t 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. F. is 16 deg. above the freezing
point; therefore half the heat received would bring us down to 8 deg. above
the freezing point, or 40 deg. F. Very few, I think, would realise that our
share of half the amount of sun-heat received by the earth would
probably result in reducing our mean temperature to about 100 deg. F. below
the freezing point, and perhaps even lower. This is about the very
lowest temperature yet experienced on the earth's surface. To understand
how such results are obtained a few words must be said about the
absolute zero of temperature.

_The Zero of Temperature._

Heat is now believed to be entirely due to ether-vibration, which
produces a correspondingly rapid vibration of the molecules of matter,
causing it to expand and producing all the phenomena we term 'heat.' We
can conceive this vibration to increase indefinitely, and thus there
would appear to be no necessary limit to the amount of heat possible,
but we cannot conceive it to decrease indefinitely at the same uniform
rate, as it must soon inevitably come to nothing. Now it has been found
by experiment that gases under uniform pressure expand 1/273 of their
volume for each degree Centigrade of increased temperature, so that in
passing from 0 deg. C. to 273 deg. C. they are doubled in volume. They also
decrease in volume at the same rate for each degree below 0 deg. C. (the
freezing point of water). Hence if this goes on to-273 deg. C. a gas will
have no volume, or it will undergo some change of nature. Hence this is
called the zero of temperature, or t