PLANETARY EVOLUTION 



the actual state of things upon the moon. For 

 the original disappearance of the lunar air and 

 water, a far more thoroughgoing explanation 

 was propounded some years since by M. Sae- 

 mann ; ' but in this explanation the extreme 

 cooling of the moon, as just illustrated, is im- 

 plicitly involved. According to M. Saemann's 

 essay, the lunar air and water have been literally 

 drunk up by the thirsty rocks. On our own 

 globe the tendency of the surface-water is con- 

 stantly to percolate through the soil of the land 

 or sea-bottom, and thence through the rocks, 

 downward towards the centre of the earth. Yet 

 with our present supply of internal heat, it is 

 not probable that any water can reach more than 

 one hundredth part of the distance towards the 

 earth's centre, without becoming vaporized and 

 thus getting driven back towards the surface. 

 In this way there is kept up a circulation of 

 water through the peripheral portions of the 

 earth's crust. But as the earth becomes cooler 

 and cooler, the water will be enabled to circulate 

 at greater and greater depths, thus materially 

 lowering the level of the ocean. In this way, 

 long before the centre has become cool, all the 

 surface-water of the earth will have been sucked 

 into the pores of the rocks, and a similar pro- 



1 In a paper on the unity of geological phenomena through- 

 out the solar system, translated by Professor Sterry Hunt, and 

 published in the American Journal of Science, January, 1862. 

 287 



