and its Physical Relations to the Glacial Epoch. 125 



snow and ice. While the ground remains covered with snow 

 and ice, as was shown at considerable length on a former occa- 

 sion*, dense fogs prevail, which cut off a great portion of the 

 sun's rays and thus lower the summer temperature. But even 

 supposing the sun's rays were to reach the earth with their full 

 intensity, they would, no doubt, melt the snow accumulated 

 during the long winter, but they would fail to raise the summer 

 temperature so long as the snow remained unmelted. In Green- 

 land, a country covered with snow and ice, the pitch has been seen 

 to melt on the side of a ship exposed to the direct rays of the sun, 

 while at the same time the surrounding air was far below the 

 freezing-point ; a thermometer exposed to the direct radiation 

 of the sun has been observed to stand above 100°, while the air 

 surrounding the instrument was actually 12° below the freezing- 

 point f. A similar experience has been recorded by travellers 

 on the snow-fields of the Alps J. 



These results, surprising as they no doubt appear, are what 

 we ought to expect under the circumstances. The diathermancy 

 of air has been well established by the researches of Professor 

 Tyndall on radiant heat. Perfectly dry air seems to be nearly 

 incapable of absorbing radiant heat. The entire radiation passes 

 through it almost without any sensible absorption. Consequently 

 the pitch on the side of the ship may be melted, or the bulb of the 

 thermometer raised to a high temperature by the direct rays of 

 the sun, while the surrounding air remains intensely cold. "A 

 joint of meat/' says Professor Tyndall, "might be roasted before 

 a fire, the air around the joint being cold as ice" §. The air is 

 cooled by contact with the snow-covered ground, but is not 

 heated by the radiation from the sun. 



When the air is humid and charged with aqueous vapour, a 

 similar cooling effect also takes place, but in a slightly different 

 way. Air charged with aqueous vapour is a good absorber of 

 radiant heat, but it can only absorb those rays which agree with 

 it in period. It so happens that rays from snow and ice are, of 

 all others, those which it absorbs best. The humid air will ab- 

 sorb the total radiation from the snow and ice, but it will allow 

 the greater part of, if not nearly all, the sun's rays to pass unab- 

 sorbed. But during the day, when the sun is shining, the ra- 

 diation from the snow and ice to the air is negative ; that is, 

 the snow and ice cool the air by radiation. The result is, the 

 air is cooled by radiation from the snow and ice (or rather, we 

 should say, to the snow and ice) more rapidly than it is heated by 



* Phil. Mag. for August 1864. 



t Scoresby's 'Arctic Regions,' vol. ii. p. 379. Daniell's 'Meteorology,' 

 vol. ii. p. 123. 



X Tyndall, ' On Heat,' article 364. § Ibid. 



