Supposing that the globe is merely a solid crust, resting upon fluid or semi-fluid matter, whether extending to the centre or not, the transfer of pressure from one part of its surface to another by the degradation of existing continents, and the formation of new ones, would be sufficient to subvert the equilibrium of heat in the interior, and occasion volcanic eruptions. For, since the internal heat of the earth is transmitted outwards by radiation, an accession of new matter on any part of the surface, like an addition of clothing, by keeping it in, would raise the temperature of the strata below, and in the course of ages would even reduce those at a great depth to a state of fusion. Some of the substances might be converted into gases; and should the accumulation of new matter take place at the bottom of the sea, as is generally the case, this lava would be mixed with water in a state of ignition in consequence of the enormous pressure of the ocean, and of the newly superimposed matter which would prevent it from expanding into steam. Now Sir Charles Lyell has shown, with his usual talent, that the quantity of matter carried down by rivers from the surface of the continents is comparatively trifling, and that the great transfer to the bottom of the ocean is produced at the coast-line by the action of the sea; hence, says Sir John Herschel, “the greatest accumulation of local pressure is in the central area of the deep sea, while the greatest local relief takes place along the abraded coast-lines. Here then should occur the chief volcanic vents.” As the crust of the earth is much weaker on the coasts than elsewhere, it is more easily ruptured, and, as Mr. Babbage observes, immense rents might be produced there by its contraction in cooling down after being deprived of a portion of its original thickness. The pressure on the bottom of the ocean would force a column of lava mixed with ignited water and gas to rise through an opening thus formed, and, says Sir John Herschel, “when the column attains such a height that the ignited water can become steam, the joint specific gravity of the column is suddenly diminished, and up comes a jet of mixed steam and lava, till so much has escaped that the matter deposited at the bottom of the ocean takes a fresh bearing, when the evacuation ceases and the crack becomes sealed up.”
This theory perfectly accords with the phenomena of nature, since there are very few active volcanoes at a distance from the sea, and the exceptions that do occur are generally near lakes, or they are connected with volcanoes on the maritime coasts. Many break out even in the bottom of the ocean, probably owing to some of the supports of the superficial crust giving way, so that the steam and lava are forced up through the fissures.
Finally, Mr. Babbage observes that, “in consequence of changes continually going on, by the destruction of forests, the filling up of seas, the wearing down of elevated lands, the heat radiated from the earth’s surface varies considerably at different periods. In consequence of this variation, and also in consequence of the covering up of the bottom of the sea by the detritus of the land, the surfaces of equal temperature within the earth are continually changing their form, and exposing thick beds near the exterior to alterations of temperature. The expansion and contraction of these strata may form rents and veins, produce earthquakes, determine volcanic eruptions, elevate continents, and, possibly, raise mountain chains.”
The numerous vents for the internal heat formed by volcanoes, hot springs, and the emission of steam, so frequent in volcanic regions, no doubt maintain the tranquillity of the interior fluid mass, which seems to be perfectly inert unless when put in motion by unequal pressure.
But, to whatever cause the increasing heat of the earth and the subterranean fires may ultimately be referred, it is certain that, except in some local instances, they have no sensible effect on the temperature of its surface. It may therefore be concluded that the heat of the earth, above the zone of uniform temperature, is entirely owing to the sun.
The power of the solar rays depends much upon the manner in which they fall, as we readily perceive from the different climates on our globe. Although the sun is about three millions of miles nearer to the earth in winter than in summer, his rays strike the atmosphere in the northern hemisphere so obliquely that it absorbs a much greater quantity of heat than when they are more direct ([N. 217]). Indeed it is so great that, when the sun has an altitude of 30°, one half of his heat is absorbed by the atmosphere, and it increases very rapidly as he sinks towards the horizon. However, that heat is not lost: it is most beneficial to the earth, being really the heat which supplies the greatest part of that which is radiated into space during the absence of the sun. Professor Dove has shown, by taking at all seasons the mean of the temperatures of points on the earth’s surface diametrically opposite to each other, that the average temperature of the whole earth’s surface in June, when we are farthest from the sun, considerably exceeds that in December, when we are nearest to him, owing to the excess of water in the southern hemisphere, and that of land in the northern, which gives a general insular climate to the former, and a continental climate to the latter.
The observations of the north polar navigators, and those of Sir John Herschel at the Cape of Good Hope, show that the direct heating influence of the solar rays is greatest at the equator, and that it diminishes gradually as the latitude increases. At the equator the maximum is 483⁄4°, while in Europe it has never exceeded 291⁄2°.
M. Pouillet has estimated with singular ingenuity, from a series of observations made by himself, that the whole quantity of heat which the earth receives annually from the sun is such as would be sufficient to melt a stratum of ice covering the whole globe 46 feet deep. Part of this heat is radiated back into space; but by far the greater part descends into the earth during the summer, towards the zone of uniform temperature, whence it returns to the surface in the course of the winter, and tempers the cold of the ground and the atmosphere in its passage to the ethereal regions, where it is lost, or rather where it combines with the radiation from the other bodies of the universe in maintaining the temperature of space. The sun’s power being greatest between the tropics, the heat sinks deeper there than elsewhere, and the depth gradually diminishes towards the poles; but the heat is also transmitted laterally from the warmer to the colder strata north and south of the equator, and aids in tempering the severity of the polar regions.
The mean heat of the earth, above the stratum of constant temperature, is determined from that of springs; and, if the spring be on elevated ground, the temperature is reduced by computation to what it would be at the level of the sea, assuming that the heat of the soil varies according to the same law as the heat of the atmosphere, which is about 1° of Fahrenheit’s thermometer for every 333·7 feet. From a comparison of the temperature of numerous springs with that of the air, Sir David Brewster concludes that there is a particular line passing nearly through Berlin, at which the temperature of springs and that of the atmosphere coincide; that in approaching the arctic circle the temperature of springs is always higher than that of the air, while, proceeding towards the equator, it is lower.
Since the warmth of the superficial strata of the earth decreases from the equator to the poles, there are many places in both hemispheres where the ground has the same mean temperature. If lines were drawn through all those points in the upper strata of the globe which have the same mean annual temperature, they would be nearly parallel to the equator between the tropics, and would become more and more irregular and sinuous towards the poles. These are called isogeothermal lines. A variety of local circumstances disturb their parallelism, even between the tropics.