of the Glacial Epoch. 327 



ice-bearer, which last occupies in fact the position of the receiver 

 in ordinary distillation ; and hence so long as the temperature of 

 the ice- bearers, inclusive of the surrounding atmosphere, does 

 not rise above 0° C, their functions remain intact. Other things 

 equal, an ice-bearer at 0° C. is scarcely surpassed in efficiency 

 by one at — 15° C. But it must be borne in mind that the actual 

 efficiency of an ice-bearer, on an annual average, is dependent 

 upon the length of time during which its temperature does not 

 rise above the freezing-point. Hence it is that those mountains 

 which penetrate furthest into the increasingly cold heights of the 

 atmosphere are most efficient, — not because their temperature is 

 sometimes far below the freezing-point, but because they main- 

 tain a temperature below that point throughout a greater por- 

 tion of the year. 



These considerations lead to the conclusion that, assuming the 

 supply of aqueous vapour to the atmosphere to remain con- 

 stant, increased condensation could only arise from greater facili- 

 ties for the radiation from that aqueous vapour into space, — a 

 condition which involves cosmical changes of which we have not 

 the slightest evidence. On the other hand, as the powers of the 

 radiating condenser are at the present moment far from being 

 taxed to the utmost, the more copious supply of aqueous vapour 

 to the atmosphere would at once produce a corresponding in- 

 crease of condensation. Such an increased supply of aqueous 

 vapour, extending over a considerable period of time, could only 

 arise from the association of a greater amount of heat with the 

 waters of the ocean. But all the hypotheses hitherto propounded 

 to account for the glacial epoch have failed to recognize this fea- 

 ture of the problem, inasmuch as they have all assumed that 

 cold alone was necessary to the development of the phenomena 

 of that epoch. This important omission must serve as my apo- 

 logy for advancing a new hypothesis, which necessarily rests in 

 some respects upon data as yet imperfectly ascertained, and 

 which has gradually elaborated itself out of the impressions I 

 received during my recent visit to Norway. Any such theory 

 must take cognizance of the following points in the history of 

 the glacial epoch. 1st. That its effects were felt over the entire 

 globe. 2nd. That it occurred, or at least terminated at a geolo- 

 gically recent period. 3rd. That it was preceded by a period of 

 indefinite duration, in which glacial action was either altogether 

 wanting, or was at least confined to regions of considerable alti- 

 tude. 4th. That during its continuance, atmospheric precipita- 

 tion was much greater, and at one period the height of the snow- 

 line considerably less than at present. 5th. That it was followed 

 by a period extending to the present time, when glacial action 

 became again insignificant. 



