

ORIGIN OF MOUNTAINS. 805 



By ascertaining, through observations on the earthquake vibration 

 and its effects, the direction of the longitudinal wave along any two 

 lines over the earth's surface, the courses of the radii from the cen- 

 tre of the region of disturbance are readily obtained ; and their inter- 

 section will give the position of this central region, situated directly 

 over the centre of propagation. Further, when the position of this 

 central region has been obtained, the angle at which a wave-path 

 reaches the surface, if an oblique angle, affords the means of deter- 

 mining the depth of the seat of disturbance or the seismic centre; for 

 the larger this angle the greater the depth of the seismic centre below 

 the centre of surface disturbance. Since the seismic centre may be a 

 region miles in breadth or length, the centre at surface may also be 

 large ; and owing to this cause, and variations in the nature and con- 

 dition of the vibrating rocks, the coseismic curies, or lines along which 

 the wave-paths reach the surface at equal angles and cause simulta- 

 neous earthquake effects, cannot be concentric circles. Isoseismic 

 curves are those along which the destructive effects are about equal. 

 The overthrowing of objects is greatest outside of the central region, 

 because an oblique thrust is more effective in this respect than a ver- 

 tical one. 



The observations made by seismometers aim to determine the di- 

 rection, intensity, and velocity of the wave at different points over the 

 disturbed region. The directions of fractures made in walls or build- 

 ings are an important means of ascertaining the direction of the wave- 

 path as it emerges at the place, and the positions of overturned or dis- 

 placed objects are an important source of information. 



Mr. Robert Mallet, by whose researches and labors the science of 

 seismology was largely developed, determined, in his study of the Nea- 

 politan earthquake of 1857, the wave-paths at twenty-six stations 

 around the seismic vertical, and obtained for the mean depth of the 

 seismic centre about of geographical miles. The amplitude of the rock 

 vibrations causing the wave was determined by Mallet to have not ex- 

 ceeded 2\ inches ; and this, movement traveled in the rocks at a rate 

 not over 15 feet per second. This velocity, although very much less 

 than that of the propagated wave (the mean for which in the Neapol- 

 itan earthquake was found to be 788 feet per second), is the chief 

 source of catastrophe in earthquakes. Mallet deduced from Hum- 

 boldt's account of the earthquake at Riobamba in 1797 (when the 

 bodies of some of the people were thrown across a river, to a height 

 of 100 feet), that the velocity of the wave was at least 80 feet a sec- 

 ond, and that the depth of the siesmic centre was 30*64 geographical 

 miles, or not far, in his view, from the greatest possible depth. Dr. 

 Oldham found for the depth of the centre in the earthquake of Jan- 



