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 degrees in little more than 12 hours.[10] In the high desert plains of Central Asia the extremes are said to be even greater.[11] Again, in his Universal Geography, Reclus states that in the Armenian Highlands the thermometer oscillates between 13 deg.  F. and 112 deg.F.  We may therefore, without any fear of exaggeration, take it as proved that a fall of 100 deg.  F. in twelve or fifteen hours not infrequently occurs where there is a very dry and clear atmosphere permitting continuous insolation by day and rapid radiation by night.

[Footnote 10:  Keith Johnston’s ‘Africa’ in Stanford’s Compendium.]

[Footnote 11:  Chambers’s Encyclopaedia, Art.  ‘Deserts.’]

Now, as it is admitted that our dense atmosphere, however dry and clear, absorbs and reflects some considerable portion of the solar heat, we shall certainly underestimate the radiation from the moon’s surface during its long night if we take as the basis of our calculation a lowering of temperature amounting to 100 deg.  F. during twelve hours, as not unfrequently occurs with us.  Using these data—­with Stefan’s law of decrease of radiation as the 4th power of the temperature—­a mathematical friend finds that the temperature of the moon’s surface would be reduced during the lunar night to nearly 200 deg.  F. absolute (equal to-258 deg.  F.).

More Rapid Loss of Heat by the Moon.