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