THE CONSTANT FACE OF THE MOON. 57 



a case it is plain that the movement has not been at all 

 like that of the moon, the characteristic feature of which is 

 that the same face is always directed towards the earth. 



It will not be a sufficient explanation to say that if the 

 moon rotated on an axis at all it must rotate in some 

 definite period, and if so, why should not that period be 

 identical with the revolution of the moon round the earth ? 

 Would it not be, it might be urged, as likely to be that period 

 as any other ? This will not satisfy the student of nature ; 

 the coincidence between these periods is so perfect that the 

 chances are many millions to one against its occurrence if 

 there were no physical reason for its existence. It is now 

 well known that there is such a reason, and as it affords an 

 instructive illustration of the mutual relations between the 

 earth and her satellite, I propose to enter into the matter 

 with some little detail. 



I must ask you to think of a very ancient epoch ; how 

 ancient it is to be I cannot express by the use of ordinary 

 chronology ; it must have been hundreds of thousands of 

 years ago, probably many millions of years ago. At this 

 period, however, the moon was not like the object that we 

 know so well. Her surface now seems to be composed 

 of rocks or stone, or at all events of materials which are 

 hard and solid. But this was not always so. There are 

 abundant traces of the remains of past volcanoes on the 

 moon ; their craters are no longer active, but some of 

 them once had dimensions and energy transcending any of 

 the volcanoes on the earth that are now known to us. It 

 is thus certain that at some former time the moon must 

 have possessed stores of internal heat by which these 

 volcanic eruptions were produced. It therefore follows 



