154 



Mr. J. B. N. Hennessey. 



[Dec. 16, 



December 16, 1880. 



THE PRESIDENT, followed by Dr. C. W. SIEMENS, 

 in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. " On Actinometrical Observations, made in India at Mus- 

 sooree and Delira in October and November, 1879. By J. 

 B. N. Hennessey, F.R.S., Deputy Superintendent. Great 

 Trigonometrical Survey of India. Received May 4, 1880 

 (the parts within square brackets August 28). 



The present actinometrical observations were taken in 1879, in con- 

 tinuation of the series of 1869, published in the " Proceedings of the 

 Royal Society," vol. 19 (1870), pp. 225 — 234; reference is suggested 

 in these pages for such particulars as are omitted here in order to 

 avoid needless repetition. The series of 1869 and 1879 were taken 

 under identical circumstances as respects stations of observation, actino- 

 meters, and observers ; in both instances the work was carried on only 

 in the entire absence of visible cloud or mist between the sun and the 

 observer, and the sun's declination during the measurements of 1869 

 was nearly the same as in the observations of 1879. Hence the two 

 series, separated by an interval of ten years, are thus highly eligible 

 for comparison with one another ; it may, however, be added that the 

 work of 1879, now under discussion, is more extensive and systematic 

 than that of the preceding series. 



2. The two actinometers used are of the kind invented by the Rev. 

 G. C. Hodgkinson, and described by him in the " Proceedings of the 

 Royal Society," vol. 15, p. 321. The instruments are the property of 

 the Royal Society. To distinguish the instruments from each other, I 

 marked one with the letter A and the other with B, in 1869 ; both are 

 in exactly the same original good condition, including their glass caps, 

 and I may add in particular that the same glasses, and in the same 

 positions, were used in 1879 as in 1869. Hence the constants deter- 

 mined and employed in 1 869 were also employed on the present occa- 

 sion ; these constants are as follows : — 



Factor No. 1 for actinometer A, to convert results with 



glass on into results glass off 1 *09 



