LIFE IN OTHER WORLDS 



The Moon weighs rather less than three and a half 

 times its bulk in water. This shows clearly that the 

 Moon is composed of material scraped off from the outer 

 surface of the earth, rather than of matter obtained from 

 a considerable depth. At the same time the layer of 

 material removed had an appreciable thickness. The 

 volume of the Moon is equivalent to a solid body whose 

 surface is equal to the area of all the earth's oceans, and 

 whose depth would be thirty-six miles. It seems probable, 

 therefore, that at the time when the Moon was torn off, 

 or shot off, from the earth, the parent body had a solid 

 crust averaging at least thirty-six miles in thickness, while 

 beneath this crust the temperature was so high that the 

 materials underneath were molten or liquid, and in other 

 places were only kept solid by the enormous pressure of 

 the material above them. When the Moon separated 

 from the earth three-quarters of this crust was carried 

 away. It has sometimes been supposed that the re- 

 mainder was torn into two parts, one of which formed the 

 great land area of the Eastern Hemisphere and the other 

 the great land area of which North and South America 

 are the relics in the Western Hemisphere. These two 

 great areas, at that time, floated on the semi-liquid 

 surface like two large ice-floes. But they were, of course, 

 a good deal heavier than ice, and the molten stuff on 

 which they floated was a good deal heavier than water. 

 Later on this liquid stuff cooled and hardened. But 

 its bottom was still a good deal lower than the surface 

 of the great areas of land which had " floated "on it ; 

 and therefore it formed great depressions all about and 



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