1881.] the Atmosphere which absorbs Radiant Heat. 



223 



•209 on the 4th November, 1869, and "174 and '185 respectively on 

 the 12th and 14th November, 1879. 



If the differences in these values of the total absorption be caused 

 entirely by variations in the quantity of aqueous vapour in the air, 

 the fraction of the total incident heat which is arrested before reaching 

 the ground will be a direct function of the vapour tension at the 

 place of observation, and an inverse function of the rate of diminution 

 of the vapour tension with height. We may suppose it to be simply 

 proportional to the total quantity of water vapour over the place of 

 observation. This has been shown by Dr. J. Hann* to be equal to a 



depth of water, /X ^-^82 x .4343 w here / is the vapour pressure 



expressed in units of height of mercury, and C is the constant of the 

 vapour formula in metres. Neglecting the temperature coefficient, 

 which should probably enter into C also, and reducing to English 

 measure, the quantity of rain that would be formed by the complete 

 condensation of the vapour over any place is given by the formula, 

 Q=fx '00014 C inches, where /is expressed in inches of mercury, Q in 

 inches of water, and C in feet. At Mussooree, on the days of obser- 

 vation in 1879, the several quantities had the following values : — 



Date. 



K 



/ 



c 2 



Q 



12th November .....<■« 



•04106 





•221" 



26,814 ft. 



•829" 



14th „ 



•04231 





•183 



25,499 



•653 



If now K=«Q, the value of a. on the 12th November is found to 

 he *0495, and on the 14th, '0648. The fraction of the total radiation 

 absorbed from day to day therefore does not bear a constant relation 

 to the quantity of aqueous vapour in the air ; and, consequently, it 

 seems probable that the quality of the sun's heat is subject to con- 

 siderable variations from day to day. If this be so, it will be impos- 

 sible to arrive at comparable actinometric results from a few observa- 

 tions about apparent noon each day, unless the observing station 

 be sufficiently elevated to lie above the greater part of vapour atmo- 

 sphere. 



Unfortunately no hygrometric observations were taken at Mussooree 

 in November, 1869, so that it is impossible to determine precisely, 

 from the actinometric observations made there by Mr. Hennessey, 

 whether the radiation emitted from the sun was more or less sus- 

 ceptible of absorption in 1869 than in 1879. At Dehra, however, the 



* " Zeitschrift fur Meteorologie," Band IX, page 199. 



