2 



Lieut. J. H. Hennessey on the 



[June 16, 



4. Position of observatory. — "With the permission of Colonel Walker, 

 R.E., Superintendent of the Great Trigonometrical Survey of India, the 

 spectroscope was placed in the small rotating dome observatory of the 

 Survey department. It stands on the southernmost range of the Hima- 

 laya Mountains, in lat. N. 30° 27' 41", long. E. 78° 6' 45", height above 

 mean sea-level 6937 feet, My observations were made in this observa- 

 tory, excepting the interval last winter, when, in order to command a 

 view of the sun down to the horizon, I shifted the instrument to an 

 adjoining ridge. 



5. Times of observation.- — I was not long in ascertaining that between 

 10 a.m. and 2 p.m. no sensible alteration in the spectrum occurred. I am 

 here speaking of the red end, which has almost exclusively been the sub- 

 ject of my study. Accordingly, the observations which may be called 

 " sun high 3 ' were made between these hours. It was, however, some time 

 before I discovered that by far the greater effect of the earth's atmosphere 

 on the spectrum did not occur until the sun was about to dip* under the 

 horizon. In fact it is only when the sun is some three or four diameters 

 from the horizon that the very considerable alteration of the spectrum 

 begins. To secure this condition, the atmosphere must be quite free from 

 dust, which, as already implied, rises in great clouds from tbe plains until 

 deposited by the heavy rains in July and August ; and the sky must be 

 clear of clouds, at least about the horizon. The lines were watched under 

 these circumstances, and mapped down to the last moment before any 

 serious diminution of light occurred. These results are given in the map 

 observed " at sunset." It is easy to see that such favourable conditions 

 cannot be expected continuously day after day ; and even when available, 

 the time for observation is so very limited, that no results can be obtained 

 without considerable perseverance. "When, however, this exceptional 

 condition of air and sky do occur, the observer who has watched and be- 

 come familiar with the spectrum sun high, is well rewarded by the decided 

 manner in which the atmospheric lines now stand out, as the; sun, still 

 quite bright, is on the point of setting. 



6. Narrative resumed. — Between May and October 1868 I employed 

 my leisure in watching the spectrum sun high, fully expecting that the 

 brilliant weather of the autumn would enable me to make every endeavour 

 towards carrying out the suggestions of the Committee with the sun low. 

 But the autumn came and the air was still hazy. In fact, while the average 

 fall of rain here is some 90 inches, there fell in the season of 1868 only 61 

 inches, or about two-thirds the usual quantity ; a drought which in the 

 first instance led to the scarcity of food, from which these provinces are 

 only now emerging. To complete my mortification, I found that the colli- 



* A range of bills to the eastward conceals the view for some degrees in altitude ; 

 so that, though I have repeatedly watched the spectrum of a morning as well, my map 

 is in preference based on the view at sunset, when the sun can be followed down to 

 the horizon. 



