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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



then two tilings would happen. One would be 

 the manifestation of vulcanian energy in conse- 

 quence of the heat generated by the crust as it 

 crushed its way downward upon the retreating 

 nucleus. The other would be the influx of water 

 wherever it found access to the cavernous spaces 

 between the crust and the nucleus. It is prob- 

 able that before this vulcanian era of the moon's 

 history was completed a considerable portion of 

 the lunar waters had taken its place permanently 

 beneath the crust. It should be noticed that this 

 era corresponds with a part of the earth's exist- 

 ence which is as yet far from being completed, 

 even if it can be regarded as much more than be- 

 gun. It is far from unlikely that the era during 

 which a planet's crust is thus kept in constant 

 activity by the retreating motion of the nucleus 

 synchronizes with the period during which life 

 exists on the planet's surface. During all this 

 period, which may have lasted tens of millions of 

 years, not only were portions of the waters of the 

 moon gradually taking up their place in cavernous 

 spaces between the crust and the retreating nu- 

 cleus, but another process must have been at work 

 to exhaust the lunar seas. When water falls 

 upon a large land-surface in the form of rain, so 

 that the surface is thoroughly drenched, a portion 

 probably disappears permanently from the water- 

 circulation of the globe. Of course, the greater 

 portion is conveyed into the sea in the form of 

 running water. Then, again, the drying of the 

 surface means that the water which had moist- 

 ened it is taken into the air again in the form of 

 aqueous vapor. And this eventually assumes the 

 form of visible cloud, and after sundry changes 

 (during which it may many times in turn appear 

 as cloud or disappear as vapor) it falls again in 

 rain, and may be either restored in this way di- 

 rectly to the sea from which it came, or so fall on 

 land-surface as to run into some stream commu- 

 nicating by brook, rivulet, river, and estuary, with 

 the ocean. And some portion of the water which 

 falls on land-surfaces, passing below the surface, 

 feeds internal streams, and eventually appears 

 again in the form of spring-water. But it cannot 

 be doubted that a portion of the water which falls 

 on dry land soaks its way downward, very slow- 

 ly, perhaps, but steadily and continuously, thus 

 removing itself from sight, and pro tanto dimin- 

 ishing the planet's surface-waters. 



How much of the water would be removed by 

 these causes, before the last stage of all began 

 (at least the last change of a planet's existence as 

 a body undergoing change) is not easily deter- 

 mined. Probably a quarter or a third of the wa- 

 ter forming the original oceans of a planet might 



be withdrawn in one or other of these ways, leav- 

 ing the rest to be removed during the refrigera- 

 tion of the nucleus itself — a process requiring 

 many millions, possibly hundreds of millions, of 

 years for its completion. 



In whatever way the withdrawal of the lunar 

 seas was accomplished, it is certain that every 

 particle of water has disappeared from the sur- 

 face of the moon ; and as there are clear signs 

 of the former existence of extensive lunar seas, 

 apart from the strong a priori considerations 

 showing that the moon must once have had water 

 on her surface, we have little choice but to admit 

 that the waters of the moon have been withdrawn 

 by such gradual processes as have been described 

 above, and consequently that the era of the moon's 

 existence as a habitable world is really removed 

 from the present epoch by the enormous time- 

 intervals required for the completion of those 

 processes. In fact, we can see clearly pictured 

 on the moon's face the evidence which shows 

 that she has passed through all the stages of 

 planetary life, from the time when her whole 

 frame was glowing with intensity of heat, down 

 to the period when she had reached the condition 

 which our earth in the remote future must attain 

 — that of a cold, dead orb, neither living itself 

 (regarding physical changes as corresponding with 

 vitality) nor capable of being the abode of living 

 creatures. Extending the range of our survey, 

 we find in the giant planets, Jupiter and Saturn, 

 the evidence of an earlier stage than any of which 

 the moon's present aspect affords direct evidence. 

 The sun presents a yet earlier stage, while the 

 gaseous nebula) or masses of luminous star-vapor 

 scattered through the immensity of space illus- 

 trate the earliest of all stages of cosmical exist- 

 ence of which we have any direct evidence. On 

 the other hand, we see in Mars, with his small 

 ocean-surface and rare atmosphere, the picture of 

 a stage intermediate between that through which 

 the earth is now passing, and the decrepit or 

 death-like condition of the moon. Mercury, if 

 we could examine his condition more satisfac- 

 torily than is the case, would probably illustrate 

 a stage somewhat nearer to the moon's present 

 condition. Venus, on the other hand, so far as 

 can be judged, though a somewhat smaller planet 

 than the earth, is in a somewhat earlier stage of 

 planetary existence. 



Although the moon may be regarded as to all 

 intents and purposes dead, it must not be sup- 

 posed that no changes whatever take place upon 

 her surface. On the contrary, some of the pecu- 

 liarities of the moon's condition must tend to 

 cause even more rapid changes of certain or- 



