HYPOGEIC WORK. 375 



4500 to 20,000 feet per second ; and Fouque' found the velocity in granite 9200 feet per 

 second. 



The position of the epicentrum is ascertained by noting the direction of throw of over- 

 turned columns, walls, houses, the converging lines pointing to the region of the surface 

 vertically over the epicentrum. An oblique thrust is most effective in overthrowing 

 objects ; and the particular belt-line around the central region along which the waves are 

 most destructive is called the meizoseismic curve, and lines of equal disturbance, isoseis- 

 mic curves. Such curves are far from circles. 



By means of evidence from fractures in walls and overturned objects, R. Mallet in- 

 ferred the angle of emergence of the wave, and so calculated the depth of the center of 

 disturbance. From 26 observations of the Neapolitan earthquake of 1857 he deduced a 

 depth of 6^ miles. C. E. Button, in his paper on the Charleston earthquake, assumes 

 that the total disturbance is inversely as the square of the distance from the center of 

 disturbance. By noting, in the Charleston earthquake, the circle about the epicentrum at 

 which the total effect diminished most rapidly on going from the epicentrum, he deduced 

 depths of 8 and 12 miles for two distinct centers of disturbance. 



The instruments by which the earthquake movements are detected (seismoscopes) , 

 measured (seismometers), and recorded (seismographs), are of many kinds. Those which 

 experience in Japan has proved to be most accurate are the so-called Duplex pendulum ; 

 the Bracket seismographs of Chaplin, Ewing, Gray, or Milne ; and conical pendulums. 



The geological effects of earthquakes are small, while those of the causes 

 which produce earthquakes are large. Vibrations loosen rocks and may tumble 

 them down precipices, as they tumble down houses and walls. Occasionally 

 they produce some rotation in the objects moved where the object is not 

 equably attached below. They may fracture the rocks and ground in the re- 

 gion of greatest disturbance. They often occasion the drying up of springs. 



In Calabria, in 1783, fissures were made that were over a mile long, 100 

 feet wide, and 200 feet deep. In the Charleston earthquake of 1886, and 

 also in that of 1892 at Quetta, in British Baluchistan, described by C. Davison, 

 railway lines were bent ; and in the latter case, on removing the bent rails 

 for repair, the new lines had to be cut 21- feet shorter than the old ones, 

 owing to a permanent displacement. 



But these rending effects and the uplifts, and other results attending 

 them, are effects rather of the deep-seated cause of the vibration and the 

 fracturing. Besides these effects, earthquakes may destroy life in the sea, 

 by impact, as a blow on the ice of a pond will stun or kill the fish. They 

 may also throw the ocean over the land in waves of 30 to 100 feet, carrying 

 in the animals of the sea, and, in these modern times, man's boats and ships, 

 besides lifting and bearing far inland sea-bottom rocks and sand, and great 

 masses of coral rock on the shores of coral islands (page 222). Further: if 

 a mountain-system of the length of an America were making, like the post- 

 Cretaceous Laramide System, and a like system cotemporaneously in the 

 other America, sea-borders, continental seas, and land-borders the world over 

 might be mostly stripped of life by earthquake waves. Or, if the mountain 

 systems in progress were of less extent, like the post-Paleozoic, a hemi- 

 sphere might experience the devastations, and austral land-borders and 

 sea-borders escape. 



