Solar Heat in the Upper Regions of the Atmosphere. 263 



much heat again from the sun as we actually do receive. And I 

 know too, by personal experience, as does Professor Tyndall, 

 how at great heights one feels sensations commonly attributed 

 to solar radiation ; and the conclusions in my last paper, which 

 seemed to contradict these and multitudes of other facts which 

 occur to everyone, surprised me and staggered mc, I confess. 

 But the truth is that the contradiction lies between, not the 

 facts, but the received interpretations of the facts; and when 

 this is the case, the interpretations must be reexamined; and the 

 process is one which leads to discoveries. 



Whatever hypothesis is offered, the fact remains, that at great 

 elevations the shaded and exposed bulb show very nearly the 

 same very low reading, so low that it may be presumed that they 

 would show the same reading exactly if they could be taken to a 

 sufficiently great height. Mr. Glaisher, I will repeat, reasserted 

 this, in the most explicit manner, as based on very numerous 

 and entirely concordant observations both of the black-bulb 

 thermometer and water-actinometer. 



The only explanation that I can suggest that is based on 

 known physical actions is the following. 



If the black-bulb thermometer does not indicate the direct 

 radiated heat, it may be that it loses it by instantaneous radia- 

 tion. It is contrary to our experience that this instantaneous 

 radiation is due to diminished pressure ; but it may be due to 

 the nearly total absence of aqueous vapour. It is the presence 

 of the atmosphere, and especially its aqueous vapour, that pro- 

 tects the earth from radiating away its heat rapidly by night. It 

 may be the presence of aqueous vapour round the bulb of the 

 exposed thermometer that prevents it also from radiating away 

 its heat by day, and that prevents our being chilled by radia- 

 tion, even in sunshine, down to the temperature indicated in the 

 shade, a temperature which, but for the aqueous vapour, would 

 be much lower than it is. Where, then, the exposed bulb is 

 protected from loss by the opake aqueous vapour, it will show 

 the heat it receives ; where it is unprotected, it will lose this 

 direct heat as fast as it receives it, and indicate the temperature 

 of the air. Should this be the case, it will be an interesting 

 result of Professor TyndalPs discoveries of the high radiating and 

 absorbing powers of aqueous vapour, that it protects the earth 

 and us from instantaneous radiation (that peculiar sensation we 

 feel at great heights and call burning) — that, in a certain sense, 

 if it does not create heat for us, but for it we should have no heat 

 to lose. 



It is quite unnecessary to point out the difficulties in accepting 

 this ; but to this we are driven if the observations are correct, 



T2 



