1881.] the Atmosphere which absorbs Radiant Heat. 225 



These data yield the following results : — 



Year. 



E. 



Km 





True K at sea-level. 



1869 



1095 



•03454 



•06094 



•1836 



1879 



1093 



•03995 



•06160 



•1741 



As far as we can judge from seven days' observations in 1869 and 

 sixteen days in 1879, it would seem that the mean radiation of the 

 sun, and the mean absorption of the atmosphere lying above Dehra 

 were practically the same in the two years ; while the absorption at 

 Mussooree was slightly greater, and that at sea-level somewhat less 

 in 1879 than in 1869. 



On the mean of all the days the quantity of water vapour in the 

 air above Dehra was equal to '846 inch of rain in 1869, and 1*174 

 inch in 1879 ; and the corresponding values of a, the coefficient of 

 absorption for common logarithms, when the water vapour is equal 

 to an inch of rain, were '072 and '0525 respectively. The vapour 

 tensions at Dehra are of too local a character to admit of any safe 

 inference from these figures ; but, as far as the evidence goes, it 

 points to the conclusion that the radiation emitted by the sun 

 during the days of observation in 1869 was more readily absorbed 

 by water vapour than that emitted in 1879. 



It is to be hoped that the observations which are to be com- 

 menced next year at Leh — a station 11,500 feet above the sea — that 

 is, lying above four-fifths of the absorbent atmosphere — will enable 

 us to solve the all-important problem whether the sun's heat varies 

 to any appreciable extent or not ; but simultaneous observations at 

 a lower station, such as Mussooree, will be required to settle without 

 doubt the further question, whether there is any sensible variation 

 in the quality of the solar rays, as tested by actinometric methods, 

 that can be compared with the changes in the absorption lines which 

 are observed in the spectra of spots. 



Since writing the above, I have seen a report of a lecture* by 

 M. Violle, Professor at the Faculte des Sciences, Grenoble, in which 

 the lecturer described some simultaneous observations made by him- 

 self and another at the summit of Mont Blanc and Grenoble, and 

 at the Grands Mulets and Glacier des Bossons, on the 16th and 17th 

 of August, 1875. M. Yiolle's values for the heat received in a minute 

 by a square centimetre of surface exposed perpendicularly to the sun's- 

 rays are given in calories in the following table : — 



* " Eevue Scientifique," April 6, 1878. 



