1883.] On the Solar Spectrum and Solar Temperature. 341 



are measures of energy, will be very nearly proportional to the 

 ordinates of the maximum or to ordinates not far from them on the 

 more refrangible side. 



Now the part of the spectrum which suffers least absorption by water, 

 and which in the solar spectrum is free from any very intense Fraun- 

 hofer lines, is near 4 on the prismatic scale. It has been shown by 

 Dewar,* and he deduced the same from Rosetti's formula, that the 

 temperature of the source is nearly proportional to the square root of 

 the total radiation, or, in other words, of the area of the thermogram 

 curve. The incandescent lamp worked by thirty cells had approximately 

 a temperature of 1,100°, and the same when worked by thirty-eight 

 cells of 1,500°, and the areas of the curves in the two cases are 

 as 106"4 to 50*4, and the galvanometer deflections when compared 

 together, as already stated, as 105 to 207, or about two to one in 

 both cases. This would give about the temperatures above stated, 

 taking either one as correct. The area of the curve of the crater is 

 very nearly sixteen times that of the thirty-eight cell curve, or the 

 temperature of the crater, using Dewar's formula, would be about 

 6,000°, a temperature corresponding to that obtained by him. 



We have already shown at what point of the solar spectrum aqueous 

 absorption has least effect, and if we may calculate from this the 

 probable area of the curve of maximum temperature. In our pris- 

 matic solar thermogram the scale of the ordinates is 4*5 times the 

 scale of the crater thermogram, but the ordinates of the point of the 

 spectrum above referred to are nearly equal. Hence the highest solar 

 temperature would be ^ 4'5 x 6000°, or about 12,700°, using the same 

 relations of temperature and radiation.f Taking, however, the square 

 root of the whole area of the solar curve, as would be the case when 

 direct and not spectrum measurements were made, we should get a 

 temperature of about ^2 *5 times that of the crater, or 9,600°, a tem- 

 perature also very similar to that obtained by Dewar under the condi- 

 tions specified. 



In conclusion we say, 1st, that the thermopile experiments have 

 confirmed our previous views as to the coincidence of the absorptions 

 by water and those shown in the solar spectrum ; 2nd, that the 

 extension of the work beyond our previous point emphasises them ; 

 3rd, that the highest temperature of the sun is not less than 12,700°, 

 a temperature far higher than that which has been recently put forward 

 by Sir W. Siemens ; 4th, that the existence of a large quantity of solar 

 radiation due to low temperature has been shown to be more than 

 probable. 



* " Brit. Assoc. Eep.," 1873, p. 465. 



f Since this paper was read we have made further experiments in regard to this 

 relation, and believe that we may have to modify it, reducing the temperature some- 

 what.— July 29. 



