436 



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



[Mar. 23,. 



vapour, and which, with the law of vertical distribution denned by 

 the simultaneous observations at Mussooree and Dehra, would each 

 produce a pressure equal to one inch of mercury. 



From the observations of the 12th November, 1879, given at page 

 218 of my previous paper, we get the following equations : — 



Mussooree. . . . 23 533 x 2-304 log* + '221 x 2*304 log p= -'09463, 

 Dehra 27*832 x 2"292 log » + '375 x 2'292 log (3= -14233 y 



and from the observations of the 14th we get : — 



Mussooree. .. 23529 x 2 339 log « +-183x2-339 log 0= — -102293, 

 Dehra 27-825 x 2-330 log * + -302 x 2*330 log £=-'151972. 



These give the following results : — 



Date. a. (3. 1 — <*. 1— /3. 



12th -99856 '76030 '00144 '23970 



14th -99855 '69536 '00145 '30464 



The absorption due to dry air of one inch pressure seems therefore to 

 be almost invariable and equal to only '1445 per cent, of the total 

 radiation — a quantity that could hardly be measured in laboratory 

 experiments ; while that due to water vapour of equal pressure varies 

 between 24 and 30 per cent, (perhaps between much wider limits) and 

 is equal, on the mean of the two days' observations, to 27*217 per cent. 

 In an atmosphere of dry air at 30 inches pressure the absorption would 

 be 1 — a 30 , or about 4^ per cent, of the total radiation. The fraction of 

 the total heat that is absorbed by dry air, though in most cases very 

 small in comparison with that absorbed by the vapour, seems therefore 

 to be an appreciable quantity, and, if the tension of vapour at sea- 

 level were only j- of an inch or less, the dry air would have the greater 

 effect of the two. 



Since the quantities of air (Q) and vapour (Q x ) in a vertical column 



of given sectional area are in the ratio Q=-X — X^-, where C and 



Qi / a \ c i 



C a are the constants of the logarithmic formulae for vertical distribu- 

 tion, the absorptive powers for equal masses of the two gases will be 



in the ratio 5 x 26106 __ 1 — "Water vapour has, therefore, 



27-217 8 66218 764-4 r 

 764*4 times the absorptive power possessed by air for the kind of radia- 

 tion emitted from the sun on the 12th and 14th November, 1879. As a 

 very large proportion of the total solar radiation is luminous, and since 

 water, both in the liquid and in the gaseous state, transmits the 

 greater part of the luminous radiation, the relative diathermancy of 

 dry air for radiant energy proceeding from sources at a lower tempera- 

 ture must be very much greater still. 



