l moon. The _radiated_
heat, on the other hand, will reach its maximum when the average
temperature of that part of the moon's surface turned toward the earth
is highest; and this must be some time after full moon, for the same
sort of reasons that make the hottest part of a summer's day come two
or three hours after noon.

The conclusion early reached by Lord Rosse was that nearly all the
lunar heat belonged to the second category--dark heat _radiated_ from
the moon's warmed surface, the _reflected_ portion being comparatively
small--and he estimated that the temperature of the hottest parts of
the moon's surface must run as high as 500 deg. F.; well up toward the
boiling-point of mercury. Since the lunar day is a whole month long,
and there are never any clouds in the lunar sky, it is easy to imagine
that along toward two or three o'clock in the lunar afternoon (if I
may use the expression), the weather gets pretty hot; for when the sun
stands in the lunar sky as it does at Boston at two P.M., it has been
shining continuously for more than two hundred hours. On the other
hand, the coldest parts of the moon's surface, when the sun has only
just risen after a night of three hundred and forty hours, must have a
temperature more than a hundred degrees below zero.

Lord Rosse's later observations modified his conclusions, to some
extent, showing that he had at first underestimated the percentage of
simple reflected heat, but without causing him to make any radical
change in his ideas as to the maximum heat of the moon's surface.

For some time, however, there has been a growing skepticism among
astronomers, relating not so much to the correctness of his measures
as to the computations by which he inferred the high percentage of
obscure radiated beat compared with the reflected heat, and so deduced
the high temperature of lunar noon.

Professor Langley, who is now engaged in investigating the subject,
finds himself compelled to believe that the lunar surface never gets
even comfortably warm--because it has no blanket. It receives heat, it
is true, from the sun, and probably some twenty-five or thirty per
cent. more than the earth, since there are no clouds and no air to
absorb a large proportion of the incident rays; but, at the same time,
there is nothing to retain the heat, and prevent the radiation into
space as soon as the surface begins to warm. We have not yet the data
to determine exactly how much the temperature