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TEE POPULAR SCIENCE MONTELY.— SUPPLEMENT. 



(through the action of some quite inconceivable 

 forces) to burst into fragments, the catastrophe 

 would be looked upon by astronomers of this 

 class as only interesting because affording an 

 illustration of the dynamical effects of explo- 

 sion, and suggesting divers matters for calcu- 

 lation. 1 The recognition of the signs of life 

 upon the moon, if we could imagine the possibil- 

 ity of life on that sterile surface, would proba- 

 bly have no interest at all for the merely mathe- 

 matical astronomer ; for it would scarcely be 

 possible to connect the discovery with d?y by 

 dx*. I would not be misunderstood as wishing to 

 imply that there is little interest in the mathe- 

 matics of astronomy. On the contrary, I not only 

 recognize great interest in the mathematical rela- 

 tions to which I have referred, but I have a strong 

 feeling of sympathy with those who are specially 

 attracted to that particular department of astro- 

 nomical research. There is a wonderful charm 

 about mathematical astronomy, as, indeed, about 

 mathematics generally ; and there is exceeding 

 interest in many of the physical discoveries which 

 have resulted from mathematical calculations. 

 It would, indeed, be a mistake of the same kind 

 as that which I decry to depreciate the interest 

 of such discoveries. For what I would insist 

 upon is, that the true astronomer should regard 

 with interest the results of all forms of astronomi- 

 cal research. And although the recognition of 

 some new feature of the moon's surface may not 

 have iuvolved any profound mental effort, or the 

 exercise of exceptional skill in mathematical cal- 

 culation or instrumental observation, yet all who 

 rightly appreciate the object of scientific research 

 must admit the great interest of the questions 

 suggested by supposed lunar changes. 



No further apology need be made, therefore, 

 I think, for a brief inquiry into the subject of the 

 new lunar crater said to have been discovered by 

 Dr. Klein, of Koeln. 



1 A few days after the recent transit of Mercury I 

 met one of the most eminent of our mathematical as- 

 tronomers, and expressed disappointment at the un- 

 favorable weather which had prevailed in England 

 during the transit. It had seemed to me a matter of 

 considerable interest to determine whether Mercury 

 has an atmosphere, and, if so, of what nature that 

 atmosphere may be ; to examine into the phenomenon 

 of the bright spot said to have been seen on Mercury ( 

 and so forth ; and to make better inquiries into the 

 physical condition of Mercury. But I found all such 

 inquiries were regarded by him as utterly trivial— the 

 only point he cared to speak about being a discrep- 

 ancy of a few seconds in the moment of contact. 

 Yet, oddly enough, he had published a formula for 

 predicting the time of contact which I found far from 

 exact. 



It may be well first to consider the a priori 

 probabilities of changes in the moon, such as 

 might be detected from the earth — but briefly, 

 because otherwise space would hardly suffice for 

 the due consideration of supposed instances of 

 change. It has always seemed to me that such 

 a priori consideration of a subject is a desirable 

 preparation for the examination of a posteriori 

 evidence, so long only as we are careful not to 

 allow the consideration of what might be expected 

 to prevent us from duly attending to wfiat has 

 actually been observed. 



Without dwelling upon the earlier stages of 

 the moon's history, we may fairly assume that 

 the moon was once an intensely-heated globe, 

 and that she has passed through many stages of 

 cooling to a far later stage of planetary life than 

 that through which our earth is at present pass- 

 ing. Nearly all who have ever investigated the 

 evidence afforded by the moon's telescopic as- 

 pect agree in this conclusion, though in other re- 

 spects they entertain widely discordant opinions. 



We may recognize three special stages in the 

 moon's cooling which correspond with stages 

 through which our earth has already passed. 

 First there was the stage in which a lunar crust 

 and a lunar nucleus were formed. (Observe, that 

 I do not here adopt any theory as to the nature 

 of either ; I infer only from what we know about 

 our earth that at a very early stage of planetary 

 cooling the nuclear regions and the inclosing 

 shell of matter became distinguished one from 

 the other, in such sort that thereafter each obeyed 

 a distinct set of influences corresponding with 

 its position and with the conditions to which it 

 was exposed.) Secondly came the stage in which 

 the exterior shell, cooling more rapidly than the 

 nuclear matter, contracted upen the nucleus — a 

 process leading to the formation of rifts and 

 clefts in the crust, precisely as though the nu- 

 cleus had expanded within the inclosing shell. 

 Thirdly, when the crust had thus parted with the 

 greater portion of its heat, there came the stage 

 when the nuclear matter, now far hotter than 

 the crust, cooled more quickly (having more heat 

 to part with), and thus shrank away from the 

 crust. 



In the case of our own earth, it was during 

 the second of these great stages, which lasted 

 probably for many millions of years, that the 

 great deformations of the terrestrial spheroid 

 had their origin. In the third stage were formed 

 those corrugations of the thickened crust which 

 constitute the various orders of mountain-ranges. 

 To the latter part of this third stage belong the 



