Several theories are advanced. One class of scientists believes that the earth is a mass of molten matter, with only a thin outer shell of cooled material. That a very [Illustration: ERUPTION OF MOUNT VESUVIUS.] high temperature exists at no very great distance from the surface is beyond a doubt. The observations made in mines and artesian wells show that the average increase in the temperature is one degree for every fifty-five feet in depth. One noted variation exists in the deep wells at Buda-Pesth, in Austria, where the temperature increased up to 3,000 feet; but beyond that depth it became cooler again. The Comstock lode in Nevada, the richest mineral vein in the world, is nearly at the limit of practicable working, the normal temperature being as high as one hundred and fifty degrees. Even if it be conceded that the material of the earth is a molten mass, there would be two theories to explain it: one, that the earth was originally in a state of fusion, and was slowly cooling; the other, that the great pressure from without keeps an otherwise solid center greatly compressed and heated, and consequently liquid. Either supposition is based on well established facts; but it does not appear clear that the molten globe with a cool shell can settle the entire question. The objections to this are several. One, the complete absence of uniformity in the increase of heat as we descend. While the total average is as given above, the variations are so many and vast, that there does not seem to be any general law, as there should be if the molten interior possessed the least uniformity. In some shafts the increase is one degree for twenty feet; in others, one for every one hundred; in some, the temperature increases much more rapidly at great depths, in others, much less rapidly. A second objection is, the vast difference in the character of lavas, even in districts very near each other. Thirdly, there seems no definite connection between volcanoes in the same region. Two adjacent ones may exhibit very different conditions. Mauna Loa is about 10,000 feet above Kilauea, a great crater of the same mountain. Yet the upper is often in a violent state of eruption when the latter is perfectly quiet. It would be difficult to conceive how these are supplied from the same source. If the interior were a molten mass in a state of equilibrium, as would be necessary if the uniformity of its motions in the solar system were to be preserved, any undue pressure would compel the molten matter to escape from the lowest opening. This would be in accordance with the simplest laws of liquids. Then we should find volcanic action most vigorous at the lowest active volcano; but such is not the case. The idea of a uniformly liquid interior seems hardly tenable.