During many long ages this process of condensation of the nebulous vapour steadily continued, being controlled by the laws of gravitation and transformation, until, at length, a number of rotating spherical nebular masses were formed, in a state of high heat from the shock of their recently-united atoms, which spheres gradually cooled by radiation, consequently contracting and becoming possessed of a more rapid rotary motion, giving off from their equatorial regions large rings of vapour, which, in their turn, condensed and, under the influence of the same two laws, formed separate spheres for themselves. This is the mode by which our planetary system was formed, as taught by Laplace and accepted by the scientists of to-day. The earth, then, in common with other planets, may be said to have passed from the condition of a gaseous to a highly-heated fluid mass, and to have gradually become plastic, and moulded by revolution on its own axis to its present shape—_i.e._, an oblate spheroid, or globe, flatter at the poles than at the equator, with a polar diameter about twenty-six miles shorter than the equatorial diameter. This is the shape that all plastic bodies which rotate on their axes must assume, as we are clearly taught by mathematics. Assuming, then, that the earth was in a state of incandescence when it began to take a definite form, we shall at once see that the denser materials composing it would gravitate towards the centre, forming a semi-plastic mass surrounded by an envelope of gases and watery vapour. The gases would be quickly disposed of in various chemical combinations, and the watery vapour would be condensed and deposited in depressions on the surface of the central mass as soon as it had become cooled sufficiently. The outer crust of this central, semi-solid mass was soon converted, under the intense heat, into a hard, granite-like rock, which was continually subject to sudden upheavals, resulting partly from the violent escape of gases, and partly from water passing through fissures on the surface to the heated interior and giving rise to steam of great expansive power. In this manner great inequalities of the surface were, no doubt, produced, whose rugged edges, after the lapse of a vast period of time, were gradually softened down by the subsequent action upon them of air and water. This first rock formation is termed by geologists the Plutonic (from Pluto, monarch of hell), on account of its being the result of intense heat, and not, as is the case with all other rock formations, laid down in layers by water. Whether the Plutonic rock forms a solid centre to our earth is matter of uncertainty; but all are agreed that the internal heat of our planet, whether caused by the friction of the particles of a solid substance or by a molten fluid, is still, even in these later times, intense. In boring through the earth’s crust, the average increase in temperature for every fifty feet of descent, after the first hundred