RADIATION AND ABSORPTION, 75 
and variable temperature of the column of air, and with the two 
unequal absorbing powers of the atmosphere. 
19, Let us now attempt to indicate how it is possible to ob- 
serve the zenithal temperature each instant of the night, nearly 
as we observe the temperature of the air. 
I have employed two methods for this purpose: one which 
rests on the employment of mirrors, and the other on the employ- 
ment of a novel instrument which I call actinometer; we know 
that this name is already applied to a very important invention 
of Sir J. Herschel, and it seems to me happily chosen by that 
illustrious astronomer to designate all kinds of apparatus the 
object of which is to measure the effects of radiation, whatever 
be the principle of their construction. 
2e53) 
NTN | 
‘ 
DM 
It will be sufficient to indicate here the second method: I 
shall only remark with regard to the first, that the cooling which 
is observed at the focus of a mirror whose axis is directed to- 
wards the zenith, does not depend upon the concentration of the 
rays, as has been hitherto supposed ; a simple plate of polished 
metal, or rather an expanded cone, produces nearly the same 
effect, so that I have found it possible to substitute for the mir- 
rors reflectors of this kind which are much more convenient. 
However, with the reflectors, as with the mirrors, the experi- 
ments are delicate and the formule very complicated ; they con- 
