92 BULLETIN OF THE 



The first part of the second member, Bftx T , expresses the amount 

 of heat radiated by the body, and the second, Bf/jJ , the amount of 

 heat received from the enclosure ; the radiating and absorbing 

 powers being usuaMy assumed to be the same, /is common to both. 



In applying this formula to bodies in space, protected from the 

 rays of the sun, r' would represent the temperature of space, by 

 which is meant the temperature at which a body would stand by 

 the heat received from the stars. In applying it to bodies on the 

 earth's surface it may be regarded as the temperature of an imagi- 

 nary enclosure, from which as much heat would be received as 

 from all surrounding objects, the earth's surface, and the atmos- 

 phere, &c, not including the sun, and hence it represents the shade 

 temperature. 



If wejiow suppose the body to be exposed to the direct rays of 

 the sun, the amount of heat thus received must be added to that 

 received from space, or from terrestrial surroundings, that is, to 

 Bf/j. T , and the preceding formula then becomes 



(1) « = — K P f + Bf(p? - & 



In which 



K = the units of heat received from the sun on a unit of surface ; 



p = the ratio between the surface receiving rays, projected on a 

 plane perpendicular to the rays, and the whole radiating sur- 

 face. 



As the body receives the rays from one direction and upon one 

 side only, and radiates from all sides, the average amount of heat, 

 Kpf, received over the whole surface and absorbed, must be com- 

 pared with the amount lost by radiation, and hence the factor / 

 must come in, since only the heat absorbed affects temperature, the 

 absorbing and radiating power here, as usual, being assumed to be 

 the same. 



In the case of <x spherical body, as the bulb of a thermometer, 

 the value of p becomes \, since the projected receiving surface of 

 the sphere is one-fourth of the whole radiating surface of the 

 sphere. In the case of a long cylinder, in which the radiation from 

 the ends could be neglected in comparison with the whole, the value 



of p becomes -, if the side of the cylinder is exposed perpendicu- 



larly to the sun's rays. In the case of a thin disk, with its surface 

 perpendicular to the sun's rays, neglecting the radiation from the 



