IS THE MO OX DEAD? 



277 



ly to coerce its motion of rotation into exact 

 agreement with its motion of revolution. It is 

 known that this would necessarily happen if the 

 original approach to agreement between these 

 motions had been sufficiently close. If we 

 adopted this view, we should find ourselves in 

 presence of the somewhat remarkable fact that 

 the small moon was in the beginning set rotating 

 so slowly that its day lasted as long as a lunar 

 month. Such a rotation, as the result of some 

 process of systematic evolution, could be readily 

 accepted ; but that this motion, which presents 

 no recognizable advantages, and many most man- 

 ifest inconveniences (for creatures living in the 

 moon), should have been specially communicated 

 to the moon by the creative hand, would not be 

 an acceptable theory, even if we were not forced 

 by overwhelming evidence to throw special crea- 

 tive acts very much farther back (to say the 

 least) than the formation of our moon, or of any 

 part of the solar system. 



Another explanation which has been offered 

 runs as follows : When the moon had oceans, 

 the earth must have acted on those oceans in the 

 same way as the moon now acts on the oceans 

 of our earth. In one respect the earth must 

 have acted more energetically, in another less. 

 Being very much (eighty-one times) more massive 

 than the moon, the earth necessarily exerts much 

 more force on the moon's substance than the 

 moon exerts on hers. 1 On the other hand, the 

 relative difference between the pull on the nearest 

 and remotest parts of the globe is less in the case 

 of the earth drawing the waters of the moon (in 

 old times) than in the case of the moon drawing 

 the waters of the earth ; for the moon is a much 

 smaller globe than the earth ; and this difference 

 is the really effective force in the production of 

 tides. Also it is probable that the moon never 

 had a relatively large ocean-surface, as will pres- 

 ently be shown, and small seas (probably discon- 

 nected) could not be swept by a great tide-wave. 

 Still we may suppose that there was once a tidal- 

 wave, greater or less, sweeping athwart the lunar 

 seas much in the manner of our own tidal-wave. 

 Now, our tidal-wave is beyond doubt slowly 



1 In one sense the moon pulls the earth just as strong- 

 ly as the earth pulls the moon, for gravity is not a force 

 which one body exerts on another solely, but a mutual 

 force. But what mathematicians call the moving- force 

 exerted by the earth on the moon is eighty-one times 

 greater than the corresponding- force exerted by the moon 

 on the earth ; for the mutual attraction between these 

 bodies has in the former case to move the moon, whereas 

 in the latter it has to move the much larger mass of the 

 earth. 



checking the earth's motion of rotation, for the 

 wave travels so as to meet the motion of rotation, 

 which therefore to some slight degree it opposes. 

 This will go on, until at length the rotation has 

 been so reduced that the tidal-wave no longer 

 affects it ; or, in other words, until the earth's 

 period of rotation corresponds with the period 

 of the tidal-wave, viz., with the lunar month. 

 Hundreds of millions of years will pass before 

 that happens ; but then we have seen that the 

 moon may fairly be regarded as illustrating the 

 earth's condition hundreds of millions of years 

 hence. Accordingly, there is nothing absolutely 

 incredible in the theory that during the remote 

 ages when the moon had seas the tidal-wave 

 which traversed them, continually retarding the 

 moon's motion of rotation, gradually coerced it 

 into absolute agreement with her motion of rev- 

 olution around the earth. Still it must be ad- 

 mitted that the theory is not very easily to be 

 accepted as it stands. The seas of the moon were 

 probably less in relative, extent, even when at 

 their largest, than those of Mars now are, and 

 such seas could have no tidal-waves which even 

 in thousands of millions of years could reduce 

 the moon's rate of rotation in any considerable 

 degree ; and, as we shall presently see, the dura- 

 tion of the era when the moon had seas can 

 hardly have been measured by periods so vast. 

 On the whole, while we may admit the probability 

 that at some very distant time in the past the 

 earth may have exerted influences on lunar seas 

 resembling those which the moon now exerts on 

 our seas, it does not appear to us probable that 

 the peculiar feature we are now considering can 

 be attributed either wholly or in very large de- 

 gree to the retarding influence of tidal-waves 

 upon the moon. 



One other theory remains which seems to 

 have more in its favor than either of those 

 hitherto considered. Before the moon became a 

 separate planet her frame, then vaporous, must 

 have been enwrapped in the vaporous frame of 

 the earth. While this continued the moon was 

 necessarily compelled to move as a portion of the 

 earth's outer envelope, and therefore, of course, 

 turned upon her axis in the same time that that 

 exterior portion of the earth revolved. So soon 

 as the contraction of the earth's vaporous frame 

 left the moon outside, she was free, if she could, 

 to change her rate of rotation ; that is to say, the 

 earth's enwrapping vapor-masses no longer pre- 

 vented the moon from changing her rotation 

 rate. And there were two causes at work, either 

 of which, if in action alone, would have markedly 



