ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES. 
529 
Integrating from 0 — 0 to 6 = ttI2, we have 
Total illumination = = 4G,000 B. 
The illumination from a hemisphere paved with suns Is therefore 46,000 X 4 X 10® 
= 1'84 X 10^^ times that from the stellar sky. 
If we assume that the (piality of the radiation is the same In both cases, that is, 
if we assume that the energy is proportional to the light part of the spectrum, we 
have hy the fourth-power law 
Effective temperature of space = 
effective temperature of sun 
(•184 X lO^Y^ 
__ effective temperature of sun 
~ 655 
As the temperature of the sun probably lies between 6000 ° A and 7000 ° A, 
this gives 
Effective temperature of space — 10° A. 
If, then, a body is raised ly the sun to even such a small multiple of 10° as, Say, 60®, 
the fourth-power law of ludiation imj)lies that it is giving out and tlierefore receiving 
from the sun more than a thousand times as mucii energy as it is receiving from 
the sky. 
The sky radiation may therefore be left out of the account when we are dealing 
with approximate estimates and not with exact results, and bodies in the solar 
system may be regarded as being situated In a zero enclosure except in so far as they 
receive I’adiatlon from the sun. 
Temperature of a Planet under Certain Assumed Conditions when at a 
distance from the Sun ecpied to that of the Earth. 
The real earth presents a problem of complexity far too great to deal with. I 
shall therefore consider an ideal earth for which certain conditions hold, more or less 
approximating to reality, and determine the temperature of its surface on the 
assumption that it receives heat from the sun only. 
Let us suppose :— 
1. That the planet Is rotating about an axis perpendicular to the plane of Its orbit, 
wliich Is circular. 
This will give us too high a temperature at the equator, and the absolute zero, 
which is too low, at the poles. The mean, however, over the planet, will probably be 
not much affected hy the supposition. 
2. That tiie effect of the atmosphere is to keep the temperature In any given 
latitude the same, day and night. 
Tills is not a great departure frcan reality. On the sea, which Is more than two- 
VOL. CCD.—A. 3 Y 
