The Problem of Terrestrial Temperatures.

In order that the problem may be understood and its importance appreciated, it is necessary to explain the now generally accepted principles as to the causes which determine the temperatures on our earth, and, presumably, on all other planets whose conditions are not wholly unlike ours.  The fact of the internal heat of the earth which becomes very perceptible even at the moderate depths reached in mines and deep borings, and in the deepest mines becomes a positive inconvenience, leads many people to suppose that the surface-temperatures of the earth are partly due to this cause.  But it is now generally admitted that this is not the case, the reason being that all rocks and soils, in their natural compacted state, are exceedingly bad conductors of heat.

A striking illustration of this is the fact, that a stream of lava often continues to be red hot at a few feet depth for years after the surface is consolidated, and is hardly any warmer than that of the surrounding land.  A still more remarkable case is that of a glacier on the south-east side of the highest cone of Etna underneath a lava stream with an intervening bed of volcanic sand only ten feet thick.  This was visited by Sir Charles Lyell in 1828, and a second time thirty years later, when he made a very careful examination of the strata, and was quite satisfied that the sand and the lava stream together had actually preserved this mass of ice, which neither the heat of the lava above it at its first outflow, nor the continued heat rising from the great volcano below it, had been able to melt or perceptibly to diminish in thirty years.  Another fact that points in the same direction is the existence over the whole floor of the deepest oceans of ice-cold water, which, originating in the polar seas, owing to its greater density sinks and creeps slowly along the ocean bottom to the depths of the Atlantic and Pacific, and is not perceptibly warmed by the internal heat of the earth.

Now the solid crust of the earth is estimated as at least about twenty miles in average thickness; and, keeping in mind the other facts just 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]