SUPPLEMENTARY REPORT ON METEOROLOGY. 79 
parate these two united effects ; their relation and absolute in- 
tensities are therefore, we presume, yet unknown; nor can we 
understand how it is possible that the higher strata of the at- 
mosphere can remain permanently colder than the strata be- 
neath and the sky abeve them, without admitting a paradox of 
the same kind with a mechanical perpetual motion. 
114. Iam aware that M. Pouillet, in his memoir so often cited, 
has been led to the same conclusion with M. Poisson respecting 
the low temperature of the aerial coating of the globe, and has 
actually computed the depression of temperature which might 
exist in various hypothetical cases. Until, however, M. Pouillet 
can exhibit an experiment in which so paradoxical a result is 
actually attained, I am inclined to think that we must consider 
it as at variance with the fundamental laws of heat as at present 
received. M. Pouillet has made many ingenious experiments 
with his instrument designed to measure nocturnal radiation ; 
and (since the cooling effect of the sky may always be assimi- 
lated to that of a hollow sphere having a determinate tempera- 
ture) he has, I believe for the first time, endeavoured to translate 
into numerical language the indications of his instrument by 
actually exposing it to the radiating action of surfaces of low 
temperature, which would have the same effect, whatever be their 
distance, and that effect being known in thermumetric degrees 
on his instrument, the eguivadent temperature of the vault of 
heaven, or what he calls zenithal temperature, becomes known 
too. All this is very ingenious and clear, and such determina- 
tions of zenithal temperatures will certainly one day be of great 
value. But the difficulty is to separate this temperature into 
that due to the atmosphere, and that due to the temperature of 
space. As already stated, we are not satisfied that such a sepa- 
ration has been, or at present can be effected; and the great 
variations of the assigned temperatures of space strengthen this 
doubt; for whilst Fourier and Swanberg make it — 40° c.*, Valz, 
— 45°+, Poisson makes it 13°f, and Pouillet between — 115° 
and — 175°§. 
115. Fourier was the first who distinctly introduced the idea 
of the proper temperature of space, as well as the first who endea~ 
voured to assign to it avalue. Our ideas about an absolute zero 
* See last Report, p. 203, and Fourier, Mém. de l'Institut, vii. p. 598. The 
grounds on which Fourier’s estimate is made, nowhere exactly appear; Her- 
schel considers his published statements unsatisfactory (Geol. Trans. III. 297); 
yet it appears that M. Fourier himself was strongly persuaded of the truth of 
his estimate, which he thought was not erroneous to the amount of 8° or 10° 
cent. (Arago’s Eloge de Fourier, p. 55.) 
+ Mahlmann, p. 14, note. t Théorie, p. 520. 
§ Mémoire sur la Chaleur Solaire, p. 38. 
