146 



♦ KNOV/LEDGE ♦ 



[Maech 1, 1886. 



miy note Tvhat the -workers themselves would be apt to 

 overlook, — the general rather than the special signifi- 

 cance of the results obtained in the various fields of 

 scientific labour. 



In two du-ections results have been obtained which 

 enable us to examine ivith advantage the perplexing 

 problems presented by the surface of our neighbour 

 planet, the moon. On the one hand, astronomical obser- 

 vations on bodies of a very different class — the stars- 

 have corrected the old idea that the various orbs peopling 

 space are probably formed of very different materials. 

 Dr. Whewell was perhaps the last to maintain this doc- 

 trine (in his "Plurality of Worlds"), though we still 

 occasionally hear a form of the doctrine advanced in the 

 utterly untenable theory that in the solar system the 

 outer planets are formed of the lighter elements, the 

 inner planets of the heavier,— a theory inconsistent with 

 physical and dynamical possibilities. We now see that 

 in all jirobability all the orbs in space are aggregations of 

 matter in which the Viirious elements though not joresent 

 in jjrecisely the same proportions, are yet not very 

 diversely represented. This doctrine applied to the moon 

 at once defines the general nature of the jiroblems we 

 have to deal with, and suggests the line on which their 

 interpretation must be sought. On the other hand, 

 geologists have recognised the fallacies underlying their 

 old ideas respecting the formation of the various features 

 of the earth's surface-contour. 



Thej- no longer regard mountain ranges as portions of 

 matter thrust upwards by the earth's interior forces, or 

 these interior forces as products of the interior fires of 

 the earth. On the contrary, they regard the great 

 mountains of the etrth as among the products of 

 the exterior forces, the action of sea, rain, wind, snow, 

 and so forth, — as resulting from processes of deposition 

 in long, trough-like hollows, the deposited masses being 

 eventually raised by side pressures. Again, they no 

 longer regard mountain r.mges as the oldest but rather 

 as among the very youngest features of the earth's crust, 

 the loftier mountains being younger generally than those 

 of less elevation, while some of the smaller hills in 

 certain regions are actually the wrecks of forms of eleva- 

 tion of which (at any rate on their old scale) no examples 

 no w_ remain 'upon the earth. Here, again, are changes 

 of view which alter entirely the character of the problems 

 presented by the moon's peculiarities of surface-contour, 

 and may also serve to direct us to the proper lines of 

 thought for their solution, as well as to new methods of 

 observation for obtaining fresh evidence in regard to our 

 companion planet. 



We should have been glid if Mr. Nasmyth, iu issuing 

 a new and cheaper edition of his valuable work on the 

 moon, had taken the opportunity of reviewing the some- 

 what crude theories of the moon's volcanic history which 

 are associated in this volume with the finest illustrations 

 of lanar features — craters and mountain-riinges, peaks, 

 ravines, and valleys — ever produced. His work in this 

 third edition is indeed as valuable as ever ; but it might 

 have been made much more valuable, if recent researches 

 and recent resalts had bjen brought to beir upon its 

 theoretical portions. These do not need discussion, now, 

 however; having already been duly weighed. They are 

 a little more out of date now than they were when the 

 first edition appeared, but not more obviously inconsistent 

 with the views of to-day than with those already attained 

 twelve years ago. Apparently there has been no attempt 

 at revision even in points of detail. For we are still told 

 that Schmidt's mip of the moon cannot probably be pro- 

 duced, though i: has now been several years in astro- 



nomers' hands ; and many little points of the same kind 

 show that the present issue is only in the publisher's 

 sense " a new edition."* 



That the moon is or rather has been a planet there can 

 be very little doubt, though whether she was ever a 

 planet like our earth may be reasonably questioned. She 

 is now so utterly unlike the earth that it becomes rather 

 difficult to imagine that there was ever even such general 

 resemblance as is implied in the remark that she was once 

 a jilanet. She is not only arid and airless, but even were 

 she clothed with sea and air she would yet be utterly 

 unlike the earth because of her long daj- of more than 

 four weeks. We know, however, that that is a result of 

 terrestrial influence, — and that in the fulness of time our 

 earth must undergo a similar change. Indeed this pecu- 

 liarity, telling us as it does of the immense age of the 

 moon, enables us more readily to understand her death- 

 like surface. It shows us that the moon has existed long 

 enough as a planet to have aged and died, even as we see 

 she has. 



There is no difficulty, now, in understanding that even 

 if formed as long ago, or later, the moon would have been 

 much older than the earth. With 81 times as much 

 mass and only 1.3.', times as large a surface, our earth 

 would have cooled through the varioiis stages of her life 

 much more slowly, — in fact each stage would have lasted 

 just as much longer as J^l exceeds 13], or be six times as 

 long. Suppose the earth and moon both white-hot 60 

 millions of years ago, then the moon would have reached 

 the earth's present stage 50 millions of years ago, corre- 

 sponding to 300 millions of years of earth-life, — so that 

 the moon would tell us of the earth's condition 300,000,000 

 years hence. And though this result is based on assump- 

 tions, it yet jjresents trulj' the general inference we may 

 safely form that the earth will not be in the same stage 

 of planetary life as the moon until many millions of years 

 have jiassed. (If eich stage of the earth s life is six times 

 as long as the corresponding stage of the moon's, then — on 

 any assumption whatever — the earth will only reach the 

 moon's condition after a period five times as long as the 

 interval which has elapsed since they were both simul- 

 taneously in the same stage, or running neck and neck in 

 the race of jjlanetary life.) 



But even with this knowledge it remains difficult to 

 understand why the moon should be so unlike the earth. 

 The waters of the earth may soak their waj' beneath the 

 crust (as our underground caves, and even our hot wells 

 and volcanic outbursts, show they are doing) till they 

 all disp-ppear. Our air can hardly, however, become 

 thinned to the condition of the lunar air. And even if 

 it did, and every trace of water had vanished, the earth 

 would not be as the moon is. There are no great craters 

 on the earth as on the moon : there are scarcely any great 

 mountain ranges on the moon as on the earth. In these 

 chiefly, but in other important respects also, the moon 



* Even the deso'iption of spectroscopic analysis as a method 

 ■nhich has been brought into use " during tliepast few years " reads 

 strangely in a work dated more than a quarter of a century later 

 than the discovery by which that method was introduced, and 

 twenty-twoyears later than the most recent of the spectroscopic dis- 

 coveries mentioned in the book. Errors too which were noted when 

 the first edition appeared might as well have been corrected. 

 Amongst slight faults, more significant perhaps than important 

 errors (which may be error.s of opinion, and still maintained) we 

 note the reproduction of a curiously absurd mistranslation of 

 Laplace's remark that he advanced his famous nebular hypothesis 

 " with the mistrust which everything that is not a result of ob- 

 servation and calculation should inspire : " this is translated as if 

 Laplace had written qui doit iiisjnrc/- (instead of qui- doit inspiri'r), 

 " the mistrust which should inspire everything," kc. 



