One of the most interesting cases of this kind is furnished by the seismic history of our own country. During the years 1811 and 1812 a remarkable series of shocks, now usually referred to as the "New Madrid earthquake," occurred in the middle Mississippi Valley. In the course of a few months no less than 1,874 shocks were felt. Eight of these were very severe, and were perceptible over the whole of the then settled portion of the United States. This earthquake produced notable geographic changes; new islands came into existence in the Mississippi, new lakes were formed in neighboring valleys (one of them one hundred miles long), and old lakes disappeared.
Light aftershocks have been experienced in this region almost every year since the original disturbance, so that this long-drawn out earthquake may be said to be still going on. The ordinary vibratory movements of the ground in an earthquake are very slight, generally amounting to a small fraction of an inch, and never to more than a few inches. In addition to these vibrations, however, there are often seen large waves, which travel over the surface of the ground no faster than a man can run. In the New Madrid earthquake the ground is said to have rolled in waves three feet high, with visible depressions between, the waves finally breaking and leaving a series of parallel fissures. Besides the earthquakes due to deep-seated movements of the earth's crust under long-continued strain, there are some directly connected with volcanic eruptions and others caused by landslides and other movements at the bottom of the sea.
Earthquakes have often been experienced by ships in mid-ocean. They sometimes give rise to great waves, which may travel for thousands of miles, and which cause a large amount of damage along low-lying coasts near the place of origin. Nowadays every earthquake of any consequence, whether occurring by land or sea, is registered by instruments known as "seismographs," which may be located hundreds or thousands of miles away from the place of disturbance. The seismograph is essentially a heavy mass of metal, suspended in such a manner that its inertia prevents it from partaking readily of the motion of the earth when the latter is shaken. A pen or sharp point attached to the suspended weight traces a record of the earth's movements on a sheet of paper carried along by clockwork, or, in some instruments, the record is made by a photographic process. Complete apparatus of this kind registers vibrations in three directions - north-south, east-west, and up-down. An earthquake sends out two principal sets, of vibrations. One set travels around the earth's surface, while the other takes a "short cut" through the interior of the globe. The latter set is registered first by a distant seismograph, not only because of its shorter path, but also because it travels at a much greater speed. Moreover, the vibrations that pass through the earth is of two kinds, one of which travels faster than the other.
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