COXDITIOXS OF COXTIXEXTAL ICE. 217 



the temperature of the ice above 32°, but the heat lost 

 by radiation might lower the temperature to far more 

 than 32° below zero. If the heat received from the 

 sun's rays should keep the surface of the ice at, say, the 

 melting-point during the summer, and the heat lost by 

 radiation should keep the surface at, say, 50° below 

 the melting-point during winter, which is not an 

 extravagant supposition, the mean temperature of the 

 surface would then be 25° below the melting-point, or 

 7° F. But the mean temperature of the underlying 

 ice would not be so low ; for the low mean temperature 

 of the surface is almost wholly due to loss by radiation 

 into stellar space during winter, and this loss would be 

 chiefly confined to the surface. Had the surface been 

 rock instead of ice, the rise of temperature during 

 summer would have been about as great as the decrease 

 during winter, and consequently the mean temperature 

 would have been much higher than in the case of ice. 

 Hence, the difference between the mean temperature 

 of a rock surface and that of the rock below would 

 not be so great as in the case of ice. The tendency of 

 direct radiation, therefore, is to maintain the surface 

 of the ice-sheet at a lower temperature than that of 

 the underlying mass. 



(3.) This tendency is strengthened by another 

 circumstance which comes into operation. During 

 summer, a large portion of the direct heat from the 

 sun is spent in melting the surface ice. The melted 

 ice passes down through crevasses and openings in the 

 sheet, thus carrying the temperature along with it. 

 The heat of summer is by this means carried down 

 below the surface, but not so the cold of winter. 



The melting of the ice on the Antarctic continent 

 will be greatly retarded, however, by the coldness of 

 the air, the temperature of which, even during summer, 



