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.