21U 



♦ KNONATLEDGE ♦ 



[May 1, 1886. 



change in u fortnight, is assuredly not a desirable place for 

 creatures so unfortunately sensitive as we are to changes of 

 temperature. 



All this, however, is not new but old. Till jSIr. Langley 

 came and set many doubting with his dreadful liolometer, 

 no one imagined that there could be life on a planet under- 

 going the vicissitudes of temperature which Sir John 

 Herschel had correctly indicated, and Lord Rosse had 

 demonstrated. That the moon, whatever lier past history 

 may have been, must now pass monthly through amazing 

 vicissitudes of heat and cold, is certain, let Mr. Langley's 

 bolometer say what it may. It is with the moon's past his- 

 tory we are concerned at present, not with these effects of 

 the sun's action on the moon's dead body. 



For dead the moon assuredly is, noiv. It is as clear that 

 there are none of the characteristics essential for life — as 

 water, air, and reasonable ranges of temperature — on the 

 moon at present, as it is that her surface has in the past 

 been the scene of tremendous disturbance. That dead body 

 of hers, carefully examined, with due i-egard to the evidence 

 which our earth also can give about planetary existence, may 

 tell us as much as a post-mortem examination might tell the 

 keenly observant anatomist of the past life of a human 

 being. What may have been the precise features of the 

 various eras of lunar life we may no more be able to tell 

 than the anatomist can tell what thoughts passed through 

 the brain which he dissects with his scalpel. But the broad 

 outline of liuman life we may trace as surely as that anato- 

 mist can follow the stages by which the body he dissects 

 had reached its final condition, before death invaded life's 

 sanctuary. 



It is here that, as it seems to me, new thoughts are sug- 

 gested by knowledge recently acquired. 



The argument from analogy has, I think, been some- 

 what too narrowly applied to the moon and other planets, 

 when they have been compared with our earth. In assuming 

 that each planet has its youth, its mid-life, its old age, and 

 finally its death, astronomers have doubtle.ss been right 

 enough ; but I think it by no means so clear that they 

 have been right in assuming (tacitly, perhaps, but still con- 

 fidently) that the various stages in the lifetime of one planet 

 resemble the con'esponding stages in the lifetime of another. 

 A dog has stages of life corresijonding to those of a man ; 

 but a puppy is not a baby oi' even like one, a young whelp is 

 unlike a lad, a dog is not a human being, and even a dead 

 dog presents no very marked features of resemblance to a 

 defunct man. 



I propose to consider here some points in which, most 

 probably, the moon's life-history must have been entirely 

 unlike the life history of our earth. The considerations 1 

 shall urge may be applied, it will be found, to other planets 

 as well to those which are larger than the earth as to those 

 which, like the moon, are very much smaller. 



I begin with some of the simple considerations in- 

 volved, such as those relating to size, surface, substance, 

 and so on. 



Ever}' one knows that the earth contains eighty-one 

 times as much matter as the moon. I might dwell on 

 the consideration that in gathering together her larger mass 

 the earth must have become very much warmer than the 

 moon ever was, even when the moon was at her youngest 

 and hottest. For the celestial bodies owe their heat to their 

 own energies in gatheiing their mass together ; and the 

 greater the gathering energy the greater the develojied 

 heat, as certainly as the stronger a blacksmith's arm the 

 greater the effect of his blows. 



It would seem even that we have evidence of a still 

 greater deficiency of original heat in the circumstance that 

 the moon not only had less energy with which to gather 



together her substance, but that having gathered it to- 

 gether, she has packed it less closely than the earth. If 

 the moon were as compact as the earth she should have only 

 an 81st part of the earth's volume. As a matter of fact 

 she has fully a 49th part. The earth put in a suitable 

 balance (I cannot indicate any suitable place for setting 

 it) would be found to weigh about five and a half times as 

 much as a globe of water of the same size. But weighing 

 the moon, three and a half globes of water as large as the 

 moon would bring down the scale on their .side. 



Starting thus in her career of life with m\ich less heat 

 than the earth, the moon would cool also much more 

 quickly. I do not mean by this that she would give out 

 more heat moment by moment than the earth did at the 

 same stage of her life ; but that she would be cooling faster 

 in the same sense that a cupful of hot water cools faster than 

 a bowlful, and a spoonful faster than a cupful. This is easily 

 seen if we compare tlie moon when red-hot with the earth 

 also red-hot, neglecting the eflect of the air of either body 

 in keeping in the heat, as clothing keeps in the heat of the 

 human body. We may be sure that the consideration thus 

 neglected does not .affect the general result ; for certainly 

 the moon was not better clothed (atmospherically) thaii the 

 earth, at any corresponding stages of their lives — but the 

 reverse. Our red-hot earth, then, had eighty-one times as 

 much heat at that red-hot time than the moon at that 

 (other) time when she was red-hot. And the earth was 

 giving out thirteen and a half times as much heat, moment 

 by moment, as the moon ; for in that degree her surface 

 exceeds the moon's. Now, if a man has 81,000L, while 

 another has but 1,000?., and expends daily 13/. 10«., while 

 his poorer friend can only afford to expend \l. daily, the 

 property of the former will last six thousand days, while 

 that of the latter will, in one thousand, be completely 

 exhausted. The richer man's money would last six times 

 as long as that of the poorer man. The earth's heat would, 

 in the same way, last six times as long as the moon's, at 

 each stage of the cooling process. 



In this way the moon would manifestly age very fast as 

 compared with the earth. If we imagine the moon and the 

 earth at the same stage of planet life six millions of years 

 ago, then in a million years from that time, or five millions 

 of years ago, the moon was where the earth is now. What 

 will five millions of yeai-s do for us, or rather for our home 1 

 But even that way of putting it is not quite strong enough. 

 Those five millions of years in the moon's history would 

 correspond to six times as long — or to thirty millions of 

 years — in the history of the earth I Our globe will show 

 marked signs of advanced age by the end of that time, I 

 venture to predict, in calm assurance that, at any rate, I 

 shall not be contradicted by the evidence of observed facts. 

 She would then bo as for advanced in planetary life as the 

 moon. 



Like the well-known calculation about wine, made under 

 (pretended) vinous influence for All ihu Year Round 

 several years ago, this calculation may be modified, yet the 

 result come out unchanged. If I remember rightly, that 

 calculation began : '' Let us suppose there are eighty casks, 

 or it may be eight hundred, or, perhaps, eight thousand," 

 and so on. We might have begun our calculation by 

 saying. Imagine the moon and the earth at the same age 

 six millions, or it may be sixty millions, or perhaps six 

 hundred millions of years ago. It really does not matter. 

 Take the longest period. Six hundred millions of years ago 

 the earth and moon were in the same stage of planetary life. 

 Then we find that five hundred millions of years ago the 

 moon had reached the stage now reached by the earth, and 

 three thousand millions of years hence the earth will have 

 reached the same condition as the moon. Is not this, the 



