Page:Popular Science Monthly Volume 2.djvu/541

Rh The diagram shows the waves radiating from the earthquake focus A to c d e f and g successively, and reaching the surface at B, where the shocks would be vertical. At 1 2 3 they become more and more oblique, and at greater distances appear almost horizontal.



Now, while the movement of the transit-wave may be very rapid, that of the particles of matter is surprisingly small. At Lisbon the velocity of the wave was 20 miles a minute, or 1,200 miles an hour. According to Mallet, where the velocity of the transit wave was 1,000



feet per second, the movement of the particles was only 12 feet per second, or eight miles an hour, and he states that three columns of the Temple of Serapis, on the shore of the Bay of Baiæ (see frontispiece), a region subject to earthquake-shocks, would be overthrown by a shock "whose wave-particles had an horizontal velocity of 3½ feet per second." The shock which threw human beings 100 feet in the air, at Riobamba, must have had a velocity of 80 feet per second. The theory of Mr.