32 REPORT — 1887. 



Fourth Report of the Gomrnittee, consisting of Professor Balfour 

 Stewart [Secretary), Professor Stokes, Professor Schuster, 

 Mr. Gr. Johnstone Stoney, Professor Sir H. E. Roscoe, Captain 

 Abney, and Mr, G. J. Symons, appointed for the purpose of 

 considering the best methods of recording the direct Intensity 

 of Solar Radiation. 



In the last report of this Committee a description was given of a copper 

 enclosure which had been constructed by them. 



This consisted of a copper cube 3^ inches square outside, the faces of 

 which were |ths of an inch thick. The cube was packed round with felt 

 T^^ths of an inch thick, and the whole was faced outside with thin 

 polished brass plates. 



Thermometers were inserted into that side of the cube intended ulti- 

 mately to face the sun, and into the opposite side, by means of which the 

 temperature of these sides could be accurately determined. Finally, a 

 thermometer was placed in the vacant space in the very centre of the 

 enclosure. 



This last thermometer occupies the position that will ultimately be 

 occupied by the internal thermometer, npon which the sun is to fall 

 through a hole; only at this stage the hole had not been constructed. It 

 is obvious that when the instrument is finally in action, with a beam of 

 solar rays (condensed by means of a lens so as to pass through the hole) 

 falling upon t!ie bulb, this thermometer will be subject to a heating 

 effect from two separate causes, 



(a) It will, first of all, be subject to radiation and convection from 

 the surrounding enclosure, which is gradually (let us suppose) getting 

 hot through exposure to the sun. 



(&) It will, secondly, have a beam of solar rays of constant size and 

 of constant intensity (except as to variations arising from atmospheric 

 absorption, seasonal change in the sun's apparent diameter, or change in 

 the sun's intrinsic radiation) continuously thrown upon it through the 

 hole. 



In fine days when there is no abrupt variation of the sun's intensity 

 the temperature of the internal thermometer will remain sensibly con- 

 stant, or at least will only vai-y slowly with the sun's altitude ; and this 

 temperature will be such that the heat lost by radiation and convection 

 from the internal hot thermometer will be equal to the heat which it gains 

 from the sources (a) and (6), save as to a small correction, calculable 

 from the slow variation of the temperature of the thermometer. 



Now, our object being to estimate accurately the intensity of source 

 (&), we must be able, notwithstanding the gradual heating of the enclo- 

 sure, to determine how much heat the internal thermometer gains from 

 source (a). That is to say, we must be able to tell what would be the 

 temperature of the internal thermometer if the instrument were still 

 made to face the sun, but without any aperture. For the solid angle 

 subtended by the hole at any point of the bulb is so small that we may 

 regard it as a matter of indifference whether there be a hole or not, 

 except as to the admission or exclusion of direct solar radiation. 



It was suggested by Professor Stokes that a simple practical method 

 of doing this would be to expose the instrument, without a hole, to an 



