1880.] On Actinometrical Observations, made in India. 155 



Factor No, 2 for actinometer B, to convert results with 



glass on into the results glass off 1 '04 



Factor No. 3, to express results obtained with actino- 

 meter B glass on in terms of actinometer A glass on. . "982 



All the results given in Tables V to VIII, attached, will be found 

 expressed in terms of A for both glass on and glass off, as was done in 

 1869 ; it will be seen that only factors Nos. 1 and 3 enter into this 

 conversion. The results of Table IX need no such conversion, as for 

 the purpose in view they are compared for each instrument with its own 

 mean. The numbers hereafter discussed are all in terms of A glass off. 



3. The co-ordinates of the stations of observations are as follows : — 



Height in feet above 

 Lat. IS. Long. E. mean sea level. 



Mussooree 30° 28' 78° 7' 6,937 



Dehra 30° 19' 78° 6' 2,229 



The direct distance is thus about nine miles between the stations, 

 which are nearly on the same meridian and are mutually visible. The 

 difference of longitude between the two stations is only 62"*5 = 4"2 

 seconds, Mussooree being east of Dehra. 



4. Dehra station is in the Dehra Dun, which is a valley some ten 

 miles wide and about forty miles long, and is bounded on the north by 

 the Himalayas and on the south by the Siwalik range of hills ; at its 

 eastern and western extremities there now respectively the well known 

 rivers Ganges and Jumna. The large native town and the civil and 

 military stations of Dehra lie west and north of the actinometer 

 station within a distance of some two miles, so that, the observer at 

 Dehra was unavoidably subject to the disadvantages of the usual smoke 

 and haze envelope which is commonly visible in the winter months 

 over all large towns when seen from a distance, and especially if viewed 

 from a height ; on the other hand, the observer at Dehra was not liable 

 to the disadvantages of strong winds. 



5. Mussooree is eminently suited for an actinometric station, espe- 

 cially in the autumn. It stands on almost the highest point of an east 

 and west ridge of hills, which falls precipitately 3,000 or 4,000 feet both 

 north and south, so that the observer is absolutely free from smoke and 

 dust, while the atmosphere in autumn is brilliantly clear. But the 

 station from its very prominence is liable to brisk breezes, which some- 

 times blow in strong gusts and always from the south. From these, 

 however, the observer and instrument were protected by means of a 

 kanat (or canvas wall, some six feet high, of a tent), erected at a 

 distance of a few feet, a protection found to be absolutely necessary, 

 for the actinometer is certainly affected by a breeze, especially if blow- 

 ing in gusts. 



[It will thus be seen that while Mussooree is highly eligible, a similar 



