58 IN STARRY REALMS. 



that even if the moon be now entirely cold, it must once 

 have been heated. No one is likely to dispute this, but 

 yet the full consequences of such an admission are not 

 always readily perceived. For if the moon were once hot 

 it is plain that earlier still it must have been yet hotter, 

 and therefore the farther we look back into the depths of 

 time past, the hotter and hotter do we see that the moon 

 must have been. At last we discover an epoch, how 

 remote I do not pretend to say, when our neighbouring 

 globe, instead of being the quiescent object that we now 

 see must have contained many active volcanoes indicative 

 of an interior containing volumes of molten lava. Earlier 

 still we perceive the moon ever hotter and hotter, until 

 at last we seem to discern a time when it must have 

 consisted of a globe, partly, or even wholly, molten. This 

 seems an inevitable deduction from the fact that the moon 

 bears traces of having once possessed internal heat. Such 

 a view of the early condition of our satellite will explain 

 many of its characteristic features. At present, however 

 I am only concerned to account for the fact that the moon 

 now always turns the same face towards the earth. 



Every one is familiar with the fact that the moon raises 

 tides on the earth ; these tides ebb and flow along our 

 coasts, and in virtue of them the satellite exercises a cer- 

 tain control on the movements of our globe. If the moon 

 had liquid oceans on its surface there cannot be a doubt 

 that the attraction of the earth would generate tides in the 

 oceans on the moon just as the attraction of the moon gene- 

 rates tides in the oceans of the earth. But there would be 

 a fundamental difference between the two cases ; the shores 

 of the lunar seas would be periodically inundated by tides 



