256 HEAT. 



in which the radiation from the sun was compared with that from a hole 

 in the wall of a constant-temperature enclosure. The enclosure was an 

 iron and porcelain tube heated in a gas furnace. Its temperature was 

 given by a Callendar platinum thermometer. One end of the tube was 

 open, and in front of the opening was a rectangular area of variable 

 width, and this was arranged so that the radiation from the sun was 

 equal to that from the enclosure. 



If p is the fraction of the sun's rays passing through the atmosphere, 

 and if q is the fraction reflected by the mirror, we may put 



pqO 4 x solid angle subtended by the sun at the receiving surface 

 = (T 4 - T 4 ) x solid angle subtended by the rectangular aperture. 



where is the effective temperature of the sun, 



T the temperature of the enclosure, 

 and T that of the receiving surface, 



T 4 was negligible in comparison with T 4 , since T was of the order of 

 1000. Then we may put 



.34 _ T 4 solid angle of aperture 

 pq solid angle of sun 



The value of depends on the value assigned to p. If, with Rosetti, 

 it is put at 0'71, the effective temperature of the sun comes out as 

 5768 absolute. If, with Langley, it is put at 0-59, the effective tem- 

 perature comes out as 6085 absolute. 



It is interesting to compare with this the value given by the solar 

 constant and the constant of radiation determined by Kurlbaum (p. 250). 

 With constant 2*1, the radiation from the sun is 6800 watts per sq. cm. 

 The radiation constant, according to Kurlbaum, is 532 x 10~ 14 watts. 



If the effective temperature of the sun is 6, then 



5x3210- 14 4 = 6800 

 whence = 5980 nearly. 



If we adopt Langley's value of the solar constant, which is 3, we get 

 = 6500 about, 



slightly above the value obtained by Wilson. 



Another value may be obtained from the equation (p. 250) 



A m = constant =2940 

 in conjunction with Langley's value for A^, viz., 



A..-O-5/fc 

 This gives = 5880 absolute. 



But this should hardly be taken as another mode of determining 

 the sun's temperature, since the equation A^fl = 2940 applies only to a 

 surface emitting full radiation. The close agreement of the result it 

 gives should rather be taken as indicating that the sun is nearly a full 

 radiator, when we consider its radiation as a whole. 



Whatever the temperature of the solar surface, the interior on the 

 average is doubtless much hotter. If, as we are entitled to suppose from 

 the high temperature, the interior of the sun is in a mobile condition, 

 the tendency will be towards a condition of "convective equilibrium," 



