§6 KEPORT~1900: 



On Solar Radiation. — Report oftJie Committee, consisting ofBv G. JOHN- 

 STONE Stoney (Chairman), Professor H. McLkod (Secretary), Sir 

 G. G-. Stokes, Professor A. Schuster, Sir H. E. Eoscoe, Captain 

 Sir W. DE W. Abney, Dr. C. Chree, Professor Gr. P. FitzGerald, 

 Professor H. L. Callendar, Mr. W. E. Wilson, and Professor 

 A. A. Rambaut, appointed to consider the best Metliods of Recordinrj 

 the Direct Intensity of Solar Radiation. (Drawn vp by Professoi' 

 H. L. Callendar.) 



As already reported, the copper-cube actinometcr constructed for this 

 Committee, and described in the Report for 1886, was entrusted to 

 Professor Callendar in August 1899 for comparison with his automatic 

 recording instrument described in the Report for 1898. In the course of 

 the past year a number of experiments have been made with this 

 apparatus by Miss W. E. Walker, 1851 Exhibition Scholar, working at 

 University College, London, under the direction of Professor Callendar. 

 The object of the work was to obtain absolute measurements of radiation 

 for the calibration of the more convenient form of continuous recorder. 

 In its original form the copper-cube actinometer was not very well 

 adapted, and probably was not intended, for absolute measurements ; but 

 with some modifications very promising results have been already obtained, 

 and it is believed that the method thus modified will lead to trustworthy 

 and valuable determinations. 



The history of the copper-cube actinometer is contained in various 

 reports communicated by this committee, of which the following is a brief 

 summary. The method originally proposed was to concentrate the rays 

 of the sun by means of a lens through a hole in one side of the cube on to 

 a central mercury thermometer with a flat bulb. The steady difference 

 of temperature between the central thermometer and the walls of the 

 cube would be approximately proportional to the intensity of solar 

 radiation, and might be taken as a measure of the same in arbitrary units. 

 To obtain the equivalent in absolute measure it would be necessary to 

 know the rate of cooling of the thermometer and the coefficient of 

 absorption of the bulb and of the lens by which the rays were concentrated 

 These might huxe been obtained by auxiliary experiments on the i-ate of 

 heating or cooling under various conditions ; but as the mercuiy thermo- 

 meters proved unsuitable in many respects, the apparatus was subsequently 

 modified by the substitution of a copper disc and a thermo-j unction for 

 the central thermometer. This permitted the observation of smaller 

 differences of temperature and the more accurate determination of the 

 thermal capacity of the irradiated disc. 



The elementary theory of the instrument, assuming that for small 

 differences of temperature the rate of cooling of the disc would be pro- 

 portional to the difference of temperature between the disc and the walls 

 of the cube, was given in the Report of the Committee for 1892. If 6 is 

 the excess of temperature of the disc over the enclosure at any time t 

 measured from the commencement of the exposure to the radiation to be 

 measured, and if r he the initial rate of rise of temperature of the disc in 



