244 Dr. J. Croll on some Controverted Points 
originally derived its cold, in like manner, from expansion. 
This is evident, for although he recognizes the effect of radia- 
tion 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 it 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 
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 con- 
sequence of loss from radiation into space. It is mainly to 
this cause, and not to the loss from expansion, as Professor 
Newcomb assumes, that the intense cold of the upper air is 
due. The air in that region has got beyond the screen which 
protected 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 noonday, as I have endeavoured to show iu 
Appendix, p. 551, the rays of a burning sun overhead would 
not be sufficient 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 con- 
clusion that in the absence of an atmosphere the earth's tem- 
perature of insolation would at any rate fall below — 50° F.; 
by which it is 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 exist- 
ing, it let radiations of all wave-lengths pass out as easily as 
they come in " (' Nature,' August 3rd, 1882). 
The temperature of the upper atmosphere, even after making 
allowance for heat received from below, must in this case 
be at least nearly 80 degrees below the freezing-point. The 
