REPLY TO CRITICS. 25 



possessed of a given temperature, say — 230° F. It 

 simply means the temperature to which a body would 

 fall were it exposed to no other source of heat than 

 that of radiation from the stars. By the temperature 

 of the upper regions I mean the temperature to which 

 air in those regions sinks in consequence of loss from 

 radiation into space. It is mainly to this cause, and 

 not to the loss from expansion, as Professor Newcomb 

 assumes, that the intense cold of the upper air is due. 

 The air in that region has got beyond the screen which 

 protected it when at the earth's surface, and it then 

 throws off its heat into space during twelve hours of 

 night, getting no return from without except from the 

 radiation of the stars. And even at noonday, as I have 

 endeavoured to show in Appendix to ' Climate and 

 Time,' p. 551, the rays of a burning sun overhead 

 would not be sufficient to raise the temperature of the 

 air up to the freezing-point. But the recent observa- 

 tions of Professor Langley prove that the loss of heat 

 from radiation is in reality far greater than I had 

 anticipated. He says : — " The original observations, 

 which will be given at length, lead to the conclusion 

 that in the absence of an atmosphere the earth's tem- 

 perature of insolation would at any rate fall below 

 — 50° F. ; by which it is meant that, for instance, mercury 

 would remain a solid under the vertical rays of a 

 tropical sun were radiation into space wholly unchecked, 

 or even if, the atmosphere existing, it let radiations of 

 all wave-lengths pass out as easily as they come in" 

 (" Nature," August 3rd, 1882). 



The temperature of the upper atmosphere, even 

 after making allowance for heat received from below, 

 must in this case be at least 80° below the freezing- 

 point. The quantity of heat lost by expansion must 

 therefore be trifling compared with that lost by radi- 



