ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES, 537 
different from those in Columns VI. and VII. These are the limits for the bright 
side, and they imply nearly absolute zero on the dark side. If we regard Mars as 
resembling our moon, and take the moon’s effective average temperature as 297° A, 
the corresponding temperature for Mars is 240° A, and the highest temperature is 
-f X 337 = 270°. But the surface of Alars has probably a higher coefficient of 
absorption than the surface of the moon—it certainly has for light—so that we may 
put his effective average temperature on this supposition some few degrees above 
240° A, and his equatorial temperature some degrees higher still. 
It appears exceedingly probable, then, that whether we regard Mars as like the 
earth, or, going to the other extreme, as like the moon, the temperature of his surface 
is everywhere below tlie freezing-point of water. The only escape from this 
conclusion that I can see is by way of a supposition tliat an appreciable amount of 
heat is issuing from beneath his surface. 
We cannot draw any definite conclusions as to the temperatures of Mercury and 
Venus till Ave knoAv Avhether they have atmospheres and whether they rotate on their 
OAvn axes. If Ave make both these suppositions and further suppose that their conditions 
approximate to those of the ideal planet at their distances giAmn in Columns IV. 
and V., then they may Avell be surrounded by hot clouds, as is sometimes supposed, 
entirely screening their solid bodies from us. If, on the other liand, their atmospheres 
are ineffective as regulators and if they alAAnys present the same face to the sun, the 
liottest part of Mercury is probably jiot ffir from 050° A, and that of Venus not far 
from 500° A. 
If a comet consist of small solid particles of diameter of the order 1 centim. or less, 
then the temperatures of these particles are giAmn in Column IX. At one-cpiarter of 
the earth’s distance, say 23 million miles from the sun, the temperature is 600°, about 
the melting-point of lead. At one-tAventy-fifth, say 3|- million miles, it will be about 
1500°, say the melting point of cast-iron. Nearer than this the temperature no 
doubt increases rapidly, but the laAv of temperature, deduced from the inverse square 
laAv for the radiation receiA’ed, requires amendment, as that laAV AA^as based on the 
supposition that a hemisphere only is lighted by the sun, and that the whole of his 
disc is visible from every part of that hemisphere. Both of these suppositions cease 
to hold when the distance from the sun is only a small multiple of his radius. 
PART II. 
Radiation Phessures. 
The pressure of radiation against a surface on AAdiich it falls, first deduced by 
Maxwell from the Electromagnetic Theory of Light, is iioav estabhshed on an 
experimental basis by the Avork of Lebedeav, confirmed hy that of Nichols 
and Hell. 
3 z 
VOL. CCII. 
A. 
