The uniform increase of temperature, as we descend into the earth, at the rate of about 24° Fahrenheit for every 100 feet, the heat of some mineral springs, leads to the conclusion that, could we advance to a place about twenty-three miles from the surface, we should attain the limits where all becomes a molten mass. The cold surface, on which we walk in such security, seems, by all analogies, to envelop a liquid caldron which has been seething from the morning of the world. This internal mass is, of course, the source of all volcanic eruptions, and of all the phenomena to which I have alluded above. A distinguished geologist has well said that “light and heat are the two extremes of being: the farther man goes away from the earth’s surface, he encounters light; the farther he recedes inwardly from its surface, he encounters heat.” It is true we are not absolutely certain that the rate of increase is uniform, at the rate of one degree Fahrenheit for every 45 feet; but if it is, we should reach the boiling point of water at less than 10,000 feet from the surface, and the melting point of iron (22° Fahrenheit) a little over 120,000 feet. The relation of this thickness to the entire diameter of the earth is about as 1 to 344, about the ratio of the thickness of an egg-shell to the egg. The Superficial Dimensions of the Land and Water on the Globe. The equatorial diameter of the earth, 7925·6 miles, multiplied into the circumference, 24898·8, equals 197,339,590, the number of square miles on the earth’s surface, reckoning as if of a true sphere. The deduction to be made, in consequence of its spheroidal shape, has not yet been estimated with any approach to nicety. The sum indicated above is exact enough to satisfy geographical purposes; enough to lead to the laws of relative rather than to a minute individualization. The proportion even of land to water has not been determined, except with approximate accuracy. It has been commonly stated that two-thirds are water and one-third land; others have computed three-fifths to be water and two-fifths land. The most accurate measurements, those instituted by Humboldt, have left it in this statement, that if the whole be taken as one, the sea occupies ·734, the land ·265, or, reduced and simplified in almost unchanged form, a little more than three-quarters water, a little less than one-quarter land. Of course it is impossible, as yet, to attain to accuracy in these estimates, as our knowledge is imperfect regarding the polar regions; there are about 17,000,000 square miles unexplored.