220 



Mr. S. A. Hill. On the Constituent of [Dec. 22 y 



4 p.m. at all elevations where observations have been made in the 

 Himalaya, as it does on the plains. In any case, the fraction of the- 

 total atmosphere which accumulates above Mussooree during the day 

 hours must be a very small one, and its influence in increasing the 

 absorptive power of the upper atmosphere may be safely neglected. 



With water vapour the case is different. This diminishes so rapidly 

 as we ascend* that, if it be the chief absorbent substance, a small 

 variation in the quantity of it lying above the higher station will con- 

 siderably affect the absorbing power there observed. In the following 

 table the value of K M for each hour of observation, computed from the 

 mean of the three series, is compared with the mean hourly values of the 

 barometric pressure and vapour tension for November at Simla, a neigh- 

 bouring station of nearly the same altitude, and with the vapour tension 

 and cloud proportion observed at Roorkee, a station 40 miles S.S W~ 

 from Mussooree and 887 feet above the sea. General Boileau's ob- 

 servations at Simla in 1843-5, from which the figures for that station 

 have been taken, were made inside a house, and do not strictly repre- 

 sent the variations in the humidity of the external air. The normal 

 variation of humidity in the upper air may, however, be inferred with 

 some approximation to the truth from the variations of cloud ; for 

 though the days of observation were without cloud, like most days in 

 November in North India, there is no reason to suppose that the 

 diurnal movements of the vapour were different in direction on those- 

 days from what they are when the humidity is so high that clouds- 

 are formed. 



Table IV. 





Absorption 



Barometric 



Vapour 



Cloud 



Vapour 



Hour. 



coefficient. 



pressure. 



tension. 



proportion. 



tension. 



Mussooree. 



Simla. 



Simla. 



Roorkee. 



Roorkee. 



8 A.M 



•03862 



23 •323 // 



•166" 



0-99 



•346" . 



9 „ .... 



•03966 



23 349 



•172 



0-90 



•367 



10 „ .... 



•03908 



23 360 



•179 



0-80 



•373 



11 „ .... 

 Noon .... 



•03969 



23 353 



•186 



0-81 



•372 



•04051 



23-333 



•194 



0-90 



•362 



1 P.M. . . . 



•04409 



23 -307 



•198 



1-10 



•344 



2 „ .... 



•04405 



23 '283 



•200 



1-25 



•332 



3 „ .... 



•04416 



23 -267 



•202 



1 -26 



•332 



4 „ .... 



•04264 



23-259 



-204 



1-23 



•354 



It will be seen from Table IV, and from the diagram below, that 

 the variation of the absorption coefficient is similar in its chief 

 feature — the increase from morning to afternoon — to that of the 



* See Strachey, " Proe. Roy. Soc." (vol. 11, p. 182). 



