1882.] On Actinometrical Observations made in India. 



47 



designate " day letters." These I briefly explain hereafter, first reite- 

 rating that I never observe when any visible interpositions exist 

 between the sun and me ; and, in fact, what can be the use of 

 measuring solar radiation through a visible varying atmospheric 

 umbrella, such as is represented by cloud, mist, haze, dust, and smoke ? 

 I thought of using a Crookes' radiometer (happening to possess one) 

 as a pioneer to the actinometer, in order to see if the former would 

 suitably indicate when the latter may be employed to good purpose, 

 but the intention unavoidably fell through. 



5. Now, in respect to my day letters, the cases I had to provide 

 for were these : the two visible causes of interposition are cloud 

 (including mist) and haze (including dust, smoke, Ac). First as to 

 cloud : — 



(1.) The whole visible sky may be perfectly clear, or there may be 

 half-a-dozen small patches here and there, but none within 50° or 60°~ 

 of the sun ; for this state my day letter is A. 



(2.) But sometimes the condition A required qualifying, because of 

 a peculiar cloud behaviour. The sun being due south, small cloud 

 balls, some 2° to 5° in diameter, appeared on the horizon about north- 

 west, and gradually rolled up and eastwards to some 30° short of the 

 sun, where they became invisible. When the circular track, if continued 

 in imagination, passed well under the sun, I indicated this by B. 



(3.) But if the track passed through the sun, the day letter used 

 is C. 



6. Then as regards haze : I stood on the outermost mountain range, 

 south of which lie the plains and to the north successive hill ridges, 

 ending in the perpetual snows. No haze is generated on the hills 

 apparently, but is blown up from below, the wind being southerly as 

 a rule : hence 



(4.) For no haze south or north I use a. 

 (5.) "For haze to south only, b. 



(6.) When the haze is both south and north, I infer that I stand in 

 it though it is invisible overhead ; for this I use c. 



Hence, by using suitable combinations of these letters, the condition 

 of the sky is fairly indicated symbolically, at least for Mussooree. 



7. The present series have been reduced in exactly the same 

 manner as those of 1879 were reduced. The individual results are 

 attached, viz. : — 



For 1880 Autumn, October 18th to 27th, in T. 1, by Mr. W. H. 

 Cole, M.A. 



For 1881 Summer, April 25th to May 9th, in T. 2, by Mr. H. W. 

 Peyehers and myself. 



For 1881 Autumn, October 11th to November 11th, in T. 3, by Mr. 

 H. W. Peyehers and myself. 



In these tables the results of A and B are expressed in terms of A, 



