1881.] the Atmosphere which absorbs Radiant Heat. 217 



absorbed by each unit thickness of the absorbent substance (supposed 

 homogeneous) is proportional to the total heat which falls upon it. 

 This rule was long ago found bj Pouillet to be approximately true of 

 atmospheric absorption. The thickness of atmosphere to be traversed 

 by the solar rays at various angles of incidence has been taken to be 

 simply proportional to the secant of the sun's zenith distance. If the 

 vertical thickness of the absorbent atmosphere be taken at ^oth of 

 the earth's radius, the error of the preceding assumption is almost 

 insensible for the inclinations in the tables ; and below an elevation 

 equal to xxroth of the radius lie 68 per cent, of the dry air, and 99 \ 

 per cent, of the water vapour of our atmosphere. If, then, R stands 

 for the total radiation that would fall upon the actinometer at the 

 limit of the atmosphere, and r for the observed radiation at the place of 

 observation, when the sun's zenith distance is z, we may put log r 

 =log R— K sec z, where K is a coefficient the antilogarithm of which 

 represents the fraction of the total radiation that would be absorbed by 

 a vertical column of the air above the place at the time of the observa- 

 tion. Assuming the K to be constant for each day at each station, I 

 have computed the values of log R independently from the sets of obser- 

 vations made at the two stations ; and then, taking the most probable 

 value of R for each day to be that derived from the mean of the two 

 log R's, in which the logarithm deduced from the observations at the 

 upper station* is given double weight, I have recomputed the coeffi- 

 cients marked K M and K D respectively, and finally worked out the 

 symmetrical values of r for the hours before and after apparent 

 noon. 



Table I.— 4th November, 1869. 







Mussooree. 







Dehra. 





Hour. 



















Appt. time. 



Sec z. 



r obs. 



r comp. 



Diff. 



Sec z. 



r obs. 



r comp. 



Diff. 



8 A.M 



3-57 



842 



817 



-25 



3-54 



715 



645 



-70 



9 „ .... 



2 21 



934 



929 



- 5 



2-20 



807 



801 



- 6 



10 „ .... 



1-71 



990 



973 



-17 



1-70 



870 



868 



- 2 



11 „ .... 



1 '50 



1000 



993 



- 7 



1 -49 



905 



898 



- 7 



Noon .... 



1-43 



988 



999 



+ 11 



143 



914 



907 



- 7 



1 P.M 



1 -50 



984 



993 



+ 9 



1 -49 



900 



898 



- 2 



2 „ .... 



1-71 



967 



973 



+ 6 



1-70 



852 



868 



+ 16 



3 „ .... 



2-21 



920 



929 



+ 9 



2-20 



786 



801 



+ 15 



4 „ .... 



3-57 



799 



817 



+ 18 



3 54 



584 



645 



+ 61 



Mean E,= 1143 log R = 3 -05804 



K M ='04078 for decimal logs 



= '09390 „ natural „ 



K D = -07027 „ decimal „ 



= "16157 „ natural „ 



* Dehra is 2,229 feet above the sea, and Mussooree 6,937 feet. 



Q 2 



