upon the top or side of more or less lofty mountains of a sugar-loaf form, these mountains being then distinguished by the name of volcanoes; a word derived from Vulcan, the heathen deity whom the old imaginative Greeks fancied to be engaged in forging thunderbolts in some of these mountainfurnaces. Volcanic mountains, indeed, are heaps of lava and cinders, which have been from time to time shot forth from cracks in the earth's surface, and which have then fallen all around the opening, very much in the same way as sand falls from a spout, arranging itself in a conical mound. Lava, when it first breaks forth from the opening of a volcanic mountain, is about as hot as melted iron, and then flows along at the rate of a mile in an hour. As it becomes cooler, it moves more slowly, and a hard crust begins to form upon its surface, which is, however, still floated along as a sort of scum upon the general mass. People can, at this time, walk over the crust of the lava-stream without scorching their boots, although red-hot material appears beneath, so soon as the crust is broken. During the day a long line of white smoke is seen hovering over the course of the stream, and no fire is visible. But at night a red-hot river appears flowing down the side of the mountain, and winding along through the lower valleys. By slow degrees the molten current forms an outer layer of solidified rock all round itself, and then it runs on in this tunnel of its own construction, very much as water streams through a pipe.

LESSON 51.-GEYSERS, VOLCANOES,
EARTHQUAKES.-Part 2.

VOLCANIC eruptions of molten rock generally occur at intervals. The mountains giving vent to the overflow are in a state of fierce activity for a few

days or weeks, and then go to sleep for years. When one of the periods of eruption is near at hand, the country round the mountain is all at once shaken by an earthquake; then comes a violent crash, and the old outlet in the mountain is cleared by an explosive burst of vapour, which has in itself the force of ten thousand cannons. Enormous blocks of half-molten rock have been seen flying through the air to the distance of many miles. One of the volcanoes of the Andes has been known to cast in this way a mighty shot, consisting of 109 solid square yards, as far as nine miles. Vesuvius, near Naples, has not unfrequently sprinkled the island of Malta, 120 miles off, with a shower of dust and ashes.

A hollow roaring sound follows the first outburst of the eruption, and occasional explosions, like the firing of cannon, are heard in the midst of this. A column of black smoke now rises high into the air, and large blocks of rock continue to play up and down over the volcanic opening. The smoke itself is indeed merely rock beaten into the state of frothy ashes, and ground down into dust and powder. At night, the fierce glow from the interior of the earth shines through the opening upon the over-hanging canopy of smoky dust, and this is then lit up with a bright red glare, in the midst of which large masses of red-hot rock are seen playing up and down. Flames never appear at the openings of volcanoes; the sugarloaf heap of previously ejected matter merely seems as if tipped with ruddy fire. As the violence of the outburst begins to get spent, the smoke rises higher and higher, and spreads itself out tranquilly at the top in the form of a giant mushroom, and during the hours of darkness the mountain appears as if surmounted by a pillar of fire, bearing upon its summit a radiant crown, jewelled with brilliant sparks, and flashing forth forked lightning all around.

The solid crust which forms on the surface of

cooling lava no doubt serves to impede the further progress of its cooling. The hardened shell acts like a great coat in holding the warmth within. By putting together all the facts of the case, men of science have come to the conclusion that the great round world is upon the large scale, even now, in the condition of the lava-stream recently poured out from the volcanic opening; that it is liquid and red-hot within, and has a cooled and solidified great coat or crust, so that if a piece of the hard shell, a few miles thick, were torn off, the red, fiery substance would become visible, as it does when a piece of the lava, a few inches thick, is picked away. The great world-drop is liquid from heat, and has simply a thin shell of solid substance on its outside where the cooling has taken place most rapidly. A Frenchman, who has given a great deal of thought and attention to this subject, Monsieur Cordier, believes that the centre of the earth is one hundred and sixty times as hot as red-hot iron, and that at a short hour's railway journey down, that is at the depth of about 23 miles, the heat is sufficiently great to make iron and granite run like water, and to melt the hardest rock like a lump of sealing-wax cast into a coal-fire. This notion that the so-called solid foundations of the globe are after all but a sea of liquid fire, and that upon a pavement laid on such a fearful abyss, the human race has to walk all its days, is certainly by no means a pleasant one. But the calculator of this terrific sum brings comfort as well as alarm with his figures. He shows that if a fierce furnace, such as the molten centre of the earth is here supposed to be, were entirely built in by a wall of granite 23 miles thick,the probable depth of the earth's solid pavement,—it would be quite impossible that any person outside should ever be sensible of the imprisoned heat. Monsieur Cordier's experiments tend to prove that it would take many thousand centuries before a single

degree of warmth could make its way from such a furnace through such a wall. There may be a vast world-large cauldron of boiling and blazing rock, 23 miles beneath man's feet, and yet he himself never know anything of its existence, protected as he is by the intervening twenty-three-miles-thick shell of solid ground, excepting as he reasons out his knowledge after feeling the throes of the earthquake, and seeing the fiery streams of the volcano. When the wrinkled shell of the earth is shaken to its foundations, and the granite rocks miles thick are bent as if they were so many sheets of paper, and mighty mountains are upheaved from the depths of the sea, men can begin to conceive that the great round world which sweeps along in space, and rolls over on itself as it goes, may be even now a large drop of mineral matter molten by heat, with a mere crust of cooled solid substance surrounding it as a shell.

THE great round world, which carries the deep and wide sea in the hollows of its surface, is not at rest. It is constantly whirling round, not in the direction of the water-flow-from tropics to poles-but in the transverse direction from west to east. North America chases Europe, and South America chases Africa in an endless circle. As then the cold polar waters advance from their comparatively small circles of rotatory motion to the wide tropical belt of the earth,

which in its mid region has a girdle twenty-five thousand miles in extent, they keep getting into spots where the eastward rush of the earth's surface is pressed on with ever-increasing speed. In the latitude of England the water of the sea is carried along, in consequence of the earth's rotation, with a speed of five or six hundred miles in an hour. In the Mid-Atlantic and Pacific, the water is carried along with a speed exceeding a thousand miles in an hour. But water, being a liquid capable of flowing, and not a hard and solid substance, does not lie passive, like a stone, on the surface of the whirling earth; in virtue of its flowing power, it hangs back a little in the basin in which it is contained. Thus the water from the poles bringing with it but the slower velocity of the small circles which it has left, is unable at first to keep pace with the more impetuously moving solid which forms the ocean-bed within the tropics. The earth consequently slips away from under it towards the east, and it lags back in an apparent_current towards the west. The water on the coast of Morocco thus moves not directly along the African coast towards Sierra Leone, but obliquely across the Atlantic towards the north shore of the South American continent.

With the dense tropical brine moving towards the pole, exactly the opposite thing occurs. It arrives in continued succession at spots where the sea-bed has less eastward movement, in consequence of the earth's whirl, than itself. It brings the speed of a thousand miles an hour to places which turn with a velocity of only four or five hundred miles an hour, and so, by its own impetuosity, goes faster than the solid substance which is beneath it. It flows towards the east, as well as towards the pole. The water off the north coast of South America, and in the Gulf of Mexico, in this way sets, not towards Greenland, but obliquely across the Atlantic towards the British Isles. The mere rotatory movement of the earth thus favours the return of the cold, light water of the poles towards the tropics, and shows