neral
Astronomy_ (1898), makes the mean temperature of Mars 223.6 deg. absolute,
by using Newton's law of heat being radiated in proportion to
temperature, and 363 deg. abs. (=-96 deg. F.) by Dulong and Petit's law; but
adds, that a closer determination has been made by Professor Moulton,
using Stefan's law, that radiation is as the _/4th_ power of the
temperature, whence results a mean temperature of-31 deg. F. These estimates
assume identity of atmospheric conditions of Mars and the Earth.

But as none of these estimates take account of the many complex factors
which interfere with such direct and simple calculations, Mr. Lowell
then proceeds to enunciate them, and work out mathematically the effects
they produce:

(1) The whole radiant energy of the sun on striking a planet becomes
divided as follows: Part is reflected back into space, part absorbed by
the atmosphere, part transmitted to the surface of the planet. This
surface again reflects a portion and only the balance left goes to warm
the planet.

(2) To solve this complex problem we are helped by the _albedoes_ or
intrinsic brilliancy of the planets, which depend on the proportion of
the visible rays which are reflected and which determines the
comparative brightness of their respective surfaces. We also have to
find the ratio of the invisible to the visible rays and the heating
power of each.

(3) He then refers to the actinometer and pyroheliometer, instruments
for measuring the actual heat derived from the sun, and also to the
Bolometer, an instrument invented by Professor Langley for measuring the
invisible heat rays, which he has proved to extend to more than three
times the length of the whole heat-spectrum as previously known, and
has also shown that the invisible rays contribute 68 per cent, of the
sun's total energy.[9]

[Footnote 9: For a short account of this remarkable instrument, see my
_Wonderful Century_, new ed., pp. 143-145.]

(4) Then follows an elaborate estimate of the loss of heat during the
passage of the sun's rays through our atmosphere from experiments made
at different altitudes and from the estimated reflective power of the
various parts of the earth's surface--rocks and soil, ocean, forest and
snow--the final result being that three-fourths of the whole sun-heat
is reflected back into space, forming our _albedo_, while only
one-fourth is absorbed by the soil and goes to warm the air and
determine our mean temperature.

(5)