94 ROUGH WAYS MADE SMOOTH. 



lunar seas. As regards the present absence of water we may 

 adopt the theory of Frankland, that the lunar oceans have 

 withdrawn beneath the crust as room was provided for them 

 by the contraction of the nucleus. I think, indeed, that 

 there are good grounds for looking with favour on the theory 

 of Stanislas Meunier, according to which the oceans sur- 

 rounding any planet our own earth or Mars, for example 

 are gradually withdrawn from the surface to the interior. 

 And in view of the enormous length of the time-intervals 

 required for such a process, we must consider that while the 

 process was going on the lunar atmosphere would not only 

 part completely with the compounds of sulphur, chlorine, 

 and carbon, but would be even still further reduced by 

 chemical processes acting with exceeding slowness, yet effec- 

 tively in periods so enormous. But without insisting on this 

 consideration, it is manifest that with very reasonable as- 

 sumptions as to the density of the lunar atmosphere in its 

 original complex condition what would remain after the 

 removal of the chief portion by chemical processes, and 

 after the withdrawal of another considerable portion along 

 with the seas beneath the lunar crust, would be so incon- 

 siderable in quantity as to accord satisfactorily with the 

 evidence which demonstrates the exceeding tenuity of any 

 lunar atmosphere at present existing. 



These considerations introduce us to the second part of 

 the moon's history, that corresponding to the period when 

 the nucleus was contracting more rapidly than the crust. 



One of the first and most obvious effects of this more 

 rapid nuclear contraction would be the lowering of the level 

 of the molten matter, which up to this period had been kept 

 up to, or nearly up to, the lips of the great ringed craters. 

 If the subsidence took place intermittently there would 

 result a terracing of the interior of the ringed elevation, 

 such as we see in many lunar craters. Nor would there be 

 any uniformity of level in the several crater floors thus 

 formed, since the fluid lava would not form parts of a single 

 fluid mass (in which case, of course, the level of the fluid 



