i$73'] Condition of the Moon's Surface. 31 



tion, in this respect — that her mean period of rotation on 

 her axis is exactly equal to her mean period of revolution. 

 (Here either sidereal rotation and revolution or synodical 

 rotation and revolution may be understood, so long as both 

 revolution and rotation are understood to be of the same 

 kind). I say "mean period of rotation," for although as a 

 matter of fact it is only the revolution which is subject to 

 any considerable variation, the rotation also is not perfectly 

 uniform. We know, furthermore, that if there had been, 

 long ago, a near agreement between the mean rotation and 

 revolution, the present exact agreement would have resulted, 

 through the effects of the mutual attractions of the earth 

 and moon. But, so far as I know, astronomers have not yet 

 carefully considered the question whether that close agree- 

 ment existed from the beginning, or was the result of other 

 forms of action than are at present at work. If it existed 

 from the beginning, that is from the moon's first existence 

 as a body independent of the earth, it is a matter requiring 

 to be explained, as it implies a peculiar relation between the 

 moon and earth before the present state of things existed. 

 If, on the contrary, it has been brought about by the amount 

 of action which is now gradually reducing the earth's rota- 

 tion period, we have first of all to consider that an enormous 

 period of time has been required to bring the moon to her 

 present condition in this respect, and, moreover, that either 

 an ocean existed on her surface or that her crust was once 

 in so plastic a condition as to be traversed by a tidal wave 

 resembling, in some respects, the tidal wave in our own 

 ocean. This, at any rate, is what we must believe if we 

 suppose, first, that the main cause of the lengthening of the 

 terrestrial day is the action of the tidal wave as a sort of 

 brake on the earth's rotating globe, and, secondly, that a 

 similar cause produced the lengthening of the moon's day 

 to its present enormous duration. It may be, as we shall 

 presently see, that other causes have to be taken into account 

 in the moon's case. 



Now we are thus, either way, brought to a consideration 

 of that distant epoch when — according to the nebular 

 theory, or any admissible modification thereof — the moon 

 was as yet non-existent as an orb distinct from the earth. 

 We must suppose, on one theory, that the moon was at that 

 time enveloped in the nebulous rotating spheroid out of 

 which the earth was to be formed, she herself (the moon) 

 being a nebulous sub-spheroid within the other, and so far 

 coerced by the motion of the other that her longer axis 

 partook in its motion of rotation. Unquestionably in that 



