They are deep borings in the earth, out of which water rises to, or even above, the surface of the ground. Wherever you bore, if you go deep enough you may get plenty of water, and this water is always found to be warmer in proportion to the depth of the well in which it rises. They are called _Artesian Wells_, from _Artois_, a town in France, where the first was constructed. In a well of this kind near Rochelle, which was 316 feet deep, the water near the surface was at a temperature of 55 degrees, and at the bottom 60 degrees. It was afterwards sunk to the depth of 369 feet, and the temperature at the bottom was then found to be 65 degrees. Do you not think it most likely that the heat continues to increase quite as fast in proportion below the deepest point that man has gone down to? Well, if so, at the depth of some miles it must be hot enough to melt anything, even granite, and all kinds of stones. If we suppose, on this evidence, that the centre of the globe is intensely heated, and that it gets gradually cooler towards the surface, there is what at once will account for the hot springs, the increasing heat in descending mines, the constant action of some volcanoes, the occasional action of others, the streams of lava, and the nature of all substances that are thrown out. It is very natural to imagine that every now and then the action of the great heated mass inside, would crack open the crust, and the sea wearing away its bottom in some places as it does, may have something to do in assisting this. Thus, new volcanoes would be formed, and earthquakes would happen where they never happened before; and over these immense cracks there would be a volcanic band. Then, after a time, these cracks might be partially, or wholly, filled up by the matter thrown into them, and the volcanoes on the surface above may become quiet for a time. [Illustration] CHAPTER IX. PARLEY TELLS SOMETHING ABOUT THE HISTORY OF MOUNT VESUVIUS. I am going to relate to you the biography of a Volcano, and I hope you will find it amusing. Before the time of the Roman Emperor Trajan, Mount Vesuvius had been quiet for a great many centuries. It is known that it had had many eruptions before then, because there are some great lava streams close by it, on which some of the most ancient Italian cities are built. In particular, Herculaneum, which was said to have been built by the Demi-god Hercules, and must therefore have been of the remotest antiquity, was built chiefly of blocks of lava, and founded upon a vast stream of the same substance.