1882.] 



Mr. S. A. Hill. On Radiant Heat, 



435 



March 23, 1882. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table and thanks ordered for 

 them. 



The following Papers were read : — 



I. " On the Constituent of the Atmosphere that Absorbs Radiant 

 Heat. II." By S. A. Hill, Meteorological Reporter, North- 

 West Provinces and Oudh. Communicated by General 

 Strachey, R.E., C.S.I., F.R.S. Received March 10, 1882. 



General Strachey has suggested to me that with the data given in 

 my first paper we may arrive at a numerical relation between the 

 absorptive powers of dry air and aqueous vapour, instead of being 

 content with merely showing that vapour is by far the most active 

 constituent of the atmosphere in this respect. The following calcula- 

 tions, worked out on the lines suggested by him, indicate that the dry 

 air has a small and, as far as we can judge, invariable effect in the way 

 of absorption, while the effect of water vapour is large and variable. 

 In other words, the air probably exercises a feeble absorption over the 

 whole range of the spectrum, while the absorption due to water vapour 

 is selective, and probably varies in amount with the nature of the 

 radiation from day to day. 



Starting with Pouillet's formula r=Hxp e , where e stands for the 

 atmospheric thickness and p for the fraction of the total radiation that 

 would penetrate vertically through an atmosphere of unit thickness, 

 we may take p to be made up of two factors, a. and /3, one of which 

 represents the diathermancy of the diy air and the other that of 

 vapour. The masses of dry air and water vapour traversed by the 

 rays will be respectively proportional, though not in the same ratio, to 

 the barometric pressure* and the vapour tension. Taking the length 

 of an oblique ray through any atmospheric stratum to be proportional 

 to sec z, we thus arrive at the formula 



log r=log R + b sec z log x+fsec z log {3, 



in which <% and /3 stand for the fractions of the total heat transmitted 

 through atmospheres composed respectively of dry air and aqueous 



* That is to say, approximately proportional to the total pressure. It will not 

 do to deduct the vapour tension from the total pressure, because the vapour thins 

 out as we ascend much more rapidly than the air does. 



