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 is 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 its surface: it would instantly have been converted into vapour; and notwithstanding the increased weight of atmosphere thus produced, and pressing on its surface, sufficient heat would have reduced 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 produced that state.