68 M. POUILLET ON SOLAR HEAT, 
larger which would not transmit heat to us, because the corre- 
sponding lines are prolonged indefinitely in space. 
We thus understand that the heat of space may be assimilated 
to the solar heat in its nature and origin, if not in its quantity, 
and that the atmosphere consequently exerts upon it the same 
absorption. This being established, the general conditions of 
equilibrium of the diathermanous envelopes which we have dis- 
cussed above find here their direct application: it is sufficient 
to admit that the globe which we have supposed of any given 
dimensions, is the globe of the earth; that the inclosure is that 
which represents the unknown temperature of space ; and, lastly, 
that the diathermanous envelope is nothing else than the atmo- 
sphere supposed at first without clouds, and possessing the pro- 
perty of absorbing only in the perpendicular direction about 20 
or 25 hundredth parts of incident heat, as we have found by the 
experiments on the solar heat related above. As the absorbing 
action which the atmosphere exerts upon the rays emitted by the 
earth is necessarily greater, it results that all the consequences 
at which we have arrived apply to the equilibrium of the terres- 
trial temperatures. 
In consequence, the phenomena which occur without the 
action of the sun and without the effects of the interior heat of 
the globe are the following :— 
1. The temperature of the surface of the earth is considerably 
higher than the temperature of space: 
2. The mean temperature of the atmosphere is necessarily 
lower than the temperature of space, and still more than the 
temperature of the earth itself: 
3. The decrease of the temperature in the atmosphere is not at 
all due to the periodical action of the sun, nor to the ascending 
and descending currents which that action may determine near 
the surface of the earth; it will even take place when the sun 
would not heat either the earth or the atmosphere, because it is 
one of the conditions of equilibrium of diathermanous envelopes, 
and its real cause lies in the unequal absorbing actions which 
the atmosphere exerts upon the rays of heat derived from space, 
and upon those which are emitted all round the globe by the 
surface of the soil or by that of the seas. 
M. Fourier is, I think, the first who has had the idea of re- 
garding the unequal absorption of the atmosphere as exercising 
