GEOLOGY. 325 



the secondary or repetitive class of thermal actions may be high multiples of 

 the incoming and outgoing values. 



According to Dines, about one-half the energy of the solar rays outside 

 the atmosphere is reflected to open space and has no heating value. In this 

 the detention is nil. Thermally speaking, the time-factor is zero, and reflec- 

 tion thus serves as the bottom of the time-factor scale. 



LEADING POINTS CONSIDERED. 



A prehminary list of the main points considered may prove convenient: 



1. Since a fraction of the solar energy is absorbed in penetrating the 

 atmosphere (60 calories per cm^ per day, according to Dines^), the question is 

 considered whether absorption in the upper levels of the atmosphere gives 

 larger or smaller total heating value before escape than would penetration 

 to the earth's surface and radiation from the bottom of the atmosphere. The 

 principles involved have several applications. 



2. Since a larger percentage of solar radiation reaches the earth's surface 

 (300 calories per cm^ per day, Dines) , questions have been considered relating 

 to (1) the part that is reflected without delay and has no heating value, and 

 (2) the part that is absorbed by soil, rock, and vegetation, which is soon given 

 to the air (a) by contact, or (6) rendered latent by evaporation, or (c) ab- 

 sorbed and re-radiated as dark heat, or (d) retained by the ocean for long 

 periods and carried far by its circulation. 



3. Since the dark long-wave terrestrial radiation plays a large part in the 

 climatic problem, questions have been considered relative to (a) the portion 

 that escapes into space without absorption, (6) the portions that are absorbed 

 by the several atmospheric constituents, and (c) the secondary courses these 

 portions take, including the time-factors involved. 



The consideration of the parts played by the several atmospheric con- 

 stituents has been treated by grouping them into three classes, as follows: 



(a) Constituents that have Uttle or no power of absorbing (at ordinary 

 atmospheric temperatures) terrestrial radiation and equally little or no power 

 of radiating their own heat, however acquired. Nitrogen is the type of this 

 group, but oxygen, argon, and the rare atmospheric elements are grouped 

 with it for present convenience, with some reservations. 



(b) Such permanent constituents as have little power of absorbing incom- 

 ing solar radiation but readily absorb certain wave-lengths of the terrestrial 

 radiation, and radiate this, as well as heat received from other sources. 

 Carbon dioxide is the type and chief member of this group. 



(c) Such temporary constituents of the atmosphere as (1) consume heat 

 in their production by evaporation, (2) absorb readily terrestrial radiation 

 and less readily solar rays, (3) readily radiate heat, however received, so long 

 as they remain in the state of vapor, (4) give forth latent heat in passing back 

 to the liquid or solid state, and (5) become efficient reflectors in this liquid 

 or sohd state. 



RELATION OF INEFFECTIVE RADIATION TO THERMAL STORAGE. 



By far the greater mass of the " permanent" constituents of the atmosphere 

 are poor absorbers of solar rays (at least such as penetrate the lower atmos- 

 phere) and poor radiators of their own heat at ordinary atmospheric temper- 



^ Quart. Jour. Roy. Meteorological Soc, vol. 43, p. 151 (1917). 



