LIFE IN OTHER WORLDS 



There remains the question of the probability of life 

 on the Moon or on Mars ; and the question of possible 

 life on the Moon is specially worth consideration, because 

 the earth's satellite was once part of the earth's mass. 

 We may first repeat briefly the explanation which 

 Sir G. H. Darwin has given of the Moon's separation 

 from the earth. If a flexible hoop or ring is spun very 

 rapidly it will be seen to flatten itself at top and bottom 

 (or, as we might say, at its poles) and broaden itself out 

 at its middle or equator. The semi-liquid earth once 

 rotated so rapidly that its swelling equatorial belt was 

 almost at the point of separation from the parent body. 

 Before this occurred, however, the tension was so great 

 that one large portion of the protuberance, where it was 

 weakest, broke away, and began to move around the 

 earth at a considerable distance from it. There are 

 several estimates of the bulk of the earth thus shot 

 off; but we may assume that about one-fiftieth of the 

 earth escaped thus. It must have consisted of a consider- 

 able portion of the earth's solid crust, and a much larger 

 quantity of the molten rocks of the earth's interior. 



The Moon is much lighter than the earth. The earth 

 taken as a whole weighs about five and a half times as 

 much as water. If we consider its surface alone, this 

 weighs rather more than two and a half times the weight 

 of water from which it can be seen that the interior of 

 the earth is very much denser than the earth's surface crust. 1 



1 The figures are : earth's specific gravity =5 '6. The specific gravity 

 of the surface material ranges from, in general, between 2*2 and 3*2, 

 with an average of 2 -7. The specific gravity of the Moon is 3*4.. 



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