1881.] the Atmosphere which absorbs Radiant Heat. 



221 



vapour pressure observed at Simla, but totally different from the 

 variation of the barometric pressure ; while the absorption curve 

 agrees closely with that of the humidity of the upper atmosphere as 

 indicated by cloud, even in minor points like the decrease at 10 a.m. 



Also, since the hourly observations at Roorkee show that the pressure 

 of vapour there is less in the afternoon than in the morning, it is 

 probable that the variations, both above and below, are caused by an 

 upward movement of the vapour during the day. Such a movement 

 would not affect the absorption of heat by the whole depth of the 

 atmosphere, if water vapour be the chief absorbent substance, and we 

 find that at Dehra the absorption in the afternoon is little, if at all, 

 greater than in the morning. The evidence of the diurnal variation, 

 therefore, points strongly to the conclusion that water vapour is the 

 chief absorbent. 



The question may be answered more directly by the second method, 

 the determination of the law of vertical distribution of the absorbing 

 matter. Since both stations lie above the dust-haze so common over 

 India in the cold weather, and since the days of observation were free 

 from, cloud, we may assume that the absorbent substance we have to 

 deal with is a gas distributed according to the barometric formula; 

 and therefore the observations at two places suffice to determine the 

 law approximately. We have, then, for each day, log K M = 

 Ah 



og K D — -Q-' where A/z is 4,708 feet and C is a constant for the day 

 Computing C in this way for each day, and also C 1 and C 2 for the 



