RADIATION AND TEMPERATURE. 255 



found a value about 2-4, and Bellia * found from his own and previous 

 work a value 2*1. 



There are indications that the so-called constant is not constant, but 

 varies with the condition of the sun. It would be better if some such title 

 as " Solar Radiation Stream " were adopted to replace " Solar Constant." 



Taking 2*1 as the value, it is equivalent to 0*147 x 10 7 ergs per 

 second or 0*147 watts. 



Multiplying by 46000, the radiation from a square centimetre at the 

 surface of the sun is 6800 watts, about 9 horse-power. 



The Effective Temperature of the Radiating Surface of the Sun. We 

 receive radiation, no doubt, from different layers of the sun, and these 

 are at different temperatures. We cannot, therefore, accurately say that 

 the radiating surface has one definite temperature. But we may find 

 the one definite temperature of a fully radiating surface which is giving 

 off energy at the same rate as the sun, and this is defined to be the 

 " effective temperature" of the sun. 



Before there was any approximation to a correct law of radiation, a 

 temperature was assigned to the sun which we now know to be immensely 

 exaggerated. Thus Watterston, using Newton's law, according to 

 which the temperature is proportional to the radiation, found a value 

 7,000,000 0. 



Violle, on the other hand, found far too low a temperature by 

 using the law of Dulong and Petit. He obtained an effective tem- 

 perature of 1500, and considered that the actual temperature might 

 be 3000. 



The first determination on what we may describe as modern methods 

 was made by Rosettit He measured the heat received by a thermo- 

 pile from a lamp-black surface at various temperatures up to 300 C., 

 then from a copper ball suspended in a flame, the temperature being 

 determined by a specific heat experiment. By a subsidiary experiment 

 he determined the ratio of emission of bare copper to that of lampblacked 

 copper, and ultimately arrived at the law of radiation already quoted 

 (p. 248). He then observed the radiation received from the sun and, 

 correcting for the absorption by the atmosphere, he obtained for the sun's 

 effective temperature a value about 10,000. 



Another important determination was made by Wilson and Gray.* 

 They compared the radiation from the sun with the radiation received 

 from a platinum strip which they could raise by an electric current to 

 any desired temperature, the radiation being measured by Boys's radio- 

 micrometer. That of the sun was reflected into the instrument by a 

 mirror of known reflecting power. They first found that the radiation 

 from the platinum increased according to Stefan's fourth-power law, up 

 to the highest temperature which they could measure. Then they used 

 a result found by Rosetti, that at high temperatures the emission from 

 bright platinum is 1/2*9 of that from lampblacked platinum, and so they 

 calculated the temperature of lampblacked platinum at which it would 

 give off the same energy per sq. cm. as the sun, and they found for 

 this a value varying from 6200 0. to 7400 C., according to the value 

 assigned to the absorption by the earth's atmosphere. 



In a later paper Wilson described a modification of the experiment, 



Science Abst., xiii., No 660. f Ph* 1 - Mag., viii., 1879. 



Phil. Trans., A., 1894, p. 361. Proc. R.S., Ixix., 1901-2, p. 312. 



