54 SCIENCE IN SHORT CHAPTERS. 



down under circumstances of free radiation without first 

 forming a heated solid crust, which, by further radiation, 

 cooling, and contraction, will assume a surface configuration 

 resembling more or less closely that of the Moon. Evidence 

 of this is afforded by a survey of the spoil-banks of blast 

 furnaces, where thousands of blocks of cinder are heaped 

 together, all of which will be found to have their upper sur- 

 faces (that were freely exposed when cooling) corrugated 

 with radiating miniature lava streams, that have flowed 

 from one or more craters or openings that have been 

 formed in the manner above described. 



The third assumption will, I think, be at once admitted, 

 inasmuch as it is but the expression of a physical necessity. 



According to this, the Earth, if it has cooled as the 

 Moon is supposed to have done, should have displayed cor- 

 responding irregularities, and generally, the magnitude of 

 mountains of solidified planets and satellites should be on 

 a scale proportionate to their whole mass. In comparing 

 the mountains of the Moon and Mercury with those of the 

 Earth, a large error is commonly made by taking the cus- 

 tomary measurements of terrestrial mountain-heights from 

 the sea-level. As those portions of the Earth which rise 

 above the waters are but its upper mountain slopes, and the 

 ocean bottom forms its lower plains and valleys, we must 

 add the greatest ocean depths to our customary measure- 

 ments, in order to state the full height of what remains of 

 the original mountains of the Earth. As all the stratified 

 rocks have been formed by the wearing down of the original 

 upper slopes and summits, we cannot expect to be able to 

 recognize the original skeleton form of our water- washed 

 globe. 



If my calculation of the atmosphere of Mercury is cor- 

 rect, viz., that its pressure is equal to about one seventh of 

 the Earth's, or 4^ inches of mercury, there can be no liquid 

 water on that planet, excepting perhaps over a small amount 

 of circumpolar area, and during the extremes of its aphelion 

 winter. Thus the irregularities of the terminator, indicat- 

 ing mountain elevations calculated to reach to -^^ of the 

 diameter of the planet, are quite in accordance with the 

 above-stated theoretical consideration. 



