d to be assumed if
Sir George Darwin had not deduced it by means of purely mathematical
argument based upon astronomical facts.

Returning now to the problem of the moon's temperature, I think the
phenomena this presents may be in part due to the mode of formation here
described. For, its entire surface being the result of long-continued
gaseous explosions, all the volcanic products--scoriae, pumice, and
ashes--would necessarily be highly porous throughout; and, never having
been compacted by water-action, as on the earth, and there having been
no winds to carry the finer dust so as to fill up their pores and
fissures, the whole of the surface material to a very considerable depth
must be loose and porous to a high degree. This condition has been
further increased owing to the small power of gravity and the extreme
irregularity of the surface, consisting very largely of lofty cones and
ridges very loosely piled up to enormous heights.

Now this condition of the substance of the moon's surface is such as
would produce a high specific heat, so that it would absorb a large
amount of heat in proportion to the rise of temperature produced, the
heat being conducted downwards to a considerable depth. Owing, however,
to the total absence of atmosphere radiation would very rapidly cool the
surface, but afterwards more slowly, both on account of the action of
Stefan's law and because the heat stored up in the deeper portions could
be carried to the surface by conduction only, and with extreme slowness.

_Very's Researches on the Moon's Heat._

The results of the eclipse observations are supported by the detailed
examination of the surface-temperature of the moon by Mr. Very in his
_Prize Essay on the Distribution of the Moon's Heat_ (published by the
Utrecht Society of Arts and Sciences in 1891). He shows, by a diagram of
the 'Phase-curve,' that at the commencement of the Lunar day the surface
just within the illuminated limb has acquired about 1/7 of its maximum
temperature, or about 70 deg. F. abs. As the surface exposed to the
Bolometer at each observation is about 1/30 of the moon's surface, and
in order to ensure accuracy the instrument has to be directed to a spot
lying wholly within the edge of the moon, it is evident that the surface
measured has already been for several hours exposed to oblique sunshine.
The curve of temperature then rises gradually and afterwards more
rapidly, till it attains its maximum (of about +3