32 Condition of the Moon's Surface. [January, 



case, as the terrestrial spheroid contracted and left the other 

 as a separate body, this other, or lunar spheroid, would 

 exhibit the kind of rotation which the moon actually pos- 

 sesses. On the other theory, we should be led to suppose 

 that primarily the lunar spheroid rotated independently of 

 its revolution ; but that the earth's attraction acting on the 

 outer shells, after they had become first fluid and then 

 (probably) viscous, produced waves travelling in the same 

 direction as the rotation, but with a continual brake-action, 

 tending slowly to reduce the rotation until it had its present 

 value, when dynamical equilibrium would be secured. 



But, as I have said, in either case we must trace back the 

 moon's history to an epoch when she was in a state of 

 intense heat. And it seems to me that we are thus led to 

 notice that the development of the present state of things 

 in the moon must have taken place during an era in the 

 history of the solar system differing essentially from that 

 which prevailed during the later and better-known geological 

 eras of our own earth. Our moon was shaped, so to speak, 

 when the solar system itself was young, when the sun may 

 have given out a much greater degree of heat than at 

 present, when Saturn and Jupiter were brilliant suns, when 

 even our earth and her fellow minor planets within the zone 

 of asteroids were probably in a sun-like condition. Putting 

 aside all hypothesis, it nevertheless remains clear that, to 

 understand the moon's present condition, we must form 

 some estimate of the probable condition of the solar system 

 in distant eras of its existence ; for it was in such eras, and 

 not in an era like the present, that she was modelled to her 

 present figure. 



It appears to me that we are thus, to some extent, freed 

 from a consideration which has proved a difficulty to many 

 who have theorised respecting the moon. It has been said 

 that the evidence of volcanic action implies the existence, 

 at least when that action was in progress, of an atmosphere 

 capable of supporting combustion, — in other words, an 

 atmosphere containing oxygen, for other forms of combustion 

 than those in which oxygen plays a part may here be dis- 

 missed from consideration. But the fiery heat of the moon's 

 substance may have been maintained (in the distant eras to 

 which we are now referring the formation of her crust) 

 without combustion. Taking the nebular hypothesis as it 

 is commonly presented, the moon's globe may have remained 

 amid the intensely hot nebulous spheroid (which was one 

 day to contract, and so form the globe of the earth) until 

 the nebula left it to cool thenceforth rapidly to its present 



