s corroborative of Rosse's work, which was more elaborate and
extensive. Rosse considered that his results show that the heat from
the moon is mainly _obscure, radiated_ heat; the _reflected_ heat,
according to him, being much less in amount.

A moment's thought will show that the moon's heat must consist of two
portions. First, there will be _reflected solar heat_. The amount and
character of this will depend in no way upon the temperature of the
moon's surface, but solely upon its reflecting power. And it is to be
noted that moon-_light_ is only a part of this reflected radiant
energy, differing from the invisible portion of the same merely in
having such a wave-length and vibration period as to bring it within
the range of perception of the human eye.

The second portion of the heat sent us by the moon is that which she
emits on her own account as a warm body--warmed, of course, mainly, if
not entirely, by the action of the sun. The amount of _this_ heat will
depend upon the temperature of the moon's surface and its radiating
power; and the temperature will depend upon a number of things
(chiefly heat-absorbing power of the surface, and the nature and
density of the lunar atmosphere, as well as the supply of heat
received from the sun), being determined by a balance between give and
take. So long as more heat is received in a second than is thrown off
in the same time, the temperature will rise, and _vice versa_.

It is to be noted, further, that this second component of the moon's
thermal radiance must be mainly what is called "obscure" or dark heat,
like that from a stove or teakettle, and characterized by the same
want of penetrative power. No one knows why at present; but it is a fact
that the heat-radiations from bodies at a low temperature--radiations
of which the vibrations are relatively slow, and the wave-length
great--have no such power of penetrating transparent media as the
higher-pitched vibrations which come from incandescent bodies. A great
part, therefore, of this contingent of the lunar heat is probably
stopped in the upper air, and never reaches the surface of the earth
at all.

Now, the thermopile cannot, of course, discriminate directly between
the two portions of the lunar heat; but to some extent it does enable
us to do so indirectly, since they vary in quite a different way with
the moon's age. The simple _reflected_ heat must follow the same law
as moonlight, and come to its maximum at ful