The bottom of the Tyrrhenian Sea—between Italy, Sicily, and Sardinia—has been lowered in rather recent ages and is still sinking. We notice on the map five dotted lines, corresponding to cracks in the crust of the earth. These lines would intersect in the volcanic district of the Lipari Islands. We further see a dotted circular arc corresponding to a fissure which is regarded as the source of the Calabrian earthquakes of 1783, 1905, and 1907. The earth-crust behaved somewhat after the manner of a windowpane which was burst by a heavy impact from a point corresponding to the Island of Lipari. From this point radiate lies of fracture, and fragments have been broken off from the earth-crust by arc-shaped cracks. The volcano Etna is situated on the intersection of the radial and circular fissures. [Illustration: Fig. 16.—Seismogram recorded at Shide, Isle of Wight, on August 31, 1898] In recognition of the high practical importance of earthquake observations, seismological stations have in recent days been erected in many localities. At these observatories the earthquakes are recorded by pendulums whose styles draw lines on tapes of paper moved by clock-work. As long as the earth is quiet the drawn line is straight. When earthquakes set in, the line passes into a wavy curve. As long as the movement of the paper is slow, the curve merely looks like a widened straight line. The subjoined illustration (Fig. 16) represents a seismogram taken at the station of Shide, on the Isle of Wight, on August 31, 1898. The earthquake recorded originated in the Centre G, in the Indian Ocean. The origin has been deduced from the moments of arrival of the different waves at different stations. We notice on the seismogram a faint widening of the straight line at 20 hrs. 5 min. 2 sec. (8 hrs. 5 min. 2 sec. P.M.). The amplitude of the oscillations then began to widen, and the heaviest concussions were noticed at 20 hrs. 36 min. 25 sec., and 20 hrs. 42 min. 49 sec., after which the amplitudes slowly decreased with smaller shocks. The first shock of 20 hrs. 5 min. 2 sec. is called the preliminary tremor. This tremor passes through the interior of the earth at a velocity of propagation of 9.2 km. (5-3/4 miles) per second. It would require twenty-three minutes to pass through the earth along a diameter. The tremor is very feeble, which is ascribed to the extraordinarily great friction characteristic of the strongly heated gases which are confined in the interior of the earth. The principal violent shock at 20 hrs. 36 min. 25 sec. was caused by a wave travelling through the solid crust of the earth. The intensity of this shock is much less impaired than that of the just-mentioned tremor, and it travels with the smaller velocity of about 3.4 km. (2.1 miles) along the earth’s surface.