s the extraordinary difference in mean and extreme
temperatures of two bodies situated at the same distance from the sun,
and therefore receiving exactly the same amount of solar heat per unit
of surface. We have learned also what are the main causes of this almost
incredible difference, namely: (1) a remarkably rugged surface with
porous and probably cavernous rock-texture, leading to extremely rapid
radiation of heat in the one; as compared with a comparatively even and
well-compacted surface largely clad with vegetation, leading to
comparatively slow and gradual loss by radiation in the other: and (2),
these results being greatly intensified by the total absence of a
protecting atmosphere in the former, while a dense and cloudy atmosphere
with an ever-present supply of water-vapour, accumulates and equalises
the heat received by the latter.

The only other essential difference in the two bodies which may possibly
aid in the production of this marvellous result, is the fact of our day
and night having a mean length of 12 hours, while those of the moon are
about 14-1/2 of our days. But the altogether unexpected fact, in which
two independent enquirers agree, that during the few hours' duration of
a total eclipse of the moon so large a proportion of the heat is lost by
radiation renders it almost certain that the resulting low temperature
would be not very much less if the moon had a day and night the same
length as our own.

The great lesson we learn by this extreme contrast of conditions
supplied to us by nature, as if to enable us to solve some of her
problems, is, the overwhelming importance, first, of a dense and
well-compacted surface, due to water-action and strong gravitative
force; secondly, of a more or less general coat of vegetation; and,
thirdly, of a dense vapour-laden atmosphere. These three favourable
conditions result in a mean temperature of about +60 deg. F. with a range
seldom exceeding 40 deg. above or below it, while over more than half the
land-surface of the earth the temperature rarely falls below the
freezing point. On the other hand, we have a globe of the same materials
and at the same distance from the sun, with a maximum temperature of
freezing water, and a minimum not very far from the absolute zero, the
monthly mean being probably much below the freezing point of
carbonic-acid gas--a difference entirely due to the absence of these
three favourable conditions.

_The Special Features of