The temperature of the planetary regions is infinitely low; according to Laplace it cannot be estimated at less than 100° below zero. The glacial regions traversed in its course by the incandescent globe would necessarily cool it, at first superficially, when it would assume a pasty consistence. It must not be forgotten that the earth, on account of its liquid state, would be obedient in all its mass to the action of flux and reflux, which proceeds from the attraction of the sun and moon, but to which the sea alone is now subject. This action, to which all its liquid and movable particles were subject, would singularly accelerate the commencement of the solidification of the terrestrial mass. It would thus gradually assume that sort of consistence which iron attains, when it is first withdrawn from the furnace, in the process of puddling. As the earth cooled, beds of concrete substances would necessarily be formed, which, floating at first in isolated masses on the surface of the semi-fluid matter, would in course of time come together, consolidate, and form continuous banks; just as we see with the ice of the present Polar Seas, which, when brought in contact by the agitation of the waves, coalesces and forms icebergs, more or less movable. By extending this phenomenon to the whole surface of the globe, the solidification of its entire surface would be produced. A solid, but still thin and fragile crust, would thus envelop the whole earth, enclosing entirely its still fluid interior. The entire consolidation would necessarily be a much slower process--one which, according to the received hypothesis, is very far from being completed at the present time; for it is estimated that the actual thickness of the earth’s crust does not exceed thirty miles, while the mean radius or distance from the centre of the terrestrial sphere, approaches 4,000 miles, the mean diameter being 7,912·409 miles; so that the portion of our planet, supposed to be solidified, represents only a very small fraction of its total mass. [Illustration: Fig. 13.--Relative volumes of the solid crust and liquid mass of the globe.] We say thirty miles, for such is the ordinary estimated thickness of the earth’s crust, usually admitted by savants; and the following is the process by which this result has been obtained. We know that the temperature of the earth increases one degree Centigrade for every hundred feet of descent. This result has been borne out by a great number of measurements, made in many of the mines of France, in the tin mines of Cornwall, in the mines of the Erzgeberge, of the Ural, of Scotland, and, above all, in the soundings effected in the Artesian wells of Grenelle and Passy, near Paris, of St. André de Iregny, and at a great number of other points.