The History of Things 55 



foundly disturbed by solar tides, and it was per- 

 haps a particularly high tide which made the earth 

 give birth to the moon. This marked the first 

 critical period in the history of our planet. "At 

 the eventful time of parturition the earth was ro- 

 tating, with a period of from two to four hours, 

 about an axis inclined at some 11 or 12 to the 

 ecliptic. The time which has elapsed since the 

 moon occupied a position nine terrestrial radii 

 distant from the earth is at least fifty-six to fifty- 

 seven millions of years, but may have been much 

 more." 1 



The moon thus arose as a sort of moult of the 

 outer envelope of the hot earth. It was charged 

 with steam and other gases under a pressure of 

 5,000 pounds to the square inch, but as it receded 

 from the earth and the pressure continuously dimin- 

 ished it became "as explosive as a charged bomb, 

 and steam burst forth from numberless volcanoes." 

 The moon, in short, was only born to die. " While 

 the face of the moon might thus have acquired its 

 existing features, the ejected material might possi- 

 bly have been shot so far away from its origin as to 

 have acquired an independent orbit" 2 and some 

 of the meteorites which now descend upon the 

 earth may be returned portions of the early 



1 Prof. W. J. Sollas, Presidential Address, Section C, Brit- 

 ish Association, 1900. "Nature," September 13, p. 482. 

 'Prof. W. J. Sollas, loc. cit. 



