GEOLOGY. 331 



In coming into the atmosphere by evaporation much heat is abstracted 

 from the surface and the basal air. The vapor thus enters the atmosphere at 

 less than the mean temperature of its locality. It begins its work as a cooling 

 agency. At this stage it contrasts strongly with CO2, which usually comes 

 into the atmosphere with an excess of heat. 



After being brought into the atmosphere at thermal loss, water-vapor 

 serves — for the short period it remains vapor — as a very effective absorbent 

 of terrestrial radiation, a partial absorbent of solar radiation, and an effective 

 radiator of its own heat. This is well recognized and much emphasized. 

 The time-factors, however, are difficult to determine. If there are any care- 

 ful estimates of the mean time between its evaporation and its condensation 

 during which a molecule of water serves as vapor in the air, they are not 

 known to the writer. A considerable portion of vapor rises rather directly 

 from the evaporating surface to the clouds, and there rather promptly passes 

 back to the liquid or solid state. Another portion remains much longer in 

 the air as subsaturating moisture. A small residue persists under any 

 natural conditions and seems to serve as though it were a permanent water- 

 gas, but this part is doubtless merely a changing remnant, not a gas in which 

 the individual molecules persist. However this may be, the mean lifetime of 

 a molecule of H2O as vapor is short, and so the thermal balance between the 

 thermal cost of birth and the thermal recovery at death is a matter of moment. 

 The amount of energy given out on condensation is equivalent to that made 

 latent by evaporation, and such difference as there may be in effectiveness in 

 heating the atmosphere is essentially the difference between thermal energy 

 applied at the bottom of the atmosphere and that applied at levels higher up. 

 As the principle is the same as in the case next to be considered, space will be 

 saved by taking them together. 



As already noted, water-vapor absorbs a minor fraction of the incoming 

 solar rays but has greater capacity for absorbing outgoing rays. It is a 

 fair presumption, then, that if water-vapor did not absorb energy from the 

 solar rays as they come in, the vapor would catch the equivalent energy on its 

 way out in the form of terrestrial radiation. The question then arises 

 whether this ability to absorb incoming rays works to advantage in heating 

 the climatic zone near the earth's surface. It is obvious that the first ab- 

 sorptions of the incoming rays would take place at higher levels on the average 

 than the first absorptions of the outgoing terrestrial radiation. Hence, 

 the secondary series of absorptions and radiations of the former would offer 

 readier escape skyward than those of the latter and their active careers in the 

 atmosphere be less prolonged and effective. 



However, the main thermal work of water-vapor is the absorption of 

 terrestrial radiation and what follows this. The first absorptions are near the 

 earth's surface, from which the radiation starts, and the secondary series 

 of absorptions and radiations start low in the atmosphere and hence have 

 high chances of further absorptions. The secondary series takes much the 

 same general courses and is subject to much the same incidental conditions 

 as the secondary series of CO2 already sketched, and these do not need repeti- 

 tion here. Such differences as exist are mainly those connected with the 

 return of the water-vapor to the liquid or solid state. When the molecules 

 of water-vapor have an excess of heat over the adjacent constituents, they 

 feed into the thermal pockets offered by the nitrogen group, as in the case of 

 CO2, and likewise draw upon them and radiate heat away when their temper- 



