on the Temperature of the Sun. 441 



Calculation of the Readings of the Galvanometer. — Four 

 readings were taken on the galvanometer before the ball was 

 immersed in the water. The first reading, with an open cir- 

 cuit, gave the zero-point ; the second was made with a closed 

 circuit, the pencil of heat-rays being shut off by means of the 

 double screen. These two readings were always nearly iden- 

 tical, showing that the case protected the pile effectually. 



In the first of the experiments mentioned the readings 

 were.— Circuit open . . . 66*5 



Circuit closed . . . 68*0 

 In the second : — 



Circuit open . . . 64*0 

 Circuit closed . . . 65*5 



The difference 1*5 in the two readings represents the effect 

 of the medium upon the face of the pile which receives the 

 radiation, the window of the case being open ; but it might 

 also arise from some slight thermo-electric current induced 

 by contact with the rheophore. Whatever may be the cause 

 of the slight deflection, it can easily be eliminated by subtract- 

 ing the second reading from the following readings which give 

 the deflection produced by the radiation. 



The third reading affords a measure of the thermal effect of 

 the flame ; the difference between the fourth and the second 

 reading gives the total effect of the radiation of the hot ball 

 and of the half of the flame which surrounds it. Thus the 

 thermal effect produced on the pile by the radiation of the 

 ball can easily be found. It is well known that the blue flames 

 of Bunsen's burners are almost entirely diathermanous. Con- 

 sequently it is only necessary to subtract the number of divi- 

 sions which represent the thermal effect of the front part of the 

 flame (which on account of its feeble emisssive power is always 

 small in comparison with the number of divisions obtained by 

 the radiation of the heated body) from the difference between 

 the fourth and second reading, which gives the combined effect 

 of the flame and the ball. 



In the first of two calorimetric experiments the difference 

 between the fourth and second readings was equal to 248*9 

 divisions, which, on subtracting 13*6 divisions, the thermal 

 effect of that part of the flame which could radiate towards the 

 pile, gives us 235*3 as the measure of the thermal effect of the 

 ball on the face of pile No. 1. 



The distance from the centre of the ball to the centre of the 

 pile was 185 millims. ; and 66 millims. was the distance from 

 the same centre to the screen, the smallest hole in which was 

 15 millims. in diameter. The diameter of the ball was 30 

 millims. It can be shown by a simple calculation that every 



Phil Mag. S. 5. Yol. 8. No. 51. Dec. 1879. 2 H 



