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.
Pages:
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143