CHAPTER II 



HISTORICAL SURVEY 1 



Insolation from the sun, on the one hand, and, on the other, radia- 

 tion out to space, are the two principal factors that determine the 

 temperature conditions of the earth, inclusive of the atmospheric 

 envelope. If we do not consider the whole system, but only a volume 

 element within the atmosphere (for instance, a part of the earth's 

 surface) this element will gain heat: (I) through direct radiation 

 from the sun; (II) from the portion of the solar radiation that is 

 diffused by the atmosphere; (III) through the temperature radiation 

 of the atmosphere. The element will lose heat through temperature 

 radiation out to space, and it will lose or gain heat through convection 

 and conduction. In addition to these processes, there will often occur 

 the heat transference due to the change of state of water : evapo- 

 ration, condensation, melting, and freezing. The temperature radi- 

 ation from the element to space, diminished by the temperature 

 radiation to it from the atmosphere, is often termed " nocturnal 

 radiation," a name that is suggested by the fact that it has generally 

 been observed at night, when the diffused skylight causes no compli- 

 cation. In this paper it will often be termed " effective radiation." 

 The effective radiation out to the sky together with the processes of 

 convection and conduction evidently under constant conditions must 

 balance the incoming radiation from sun and sky. The problem of 

 the radiation from earth to space is therefore comparable in impor- 

 tance to the insolation problem in determining the climatic conditions 

 at a certain place. 



The first observations relating to the problem of the earth's radia- 

 tion to space are due to the investigations of Wilson, 2 Wells, 8 Six, 4 

 Pouillet, 5 and Melloni, 6 the observations having been made between 

 the years 1780 and 1850. These observers have investigated the 



1 Large parts of this chapter as well as of chapters III, IV and V: 1 have 

 appeared in the Astrophysical Journal, Vol. 37, No. 5, June, 1913. 

 a Edinburgh Phil. Trans., Vol. 1, p. 153. 



3 Ann. de chimie et de physique, tome 5, p. 183, 181 7. 



4 Six, Posthumous Works, Canterbury, 1704. 



5 Pouillet, Element de physique, p. 610, 1844. 



Ann. de chimie et de physique, ser. 3, tome 22, pp. 129, 467, 1848. 

 Ibid., ser. 3, tome 21, p. 145, 1848. 



