252 J. Croll—Geological Climatology. 
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 it lost 
when it rose from the equator. 
Professor Newcomb has misapprehended not only my mean- 
ing, but also the chief reason why the air in the upper regions 
is so intensely cold. Any one who has read what I have 
stated in pp. 35-40, ‘Climate and Time,’ regarding the tem- 
perature 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 
possessed of a given temperature, say —239°F. It simply 
means the temperature to which a body would fall were it 
exposed to no other source of heat than that of radiation from 
the stars. By the temperature of the upper regions, I mean 
the temperature to which air in those regions sinks in conse- 
quence of loss from radiation into space. It is mainly to this 
‘cause, and not to the loss from expansion, as Professor New- 
comb assumes, that the intense cold of the upper air is due. 
The air in that region has got beyond the screen which pro- 
tected it when at the earth’s surface, and it then throws off its 
heat into space during twelve hours of night, getting no return 
from without except from the radiation of the stars. And even 
at noon-day, as I have endeavored to show in Appendix, p. 
551, the rays of a burning sun over head would not be sufli- 
cient to raise the temperature of the air up to the freezing 
point. But the recent observations of Professor Langley prove 
that the loss of heat from radiation is in reality far greater 
than I had anticipated. He says: “ The original observations, 
which will be given at length, lead to the conclusion that 1 
the absence of an atmosphere the earth’s temperature of insola- 
tion would at any rate fall below —50° F., by which it 1s 
meant that, for instance, mercury would remain a solid under 
the vertical rays of a tropical sun were radiation into space 
wholly unchecked, or even if, the atmosphere existing, it let 
radiations of all wave-lengths pass out as easily as they come 
in.”—* Nature,’ Aug. 3d, 1882. 
The temperature of the upper atmosphere, even after mak- 
_ ing allowance for heat received from below, must in this case 
be at least nearly 80 degrees below the freezing point. The 
quantity of heat lost by expansion must, therefore, be . trifling 
compared with that lost by radiation ; and although the heat 
lost by expansion is fully restored by compression, yet the ait 
would reach the earth deprived almost entirely of the heat with — 
& 
