Physical Theory of Secular Changes of Climate. 183 



direct radiation to lower the temperature of all surrounding 

 bodies to 32°. 



In Greenland, 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*. A similar experience has been recorded by tra- 

 vellers on the snow-fields of the Alpst. 



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 

 iucapable 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 "J. 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 the 

 sun ; and, as a consequence, in a country like Greenland, covered 

 with an icy mantle, the temperature of the air, even during sum- 

 mer, seldom rises above the freezing-point. 



Were it not for the ice, the summers of North Greenland, 

 owing to the continuance of the sun above the horizon, would be 

 as warm as those of England; but, instead of this, the Greenland 



* Scoresby's ' Arctic Regions,' vol. ii. p. 379. Darnell's ' Meteorology,' 

 vol. ii. p. 123. 



t Tyndall, ' On Heat,' article 364. 

 t Ibid. 



