When we are presented with a complex problem depending on a great number of imperfectly ascertained data, we may often check the results thus obtained by the comparison of cases in which some of the more important of these data are identical, while others are at a maximum or a minimum. In the present case we can do this by a consideration of the Moon as compared with the Earth and with Mars. _Langley's Determination of the Moon's Temperature._ In the moon we 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° 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._