46 



Mr. J. B. N. Hennessey. 



[May 25 r 



change, moreover, enabled me to adopt tlie desirable condition that 

 two observers and instruments, placed side by side, should work with 

 one and the same chronometer between them, so that two independent 

 results might be obtained simultaneously. It was under these condi- 

 tions that the auturrtn series of observations in 1881 were made ; other- 

 wise, both the summer and autumn series of 1881, as well as the series 

 of 1880, were taken exactly in keeping with the observations of 1879. 

 The procedure followed in the last-named series is fully described in 

 the " Proceedings of the Hoyal Society," vol. 31, p. 154, to which 

 I may be permitted to refer, instead of making repetitions here, espe- 

 cially since my leisure is at present limited, so that it had perhaps 

 better be devoted to what follows. 



2. Returning to the autumn simultaneous observations of 1881, not 

 only were two actinometers employed side by side, but the two instru- 

 ments were of different patterns. One of them was A (Hodgkinson's), 

 which has been amply described in my previous communications ; the 

 other instrument was one * of those constructed according to designs 

 by Professor Balfour Stewart. It is marked No. 2, and is supplied 

 with two similar tubes, or thermometers, viz., No. 1306 and No. 1307, 

 both of which read from 0° to 100° F., and are graduated on the glass 

 to degree fifths. Tube 1306 alone was employed : it was readily 

 adapted to give readings for so long a time as thirty consecutive 

 minutes of observation, by casting off sufficient mercury into the pear- 

 shaped receptacle above. This was done once for all before the series 

 was begun, and in fact the surplus mercury is still in the receptacle. 

 Otherwise, particular attention was paid to the settings of the ther- 

 mometer, in rotation and depth, and to those of the lens ; the latter 

 was kept scrupulously clean, and was used to its full extent, i.e., no 

 stops were employed. It will thus be seen that the observations were 

 all strictly differential. The graduations were read as usual with a 

 magnifying-glass ; the exposures were of sixty seconds alternately in 

 sun and shade, with thirty seconds' intervals between, so that the 

 instruments were read botli at the beginning and end of each sixtif 

 seconds' exposure, all of which is in keeping with what was done during 

 previous work. In poiut of observers, Mr. Peychers worked A and I 

 used No. 2. 



3. My observations before 1881 were restricted to the autumn, but 

 an opportunity occurred in the summer of 1881, which was utilised so 

 long as it lasted. The results are of an unexpected nature. 



4. There is another point to be mentioned. Increased experience 

 has suggested keener discrimination in respect to atmospheric condi- 

 tions, and this has called for some concise mode of indication, which I 



* For the loan of this instrument I am indebted to the kindness of H. F. Blan- 

 ford, Esq., F.E.S., Meteorological Reporter to the Government of India. 



