of the Glacial Epoch. 339 



in the glacial drift found a congenial temperature. The com- 

 mon notion, therefore, that the glacial epoch was a cold period 

 is correct, although heat, and not cold, was the cause of that 

 epoch. This apparent paradox, that heat should be the cause of 

 cold, finds its parallel in the ice-making machines which were 

 in operation at the last Great Exhibition. In those machines, 

 which produced from 2 to 12 tons of ice per ton of coal, the 

 glacial produce was directly proportional to the amount of heat 

 developed by the combustion of coal. 



Such are the principal objections to my hypothesis which 

 have occurred to myself or been suggested by others; and 

 although I have thus endeavoured to remove them, yet it would 

 be idle to deny that some of them retain considerable force, or 

 that others, perhaps even more formidable, may occur to the 

 minds of those who possess a far more extensive knowledge of 

 the phenomena of the glacial epoch than I can pretend to. No 

 hypothesis can be considered as of any value until it has been 

 thoroughly sifted, and by the result of this necessary process the 

 one I have now ventured to suggest must either stand or fall. 

 Meanwhile it commends itself by requiring the assumption of no 

 natural convulsion or catastrophe, no vast or sudden upheavals or 

 depressions, and no change in the thermal relations of our earth 

 to the sun or to space. On the contrary, it insists that the gla- 

 cial epoch was normally and gradually evolved from a thermal 

 condition of the interior of our globe which can scarcely be said 

 to be any longer the subject of controversy. 



In conclusion, this hypothesis suggests the probability that 

 the other bodies belonging to our solar system have either already 

 passed through a similar epoch or are destined still to encounter 

 it. With the exception of the polar ice of Mars and the bright 

 clouds of Jupiter, we have hitherto obtained no certain glimpse 

 into the thermal or meteorological condition of the planets : 

 neither is the physical state of their surfaces accessible to our 

 best telescopes. It is otherwise, however, with the moon, whose 

 distance is not too great to prevent the visibility of comparatively 

 minute details. A careful observation of the lunar surface for 

 more than a year with a silvered-glass reflector of 7 inches' aper- 

 ture and of good defining power, has created in my mind an 

 impression that our satellite has, like its primary, also passed 

 through a glacial epoch, and that several at least of the valleys, 

 rills, and streaks of the lunar surface are not improbably due to 

 former glacial action. Notwithstanding the excellent definition 

 of modern telescopes, it cannot be expected that other than the 

 most gigantic of the characteristic details of an ancient glacier- 

 bed should be rendered visible. Under favourable circumstances 

 the terminal moraine of a glacier attains to enormous dimensions ; 



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