24 DISCUSSIONS IN CLIMATOLOGY. 



of the rising air. This temperature is, indeed, much 

 below that which maintains at the surface, for the 

 simple reason that air hecomes cold by expansion 

 according to a definite and well-known law. Having 

 thus got his rising current constantly cooled off by 

 contact with the cold air of the upper regions, it has 

 to pass on its journey towards the poles," etc. (p. 267).* 



Here the cooling of the ascending air is attributed 

 to two causes — (1) the heat lost by expansion as the 

 air rises ; (2) the heat lost by contact with the colder 

 air through which the ascending air passes and with 

 which it mixes in the upper regions. But the two 

 may be resolved into one, viz., the heat lost by expan- 

 sion ; for the cold air, to which the ascending air 

 communicates its heat by contact, is assumed to have 

 originally derived its cold, in like manner, from 

 expansion. This is evident, for, although he recognizes 

 the effect of radiation into space, he assumes that this 

 loss is compensated by counter-radiation. The upper 

 regions are, he says, exposed to the radiation of the 

 sun on the one side, and of the earth's lower atmosphere 

 on the other, and there is no proof that these do not 

 equal the surface-temperature. And again, when the 

 air descends in high latitudes to the earth's surface, an 

 amount of heat will be evolved by compression equal 

 to that which is lost when it rose from the equator. 



Professor Newcomb has misapprehended not only 

 my meaning, but also the chief reason why the air in 

 the upper region is so intensely cold. Any one who 

 has read what I have stated in pp. 35-40, ' Climate 

 and Time,' regarding the temperature of space will 

 readily understand what I mean by the temperature 

 of the upper regions. By the temperature of stellar 

 space, it is not meant that space itself is a something 



* The italics are mine. 



