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 deg.

F.
Supposing the earth received only half the sun-heat it now receives, what would then be the probable mean temperature of the South of England?
The majority would, I think, answer at once--About 24 deg.

F.
Nearly as many would perhaps say--48 deg.