28 report— 1884. 



On receipt of this information from the Royal Society your Com- 

 mittee, taking into consideration, not only the late period of the present 

 Session of Parliament, bnt also the &hort time that remains before the 

 dissolution of the present Parliament, have determined that it will be- 

 more advantageous to defer approaching Government on the subject till a 

 later period. 



Your Committee still wish to hold the matter under further considera- 

 tion, and they therefore ask that the Committee be reappointed without a 

 grant of money. 



Note. — Since the above Report was drafted the Government have- 

 agreed to join the ' Bureau International des Poids et Mesures ' 



Report of the Committee, consisting of Professor Balfour Stewart 

 (Secretary), Professor Stokes, Mr. G. Johnstone Stoney, Professor 

 SirH. E. Roscoe, Professor Schuster, Captain Abney, andMr. G. J. 

 Symons, appointed for the purpose of considering the best methods 

 of recording the direct intensity of Solar Radiation. 



This Committee, acting on a suggestion made by General Strachey, have 

 chiefly devoted their attention to the subject of a self-recording actino- 

 meter. 



The self-recording actinometer of Mr. Winstanley would not be 

 suitable, 1 because it is influenced by radiation from all quarters. Other 

 actinometers require manipulation on the part of the observer which would 

 make it almost impossible to make them self-recording. It was suggested 

 by Professor Balfour Stewart that a modification of his actinometer might 

 be adapted to self-registration by taking for the quantity to be observed, 

 not the rise of temperature of tbe enclosed thermometer after exposure for 

 a given time, but the excess of its temperature when continuously exposed 

 over the temperature of the envelope. After making some calculations 

 as to the behaviour of such an instrument, Professor Stokes came to the 

 following conclusions : 



(1) The enclosure should be of such a nature as to change its tem- 

 perature very slowly, and of such a material that the various portions of 

 the interior should be at the same moment of the same uniform tempera- 

 ture. For this purpose an arrangement somewhat similar to that used in 

 Prof. Stewart's actinometer is suggested ; the outside to consist of polished 

 metallic plates, then a layer of some non-conducting substance, such as- 

 felt, then a thick copper interior which need not be polished. Into this 

 copper is to be inserted a thermometer which will give the temperature- 

 of the copper interior from moment to moment. 



(2) In the middle of the enclosure is to be placed the thermometer, 

 upon which the heat of the sun is made to fall by means of a hole in the 

 enclosure, either with or without a lens. This thermometer should be so 



1 ' This is the case at present, but there would not be any great difficulty in 

 modifying it so as to act as required. It is quite a matter worth consideration 

 whether a differential air- thermometer would not be very suitable, one bulb silvered 

 and the other blackened or of green glass, as I suggested to the Meteorological Council 

 some years back. By this means only one reading would be necessary, whilst in the 

 plan suggested two would have to be recorded, and the measurements would be 

 more difficult.' {Note hi/ Captain Abney.) 



