226 



Lieut. Hennessey on Actinometrical [Dec. 22, 



continue a series of observations for any lengthened period (as, say, two 

 hours) without introducing breaks of several minutes in its continuity. 



(3) The two instruments used have been lettered by the observers A 

 and B. The glasses, too, have been suitably marked as suggested by Sir 

 John Herschel in the 'Admiralty Manual of Scientific Enquiry.' Actino- 

 meter B was used at Dehra by Mr. W. H. Cole, M.A., whose observations 

 in sun and shade number 405 in all. The observations at Mussoorie were 

 made by myself with actinometer A; they are 315 in number. And as 

 respects the chronometers, barometers, and thermometers employed, I 

 need hardly add that these instruments were of a superior order and well 

 verified, or that they are in ordinary use at the head-quarters of Colonel 

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

 India. 



(4) And with every facility for reading the last-named instruments, I 

 regret having omitted to arrange for a more frequent reading of the baro- 

 meter, and that the wet- and dry-bulb thermometers were not recorded. 

 It, however, so happened that during the days of observation in October 

 and November last the sky was beautifully clear, with the trifling excep- 

 tions noted in the records of observations ; and there is no reason to sup- 

 pose that any sudden changes occurred in the hygrometric conditions of 

 the atmosphere. In future, however, when the actinometers can again be 

 worked, more numerous readings of the barometer and thermometers will 

 be d unrecorded. 



(5) The stations of observation were Mussoorie and Dehra. The direct 

 distance^between them is about nine miles. The former stands on one of 

 the southernmost ranges of the Himalayas ; the latter is in the valley Dehra, 

 or Dehra Dhoon. The hypsometrical elements for these stations, given in 

 the result-abstract and elsewhere, are taken from the records of the Trigo- 

 nometrical Survey of India. It appears from these values that Mussoorie 

 station is above that at Dehra about 4700 feet. 



(6) The procedure agreed on between Mr. Cole and myself was to ob- 

 serve daily a simultaneous series at 1 l h 40 m a.m. (mean of all the observed 

 times), another series at noon, and a third at h 20 m p.m., the reckoning 

 being in apparent time. The slight deviations from these times which 

 appear in the result-abstract are due to little accidental causes almost in- 

 separable from simultaneous work. After four days of these series, each ob- 

 server was to determine the amount of heat stopped by the glass of his instru- 

 ment employed. In these experiments I was too busy otherwise to recipro- 

 cate Mr. Cole's observations of November 1. On the 3rd of November, 

 however, we both observed the intended succession of groups (nearly) ; so 

 that several of these are made to discharge a double duty, and are intro- 

 duced in the after- discussion of relative radiation. On November 4 we 

 each obtained a complete hourly series about the hours of 8 a.m. to 

 4 p.m. These results terminated for the time the reciprocal series of ob- 

 servations Mussoorie-Dehra. Subsequently, in April 1870, when we were 



