rresponding
temperature of the moon's sunlit surface Professor Langley considers to
be about that of freezing water.[950] Repeated experiments having failed
to get any thermal effects from the dark part of the moon, it was
inferred that our satellite "has no internal heat sensible at the
surface"; so that the radiations from the lunar soil giving the low
maximum in the heat-spectrum, "must be due purely to solar heat which
has been absorbed and almost immediately re-radiated." Professor
Langley's explorations of the terra incognita of immensely long
wave-lengths where lie the unseen heat-emissions from the earth into
space, led him to the discovery that these, contrary to the received
opinion, are in good part transmissible by our atmosphere, although they
are completely intercepted by glass. Another important result of the
Allegheny work was the abolition of the anomalous notion of the
"temperature of space," fixed by Pouillet at -140 deg. C. For space in
itself can have no temperature, and stellar radiation is a negligible
quantity. Thus, it is safe to assume "that a perfect thermometer
suspended in space at the distance of the earth or moon from the sun,
but shielded from its rays, would sensibly indicate the absolute
zero,"[951] ordinarily placed at -273 deg. C.

A "Prize Essay on the Distribution of the Moon's Heat" (The Hague),
1891, by Mr. Frank W. Very, who had taken an active part in Professor
Langley's long-sustained inquiry, embodies the fruits of its
continuation. They show the lunar disc to be tolerably uniform in
thermal power. The brighter parts are also indeed hotter, but not much.
The traces perceived of a slight retention of heat by the substances
forming the lunar surface, agreed well with the Parsonstown observations
of the total eclipse of the moon, January 28, 1888.[952] For they
brought out an unmistakable divergence between the heat and light
phases. A curious decrease of heat previous to the first touch of the
earth's shadow upon the lunar globe remains unexplained, unless it be
admissible to suppose the terrestrial atmosphere capable of absorbing
heat at an elevation of 190 miles. The probable range of temperature on
the moon was discussed by Professor Very in 1898.[953] He concluded it
to be very wide. Hotter than boiling water under the sun's vertical
rays, the arid surface of our dependent globe must, he found, cool in
the 14-day lunar night to about the temperature of liquid air.

Althoug