380 CONDITION OF THE MOON ’S SURFACE.

telescopic means. It is not difficult to show reasonsat least for believing that the face of the moon mustbe changing more rapidly than that of our earth. Onthe earth, indeed, we have active subterranean forceswhich may, perhaps, be wanting in the moon. On theearth again, we have a sea acting constantly upon theshore,—here removing great masses, there using thedebris to beat down other parts of the coast, and bythe mere effect of accumulated land-spoils acquiringpower for fresh inroads. We have, moreover, windand rain, river action, and glacier action, and, lastly,the work of living creatures by land and by sea, whilemost of these causes of change may be regarded asprobably, and some as certainly, wanting in the caseof our satellite. Nevertheless, there are processes atwork out yonder which must be as active, one cannotbut believe, as any of those which affect our earth.In each lunation, the moon’s surface undergoes changesof temperature which should suffice to disintegratelarge portions of her surface, and with time to crumbleher loftiest mountains into shapeless heaps. * In thelong lunar night of fourteen days, a cold far exceed-ing the intensest ever produced in terrestrial ex-periments must exist over the whole of the unillu-minated hemisphere; and under the influence of thiscold all the substances composing the moon’s crustmust shrink to their least dimensions—not all equally(in this we find a circumstance increasing the energy

* Nasmyth pointed this out long since.