It receives heat, it
is true, from the sun, and probably some twenty-five or thirty per
cent. more than the earth, since there are no clouds and no air to
absorb a large proportion of the incident rays; but, at the same time,
there is nothing to retain the heat, and prevent the radiation into
space as soon as the surface begins to warm. We have not yet the data
to determine exactly how much the temperature of the lunar rocks would
have to be raised above the absolute zero (-273 deg. C. or -459 deg. F.) in
order that they might throw off into space as much heat in a second as
they would get from the sun in a second. But Professor Langley's
observations, made on Mount Whitney at an elevation of fifteen
thousand feet, when the barometer stood at seventeen inches
(indicating that about fifty-seven per cent. of the air was still
above him), showed that rocks exposed to the perpendicular rays of the
sun were not heated to any such extent as those at the base of the
mountain similarly exposed; and the difference was so great as to make
it almost certain that a mass of rock not covered by a reasonably
dense atmosphere could never attain a temperature of even 200 deg.
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