RADIATION AND ABSORPTION. 79 
the limits for the temperature of space; but the phenomena 
which are manifested in the equatorial regions, and which there 
prevail in a constant manner throughout the year, lead to another 
fundamental equation, from which we may derive the tempera- 
ture of space without having recourse to the mean temperature 
of the atmospheric column. 
In fact, in the equatorial zone, the surface of the earth, in- 
cluding the atmosphere which covers it, may be considered as 
a cylinder, the tropical circles of which would form the two bases, 
and the half of which is always illuminated by the sun. This 
cylinder receives each instant all the heat which falls upon the 
rectangle of its projection, the surface of which is 27h; it re- 
ceives therefore each minute a quantity of heat 
1:7633 .2rh. 
But this quantity of heat being distributed over the whole late- 
ral surface of the cylinder or upon an extent 2 77, it is evident 
that each unity would only receive for its share 
£7638 == O56. 
7 
Such is the quantity of solar heat which on an average falls 
daily every minute on each square centimetre of the equatorial 
zone. 
At the same time the heat of space also exerts its action, and 
if we designate by ¢/ the unknown temperature of space, it is 
easy to see that the quantity of heat received per minute and 
per square centimetre is 
Ba". 
Consequently the sum of the quantities of heat received is 
Ba" + 0°56. 
But the combined effects of space and of the sun may be re- 
placed by a single inclosure, of a maximum emissive power ; and 
if we represent by v the unknown temperature of this inclosure, 
capable of producing the same effects, or rather capable of trans- 
mitting the same quantity of heat, we shall have 
Ba’ = Ba" +056; 
it is true that the action of the sun is intermittent, since it ceases 
to be felt during the night, and during the day it is felt with 
different intensities at different hours ; but these intermittences, 
producing the variations of temperature which we observe du- 
ring the day and the night, do not interfere with the exactness 
of the preceding equation ; nor do they prevent the conditions 
