We now set the machines going very slowly and moved toward the northern part of the moon, where I pointed out the position of the lunar north pole, and explained that, owing to the very slight inclination of the lunar axis, there can be but very little variation of seasons in any one particular part of the moon. Thus, if at one place it were spring, it would practically always be spring there, but with very cold nights all through the lunar year. Where it was summer it would practically always be summer, also with very cold nights, and so on.
I further explained that, as the moon revolves on its axis in the same time that it takes to make one revolution round the earth, those on the earth always see the same side of the moon, except when occasionally, owing to inequalities in the lunar motions, they are afforded a peep just round portions of the edges at different periods. The remainder of the other side of the moon has never yet been seen from the earth by human eyes, and in all probability never will be seen for millions of years to come.
John, who as usual was smoking like a factory chimney, here removed his pipe from his mouth and said, "Professor, you stated just now that the nights on the moon would always be intensely cold, and I should like to know whether there is any really reliable information respecting the temperature of the lunar days and nights. I have seen so many contradictory theories on the subject that I scarcely know what to believe."
"In that respect," I answered, "I fear that my position is much the same as yours, for I have absolutely no certain knowledge on the point, but will just state shortly how the matter stands at present.
"During the past century many investigations have been made by scientific men respecting the temperatures on the moon, and their results have differed to an amazing extent. It would take too long, and be too wearisome, to quote all the authorities, so a few must suffice. Lord Rosse, who used a thermopile in his experiments, found that in order to produce the results he obtained, the sunlit surface of the moon must be heated to a temperature of 500 degrees on Fahrenheit's scale. Sir John Herschel had previously concluded that the temperature must be much greater than that of boiling water. On the other hand Ericsson and, more recently, Professor Langley—who used a bolometer of his own invention for measuring the heat of the sun's invisible rays—came to the conclusion that even under continued sunshine the temperature would rarely, if ever, rise above the freezing point of water.
"Professor Very, however, who has continued the delicate experiments with the bolometer, and also made other experiments and calculations of quite recent date, has obtained results more nearly in accordance with those first quoted, for he found that near the end of the second week's sunshine on the moon the temperature of the rocks, soil, &c., must rise to at least 80 degrees Centigrade above the heat of boiling water."
"My word!" said M'Allister, "that's hotter than a ship's engine-room, and I shouldn't care for such a very high temperature."
"As this is so recent," I proceeded, "and the work of one of our highest authorities, I think we must accept it as being more correct, especially as Professor Very has taken into consideration some factors which had not previously been allowed due weight.