The investigation of the interior of the earth is more difficult to us than that of the atmosphere even. We cannot say that we _know_ thoroughly more than we can learn by penetrating ¹⁄₆₀₀₀ part of the distance to the center of the globe. Deeper than that our lowest shafts have not sunk. The coal mines of England penetrate perhaps the farthest below the sea level; for the deep mines in Germany, in the Hartz district, for instance, have their entrance hundreds of feet above it. One coal mine near Durham, England, descends to a depth of about 1584 feet, and reaches a point where the thermometer is 79° Fahrenheit. The deep coal mines of England have, however, one rival, in a shaft at Liege, which is sunk 1800 feet. The modern Artesian well has gone to still greater depths, in the effort to procure brine for the manufacture of salt, or fresh water for the use of cities. At Rehme, in Porta Westphalica, a point 2160 feet from the surface has been reached, and water brought up at a temperature of 90° Fahrenheit, containing four per cent. of salt. By an ingenious application of mechanics to the process of well boring, doubtless a depth of 5000 feet could be attained. At Mondorf, in Luxemburg, a bore has been made through sandstone and the mineral formations lying beneath it, for a distance of 2700 feet, and water reached at 82° Fahrenheit. The great upheavals caused by earthquakes and volcanoes disclose still vaster depths. In the eruptions of the latter, immense masses of the inner contents of the earth are thrown out, sometimes enough to form a not insignificant mountain, and to desolate large regions with their debris. As a general thing, the original mineral forms are lost and indistinguishable in the molten mass. Yet not seldom perfect specimens are hurled out, imbedded in lava and cinders; not always minute fragments, sometimes huge blocks, testifying not with any degree of completeness, yet clearly, to a certain extent, of the composition of the region bordering on the great inner sea of molten matter. Yet most of our knowledge upon this subject is hypothetical, and what we know only indicates painfully the great extent of that of which we are entirely ignorant.