RADIATION AND ABSORPTION. 7a 
whilst the air is in general very humid under the equator, on 
account even of its temperature. 
18. A thermometer which is exposed on the ground to the 
nocturnal radiation receives heat from two different sources, 
namely from space and from the atmosphere. The heat of 
space being submitted to absorption like the solar heat during 
its atmospheric passage, only three- or four-tenths of it can in 
general reach the thermometer, at least supposing that the ex- 
periments are not made on high mountains. With respect to 
the heat emitted by the atmosphere itself in the course of the 
night, it is the effect of the individual radiation of all the con- 
centric strata which we can imagine from the level of the sea 
up to the limits of the atmosphere, and it depends consequently 
on the distribution of the temperatures throughout the whole 
height of the atmospheric column; we may add, that its influ- 
ence is much more considerable than has hitherto been supposed. 
For the rest, whatever be the relation of the intensities of these 
two causes, it is evident that we may conceive a single cause 
capable of producing an effect equal to that which results from 
their simultaneous action; or, in other terms, we may suppress 
in thought the heat of space and that of the atmosphere, and 
conceive an inclosure, of a maximum emissive power, the tem- 
perature of which is such that it imparts to the thermometer 
and to the ground precisely as much heat as they receive at once 
from the atmosphere and from space; this is the unknown tem- 
perature of that zenithal inclosure which I term the zenithal 
temperature. 
The object of this manner of viewing the phenomena is not 
to represent the peculiar and perhaps unequal actions which 
the thermometer experiences in such or such a direction, but 
only to represent with exactness the definitive and total action 
to which it is submitted, so that its depression below the am- 
bient temperature is the same with the zenithal inclosure as with 
the atmosphere and space united. It is under this condition 
that we are now permitted to assign to the zenithal inclosure 
a uniform temperature in all the portions of its extent. In short, 
it is evident that the zenithal temperature is necessarily variable 
at each instant for the same point of the surface of the earth, 
and still more variable for one point than another, because it is 
composed of a fixed element, which is the temperature of space, 
and of an element incessantly changing, which is the tempera- 
ture of the different atmospheric strata. 
