224 



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



psychrometer was observed every day at 9.30 a.m. and 3.30 p.m., and 

 from these observations, in conjunction with the figures already given, 

 we can find an approximate value of a for the 4th November, 1869, as 

 well as for the two days in 1879. In the following table the quan- 

 tities from which a is computed and its value for each day are shown : — 



Date. 



K 



/ 



c 2 



Q 



a 



4th November, 1869 . . 

 12th „ 1879 . . 

 14th „ „ .. 



•07027 

 •06209 

 •06546 



•344" 



•375 



•302 



17,210 ft. 



26,814 



25,499 



•829" 

 1 -407 

 1-078 



•0848 

 •0441 

 •0607 



The vapour tension observed at Dehra is a strictly local phenomenon, 

 as has been above pointed out, but still the values of a for the two days 

 in 1879 accord so well with those deduced from the observations at 

 Mussooree, that there can be little doubt that water vapour had a 

 greater absorbent effect on the radiation of the 4th November, 1869, 

 than on that emitted from the sun on the other days. 



From the observations made simultaneously at Mussooree and 

 Dehra, for several days near apparent noon, we may perhaps deter- 

 mine approximately whether the mean emissive power of the sun 

 was greater at the beginning of November, 1869, or in November, 

 1879. There are only three quantities to determine for each year, 

 the total radiation at the limit of the atmosphere, the absorption co- 

 efficient for Mussooree and the same for Dehra, and we have three 

 relations between these quantities : — 



(1) LogR=logr M + K M sec%, 



(2) LogR=logr D -f-K D sec2 D , 



/on t tt y Tr 4,708 feet 



(3) LogK M =logK D --?_ 



In (3) we may take C to be the constant of the formula for water 

 vapour, which, for 1869, may be computed from the observations at 

 Roorkee, Dehra, and Chakrata. From, Mr. Hennessey's tables and 

 the Meteorological Observatory records, I find the following mean 

 values for the required data : — 



Year. 







Sec Zjyj 



Sec zd 



C. 



1869 



980 



902 



1-401 



1-398 



19,077 



1879 



954 



887 



1-475 



1-471 



25,030 



