380 COSMIO PHILOSOPHY. [ft. ii. 



mouths, are now clean gone, radiated off into space. This 

 cessation of volcanic activity indicates that the planet has 

 reached its limit of consolidation, and is no longer generating 

 heat from within. 1 Now the degree of cold implied by this 

 stoppage of further lunar consolidation must immeasurably 

 exceed anything within terrestrial experience. It may well 

 have been great enough to freeze all the lunar oceans, and 

 even to liquefy, or perhaps to solidify, the gases of the lunar 

 atmosphere. The moon is indeed subjected at each rotation 

 to the fierce noontide heat sent from the sun ; but however 

 this may scorch and blister the rocky surface, it can exercise 

 but little melting power. An atmosphere, as Mayer has 

 happily observed, is like a valve which lets water run through 



1 " Nevertheless, there are processes at work out yonder which must he as 

 active, one cannot hut believe, as any of those which affect our earth. In 

 each lunation, the moon's surface undergoes changes of temperature 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 days, a cold far exceeding the intensest ever produced in terres- 

 trial experiments must exist over the whole of the unilluminated hemisphere ; 

 and under the intluence of this cold all the substances composing the moon's 

 crust must shrink to their least dimensions — not all equally (in this we hud 

 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 all the substances which had 

 shrunk to their least dimensions must expand according to their various 

 degrees ; not greatly, indeed, so far as any small quantity of matter is 

 aflected, but to an important amount when large areas of the moon's surface 

 are considered. Remembering the effects which take place on our earth, in 

 the mere change from the frost of winter to the moderate warmth of early 

 spring, it is difficult to conceive that such remarkable contraction and expan- 

 sion can take place in a surface presumably less coherent than the relatively 

 inoist and plastic substances comprising the terrestrial crust, without gradually 

 effecting the demolition of the steeper lunar elevations. When we consider, 

 further, that these processes are repeated not year by year, but month by 

 month, and that all the circumstances attending them are calculated to 

 lender them most effective because so slow, steadfast, and uniform in their 

 progression, it certainlv does not seem wonderful that our telescopists should 

 from time to time recognize signs of change in the moon's lace." — Proctor 

 The Moon, pp. 380-382. 



