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



it appears to me, to determine whether the moon's surface 

 is still undergoing changes of configuration. I cannot but 

 think that such an inquiry would be made under more 

 promising circumstances than those imagine who consider 

 that the moon's surface has reached its ultimate condition, 

 and that therefore the search for signs of change is a hope- 

 less one. So far am I from considering it unlikely that the 

 moon's surface is still undergoing change, that, on the con- 

 trary, it appears to me certain that the face of the moon 

 must be undergoing changes of a somewhat remarkable 

 nature, though not producing any results which are readily 

 discerned by our imperfect telescopic means. It is not 

 difficult to show reasons at least for believing that the face 

 of the moon must be changing more rapidly than that of 

 our earth. On the earth, indeed, we have active sub- 

 terranean forces which may, perhaps, be wanting in the moon. 

 On the earth, again, we have a sea acting constantly upon 

 the shore — here removing great masses, there using the debris 

 to beat down other parts of the coast, and by the mere 

 effect of accumulated land-spoils acquiring power for fresh 

 inroads. We have, moreover, wind and rain, river action 

 and glacier action, and, lastly, the work of living creatures 

 by land and by sea ; while most of these causes of change 

 may be regarded as probably, and some as certainly, wanting 

 in the case of our satellite. Nevertheless there are processes 

 at work out yonder which must be as active, one cannot but 

 believe, as any of those which affect our earth. In each 

 lunation the moon's surface undergoes changes of tem- 

 perature which should suffice to disintegrate large portions 

 of her surface, and with time to crumble her loftiest 

 mountains into shapeless heaps. In the long lunar night 

 of fourteen hours a cold far exceeding the intensest ever 

 produced in terrestrial experiments must exist over the 

 whole of the unilluminated hemisphere ; and under the in- 

 fluence of this cold all the substances composing the moon's 

 crust must shrink to their least dimensions, — not all equally 

 (in this we find a circumstance increasing the energy of 

 the disintegrating forces), but each according to the quality 

 which our physicists denominate the coefficient of expansion. 

 Then comes on the long lunar day, at first dissipating the 

 intense cold, then gradually raising the substance of the 

 lunar crust to a higher and higher degree of heat until (if 

 the inferences of our most skilful physicists and the evidence 

 obtained from our most powerful means of experiment can 

 be trusted) the surface of the moon burns (one may almost 

 say) with a heat of some 500° F. Under this tremendous heat 



