rth's surface solid copper or
solid silver, both most admirable conductors of heat, the cooling down
of this vast globe would be an extremely tardy process; how much more
tardy must it therefore be when such exceedingly bad conductors as
rocks form the envelope? How imperfectly material of this kind will
transmit heat is strikingly illustrated by the great blast iron
furnaces which are so vitally important in one of England's greatest
manufacturing industries. A glowing mass of coal and iron ore and
limestone is here urged to vivid incandescence by a blast of air
itself heated to an intense temperature. The mighty heat thus
generated--sufficient as it is to detach the iron from its close
alliance with the earthy materials and to render the metal out as a
pure stream rushing white-hot from the vent--is sufficiently confined
by a few feet of brick-work, one side of which is therefore at the
temperature of molten iron, while the other is at a temperature not
much exceeding that of the air. We may liken the brick-work of a blast
furnace to the rocky covering of the earth; in each case an
exceedingly high temperature on one side is compatible with a very
moderate temperature on the other.

Although the drainage of heat away from the earth's interior to its
surface, and its loss there by radiation into space, is an extremely
tardy process, yet it is incessantly going on. We have here again to
note the ability for gigantic effect which a small but continually
operating cause may have, provided it always tends in the same
direction. The earth is incessantly losing heat; and though in a day,
a week, or a year the loss may not be very significant, yet when we
come to deal with periods of time that have to be reckoned by millions
of years, it may well be that the effect of a small loss of heat per
annum can, in the course of these ages, reach unimagined dimensions.
Suppose, for instance, that the earth experienced a fall of
temperature in its interior which amounted to only one-thousandth of a
degree in a year. So minute a quantity as this is imperceptible. Even
in a century, the loss of heat at this rate would be only the tenth of
a degree. There would be no possible way of detecting it; the most
careful thermometer could not be relied on to tell us for a certainty
that the temperature of the hot waters of Bath had declined the tenth
of a degree; and I need hardly say, that the fall of a tenth of a
degree would signify nothing i