circumstance that the moon's rotation on her axis 

 takes place in precisely the same time as her rev- 

 olution around the earth. This is, in reality, a 

 very strange feature, though it is often dismissed 

 as if there were nothing very remarkable about 

 it. In whatever way the arrangement was brought 

 about, it is absolutely certain that the earth had 

 her share in the work ; and, again, no matter 

 what explanation or set of explanations we ac- 

 cept, we find most interesting evidence suggested 

 as to the moon's past condition. 



According to one account, the moon was 

 originally set spinning at a rate closely corre- 

 sponding to her present rotation rate, and the 

 earth, having by her attractive power somewhat 

 elongated the moon toward herself, acted on this 

 not-perfectly round body in such sort as gradual- 



