APPENDIX. yji 



at the time, and the length of the day at the place of observa- 

 tion. When the sun is below the horizon the air loses heat by- 

 radiation, and still more, in the strata near the surface, by 

 convection to surfaces cooled by radiation. 



It was until lately believed that the experiments of Herschel 

 and Pouillet had given an approximate measure of the absolute 

 intensity of solar radiation, and that the proportion absorbed 

 by the atmosphere at the sea-level at a vertical incidence might 

 be estimated at about one-fourth of the whole. It is not too 

 much to say that the recent researches of Mr. Langley, especially 

 those detailed in his Report of the Mount Whitney expeditio7i* 

 have completely revolutionized this department of physics. It 

 now appears that the true value of the solar constant is not 

 much less than twice as great as the previous estimate, and that 

 rather more than one-third is absorbed by the atmosphere 

 before reaching the sea-level. Mr. Langley has further proved 

 that the absorptive action of the atmosphere varies with the 

 wave-length of the rays, and that, omitting the " cold bands " 

 which correspond to the dark bands in the visible spectrum, it 

 diminishes as the wave-length increases. It further appears 

 highly probable that the larger part of the absorptive action of 

 the atmosphere is due to the aqueous vapour, the carbonic-acid 

 gas, and the minute floating particles of solid matter, which are 

 present in variable proportions. Allowing for the probable 

 extension of our knowledge by further research, it is yet evident 

 that, even if we had not to take into account the further elements 

 of the problem next to be specified, the distribution of heat in 

 the atmosphere, as dependent on solar radiation, is a question of 

 extreme complexity. 



The action of winds has an important effect in modifying the 

 temperature of the air. It is not possible to draw a distinct line 

 between the great air-currents, which affect large areas, and 

 shght breezes, depending on local causes, and limited to the 

 lower strata of the atmosphere ; but in relation to the present 

 subject it is necessary to distinguish between them. There is a 

 general circulation in the aerial envelope covering the earth, 

 caused by unequal heating of different parts of the surface. 



* Published by the War Department, United States Army, Pro- 

 fessional Papers of the Signal Service, No. xv. 



