referred to, we can understand that the heat from the interior passes
through it with such extreme slowness as not to be detected at the
surface, the varying temperatures of which are due entirely to solar
heat. A large portion of this heat is stored up in the surface soil, and
especially in the surface layer of the oceans and seas, thus partly
equalising the temperatures of day and night, of winter and summer, so
as greatly to ameliorate the rapid changes of temperature that would
otherwise occur. Our dense atmosphere is also of immense advantage to us
as an equaliser of temperature, charged as it almost always is with a
large quantity of water-vapour. This latter gas, when not condensed into
cloud, allows the solar heat to pass freely to the earth; but it has the
singular and highly beneficial property of absorbing and retaining the
dark or lower-grade heat given off by the earth which would otherwise
radiate into space much more rapidly. We must therefore always remember
that, very nearly if not quite, the _whole_ of _the warmth we experience
on the earth is derived from the sun._[8]

[Footnote 8: Professor J.H. Poynting, in his lecture to the British
Association at Cambridge in 1904, says: "The surface of the earth
receives, we know, an amount of heat from the inside almost
infinitesimal compared with that which it receives from the sun, and on
the sun, therefore, we depend for our temperature."]

In order to understand the immense significance of this conclusion we
must know what is meant by the _whole_ heat or warmth; as unless we know
this we cannot define what half or any other proportion of sun-heat
really means. Now I feel pretty sure that nine out of ten of the average
educated public would answer the following question incorrectly: The
mean temperature of the southern half of England is about 48° F.
Supposing the earth received only half the sun-heat it now receives,