e see the conditions that prevail in Mars both exaggerated
and simplified. Mars has a very scanty atmosphere, the moon none at all,
or if there is one it is so excessively scanty that the most refined
observations have not detected it. All the complications arising from
the possible nature of the atmosphere, and its complex effects upon
reflection, absorption, and radiation are thus eliminated. The mean
distance of the moon from the sun being identical with that of the
earth, the total amount of heat intercepted must also be identical; only
in this case the whole of it reaches the surface instead of one-fourth
only, according to Mr. Lowell's estimate for the earth.

Now, by the most refined observations with his Bolometer, Mr. Langley
was able to determine the temperature of the moon's surface exposed to
undimmed sunshine for fourteen days together; and he found that, even in
that portion of it on which the sun was shining almost vertically, the
temperature rarely rose above the freezing point of water. However
extraordinary this result may seem, it is really a striking confirmation
of the accuracy of the general laws determining temperature which I have
endeavoured to explain in the preceding chapter. For the same surface
which has had fourteen days of sunshine has also had a preceding
fourteen days of darkness, during which the heat which it had
accumulated in its surface layers would have been lost by free radiation
into stellar space. It thus acquires during its day a maximum
temperature of only 491 deg. F. absolute, while its minimum, after 14 days'
continuous radiation, must be very low, and is, with much reason,
supposed to approach the absolute zero.

_Rapid Loss of Heat by Radiation on the Earth._

In order better to comprehend what this minimum may be under extreme
conditions, it will be useful to take note of the effects it actually
produces on the earth in places where the conditions are nearest to
those existing on the moon or on Mars, though never quite equalling, or
even approaching very near them. It is in our great desert regions, and
especially on high plateaux, that extreme aridity prevails, and it is in
such districts that the differences between day and night temperatures
reach their maximum. It is stated by geographers that in parts of the
Great Sahara the surface temperature is sometimes 150 deg. F., while during
the night it falls nearly or quite to the freezing point--a difference
of 118 degre