Think also of earthquake-shocks felt through twelve hundred miles of country. Think of fierce tremblings and heavings lasting in constant succession through days and weeks of terror. Think of hundreds of miles of land raised several feet in one great upheaval. Think of the earth opening in scores of wide-lipped cracks, to swallow men and beasts. Think of hot mud, boiling water, scalding stream, liquid rock, bursting from such cracks, or pouring from rents in a mountain-side. Truly these are signs of a state of things in or below the solid crust on which we live, that may make us doubt the absolute security of "Mother Earth." Different explanations have been put forward to explain this seemingly fiery state of things underground. Until lately the belief was widely held that our earth was one huge globe of liquid fire, with only a slender cooled crust covering her, a few miles in thickness. This view was supported by the fact that heat is found to increase as men descend into the earth. Measurements of such heat-increase have been taken, both in mines and in borings for wells. The usual rate is about one degree more of heat, of our common thermometer, for every fifty or sixty feet of descent. If this were steadily continued, water would boil at a depth of eight thousand feet below the surface; iron would melt at a depth of twenty-eight miles; while at a depth of forty or fifty miles no known substance upon earth could remain solid. The force of this proof is, however, weakened by the fact that the rate at which the heat increases differs very much in different places. Also it is now generally supposed that such a tremendous furnace of heat--a furnace nearly eight thousand miles in diameter--could not fail to break up and melt so slight a covering shell. Many believe, therefore, not that the whole interior of the earth is liquid with heat, but that enormous fire-seas or lakes of melted rock exist here and there, under or in the earth-crust. From these lakes the volcanoes would be fed, and they would be the cause of earthquakes and land-upheavals or land-sinkings. There are strong reasons for supposing that the earth was once a fiery liquid body, and that she has slowly cooled through long ages. Some hold that her centre probably grew solid first from tremendous pressure; that her crust afterwards became gradually cold; and that between the solid crust and the solid inside or "nucleus," a sea of melted rock long existed, the remains of which are still to be found in these tremendous fiery reservoirs. The idea accords well with the fact that large numbers of extinct or dead volcanoes are scattered through many parts of the earth. If the above explanation be the right one, doubtless the fire-seas in the crust extended once upon a time beneath such volcanoes, but have since died out or smouldered low in those parts.