106 SCIENCE IX SHORT CHAPTERS. 



least afford material support to the otherwise-indicated infer- 

 ence, that the materials of the Moon's crust are similar to 

 those of the Earth's, and that they have been cooled from a 

 state of fusion. 



I think I jnay safely generalize to the extent of saying, that 

 no considerable mass of fused earthy silicates can cool 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 surfaces (that were 

 freely exposed when cooling) corrugated with radiating mini- 

 ature lava streams, that have flowed from one or more craters 

 or openings that have been formed in the manner above de- 

 scribed. The third assumption will, I think, be at once ad- 

 mitted, 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 corresponding 

 irregularities, and generally, the magnitude of mountains of 

 solidified planets and satellites should be on a scale proportion- 

 ate 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 customary measurements of 

 terrestrial mountain-heights from the sea-level. As those por- 

 tions 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 measurements, 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 correct 

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

 Earth's, or 4J- inches of mercury, there can be no liquid water 

 on that planet, excepting perhaps over a small amount of cir- 

 cunipolar area, and during the extremes of its aphelion winter. 

 Thus the irregularities of the terminator, indicating mountain 

 elevations calculated to reach to -- of the diameter of the 



