RADIATION AND TEMPERATURE. 251 



So far no general formulae have been obtained for such surfaces, 

 though special cases, such as bright and black platinum, have been studied, 

 and formulae have been devised to suit the results obtained. 



The Rate of Solar Radiation. 



The Solar Constant. One of the most interesting problems in solai 

 physics consists in the determination of the rate at which the sun is 

 radiating energy. The rate is expressed in terms of the amount falling 

 on 1 sq. cm. at the distance of the earth but outside the earth's atmo- 

 sphere. When this is stated in calories per minute it is called the solar 

 constant If we multiply by (radius of earth's orbit/radius of sun's 



lamp 

 Blacked 



FIG. 144. Pouillet's Pyrheliometer. 



radiating surface) 2 , i.e. by (92400000/430000) 2 = 46000 we get the 

 amount passing out from each square centimetre of the sun. 



Pouillefs Pyrheliometer. The first successful research on the solar 

 constant was made by Pouillet by means of the pyrheliometer. This 

 consists of a flat, thin, cylindrical vessel A, Fig. 144, of which the upper 

 face is lampblacked and the rest is silvered. This contains water and 

 serves as a calorimeter. It is mounted at one end of an axis D round 

 which it can be rotated to secure mixing of the contents, and the thermo- 

 meter stem lies along this axis, so that the temperature of the calorimeter 

 can be read easily. 



The instrument is movable about another axis on the stand, so that 

 it can be pointed in any direction, and a disc A' of the same diameter as 

 A, is fixed near the other end of the axis D, and this is exactly covered 

 by the shadow of A when D is directed towards the sun. 



Pouillet's method of observation was as follows. He first directed 

 A to a part of the sky away from the sun, protected the instrument from 

 sunlight, and noted its fall of temperature, say 6 V during five minutes. 

 He then directed it towards the sun for five minutes, and noted the rise, 

 6. Finally he again directed it away from the sun, and noted the fall # 2 

 during five minutes. He assumed that the mean cooling during the time 



