[ 537 ] 



LXII. Experimental Researches on the Temperature of the Sun. 

 By F. Rosettt, Professor of Physics in the University of 

 Padua. 



[Concluded from p. 449.] 



VIII. Thermal Radiation of the Sun. 



THE observations on the thermal effect of the radiation of 

 the sun were made with pile No. 1 (Duboscq), placed upon 

 a platform near to the galvanometer. The pile could easily be 

 directed so that the rays of the sun fell perpendicularly on its 

 face. Four readings of the galvanometer were made for each 

 observation : — the first with an open circuit, in order to deter- 

 mine the point of rest on the scale ; the second with a closed 

 circuit and the case of the pile closed ; the third with the case 

 open and the rays admitted ; the fourth with the window of 

 the case open, but intercepting the rays of the sun with a 

 small circular screen with double walls placed at about a metre 

 from the pile, so that its shadow was thrown on the window. In 

 this fourth observation the face of the pile receives the radia- 

 tion from the part of the sky surrounding the sun, which ex- 

 ercises an influence also in the third observation. Thus, by 

 taking the difference between the third and fourth readings, 

 the thermal effect of the solar radiation is obtained in scale- 

 divisions. I should remark in passing, that the fourth reading 

 was nearly always negative. 



The electric current produced by the solar radiation was so 

 great, that I had to introduce into the circuit sixteen Siemens's 

 resistance units. 



In order to determine the depth of the atmospheric layer 

 traversed by the solar rays before arriving at the pile, in the 

 first experiments the time of the third observation was noted : 

 thus by means of the ephemerides the zenithal distance could 

 be calculated. In the later experiments the determinations 

 were made directly with a theodolite. When one of the obser- 

 vers had made the first two readings, the other (who was on the 

 platform) opened the window of the case containing the pile, 

 observing at the same time by means of a theodolite the height 

 of the sun above the horizon ; in the meantime the first ob- 

 server had made the third reading with the galvanometer. 

 The hygrometric state of the air was also ascertained each 

 time ; and in order to obtain an immediate indication, a hair 

 hydrometer was used. This hygrometer had been graduated 

 by comparing it with a condensation hygrometer. The fol- 

 lowing Table shows relative humidity corresponding to the 



Phil. Mag. S. 5. No. 52. SuppL Vol. 8. 2 



