Page:The fairy tales of science.djvu/343

 of sea and land are changed, and rivers quit their former courses and ancient beds, seeking other channels and forming new beds.

The action or movement of earthquakes is threefold—vertical, horizontal, and gyratory or circular.

The vertical movement proceeds from below upwards, and may be likened to the explosion of a mine in a stone quarry. It produces cracks and fissures in the earth's crust. In many instances, the earth opens and closes rapidly; in others, portions of the crust slip down into the chasm, and disappear for ever. It was by a vertical earthquake movement that the city of Messina, in Sicily, was destroyed in the year 1783. These vertical movements are felt even at sea. Thus, for instance, during the celebrated earthquake at Lisbon, in 1755, many ships at considerable distances from the actual focus of the movement, were violently shaken, the concussion in one ship far out in the Atlantic being so great, that the men were tossed up into the air a foot and a half perpendicularly from the deck.

In the horizontal movement, the shock is propagated in a linear direction, producing undulations in the surface of the earth, bearing some resemblance to the waves of the sea, and the sight of which, curious enough, causes a swimming in the head, like sea-sickness.

These undulatory shocks in a linear direction must of course be understood to move in waves of great breadth as well as length. The horizontal