The increase of temperature observed as we descend below the earth's surface, as well as other phenomena, have led to a very general opinion, that great heat exists in the interior of the earth, and that the body of our planet, having been at one time intensely heated, has cooled down to its present temperature. With the view of pointing out courses of inquiry, by which these opinions may ultimately be tested by observation, it may be expedient to take a cursory view of some of the consequences of such an hypothesis.
And first, let us imagine the exterior of our globe to have once been in a state of intense heat. No fluid such as water could then have existed on the surface: it would instantly have been converted into vapour; and [204/205] notwithstanding the increased weight of atmosphere thus produced and pressing on the surface of the globe, sufficient heat would reduce all fluids to the gaseous state. Let us, however, inquire as to the possible extent of such an atmosphere. In the first place, it could not extend beyond that point at which the moon's attraction is equal to that of the earth.
In the next place, much more contracted limits would be prescribed by the effect of centrifugal force, and of the cooling of the vapour by expansion, and by its distance from the source of radiant heat, which had caused its evaporation.
It would be interesting to inquire, what would be the nature of the surface of the atmosphere under such circumstances. At the distance at which the centrifugal force is equal to that of gravity, it might happen that the temperature was scarcely sufficient to maintain the water in a gaseous state. Should this have been the case, a belt of perpetual clouds might have been formed, resembling those of Jupiter. If, at this limit, a still lower degree of temperature prevailed, instead of a belt of clouds, a ring of ice might be formed.
This ring of ice, being exposed to different effects of radiation from variations in the radiating power of various parts of the earth's surface, might, by the superior heat at some parts, become diminished, whilst [205/206] the condensation of the vapour might augment parts less exposed, and situated nearer to the body of the planet: and these conditions might continue, until at last the ring itself was melted or partially melted through at one or more points, and the whole might break up, and the fragments moving in a resisting medium, would ultimately fall down on the surface of the planet. The tearing up of that surface from such an event, would be augmented by the sudden conversion of the solid ice into steam; and after a time, the fragments of the ring would be absorbed again into the atmosphere of the planet.
Let us now suppose, owing to the gradual cooling down of the whole globe, the limit of condensation oi steam into water, to occur at a nearer point than that at which the centrifugal force equals that of gravity. As soon as the steam is condensed into water, it will descend towards the surface of the earth; but that surface being still very hot, will, by its radiation, again convert the descending shower into steam; and this will happen at different heights above the surface, according to the radiating power of the part below. We may, therefore, conceive a shell surrounding the earth, the outer surface of which has just been condensed into water, and the inner consists of vapour, just re-converted into that state by the earth's radiation. These surfaces will attain different heights in different places. Between these two surfaces there will exist a perpetual [206/207] rain, descending from the upper as a gentle shower, becoming gradually a violent torrent, and then as it falls re-absorbed into another gentle shower, which is entirely converted into vapour in approaching the heated surface.
Such being the state of things, let us imagine the globe to cool down uniformly. The lower surface of the descending rain, which is placed at irregular heights, will at length be brought down to the earth's surface in one or more points. The effect of this, which will in the first instance be a gentle shower, would be to cool that portion of the surface on which it falls, and hence to diminish its radiating power. This change, in its turn, will lower the under surface of the watery shell, so that a more violent rain, and ultimately an impetuous torrent will continue, perhaps, for thousands of years, its unremitted vertical action on the surface exposed to its force. The excavation of the largest valleys, or even of ocean beds, is not too much to expect from such forces.
But let us take another view of the consequences of such an original state of incandescence. The whole of the fluids now on the surface of the earth must then have been suspended in its atmosphere. But the extent of that atmosphere is itself limited by various causes: the attraction of other bodies, the effects of centrifugal force, the decrease of temperature, and the [207/208] distances at which the particles of gaseous bodies cease to repel each other, all have their influence in determining its form and magnitude. Let us suppose that we possessed data from which the approximate amount of vapour contained in the entire atmosphere were known, and consequently the whole quantity of water in it; then, since we know the area of the present seas, we might easily ascertain their average depth. If the result of such a computation should give a mean depth much less than that which we know the ocean to possess, — as, for instance, only a hundred feet, — then we might conclude, either that the surface of the earth had never been in such a state of incandescence as has been supposed, or if it had, that a new source of aqueous vapour had been supplied to it, subsequently to its cooling down. [208/209]