EARTHQUAKE PHENOMENA. 409 



a few hundred feet distant upon the iron rail, he will almost instantly 

 hear the wave through the iron rail; directly after he will feel another 

 wave through the ground on which he stands; and, lastly, he will 

 again hear another wave through the air; and if there were a deep 

 side-drain to the railway, a person immersed in the water would hear 

 a wave of sound through it, the rate of transit of which would be 

 different from any of the others, all these starting from the same point 

 at the same moment. 



The size of such a wave — that is, the volume of the displaced parti- 

 cles of the material in motion at once — depends upon the elastic limits 

 of the given substance, and upon the amount or power of the origi- 

 nating impulse. By the elastic limit in solids is meant the extent to 

 which the particles may be relatively displaced without fracture or 

 other permanent alteration; thus glass, although much more perfectly 

 elastic than India-rubber, has a much smaller elastic limit. 



Nearly all such elastic waves as we can usually observe originate 

 in impulses so comparatively small that we are only conscious of them 

 by sounds or vibrations of various sorts, the advancing forms of whose 

 waves are imperceptible to the eye ; but when the originating im- 

 pulse is very violent, and the mass of material suddenly acted on very 

 great, as in an earthquake, the size of the wave may become so great 

 as to produce a perceptible undulation of the surface of the ground, 

 often visible to the eye, and by whose transit bodies upon the earth 

 are disturbed, (chiefly through their own inertia,) thrown down, &c. 



There is every reason to consider it established that an earthquake 

 is simply '•'■the transit of a wave or leaves of elastic compression in any 

 direction, from vertically upwards to horizontally in any azimuth, through 

 the crust and surface of the earth, from any centre of impulse, or from 

 more than one, and which may he attended with sound, and tidal waves, 

 dependent upon the irnpjulse and upon circumstances of position as to sea 

 and land. 7 ' 



Until this was clearly grasped the observation of earthquake phe- 

 nomena, in the absence of a "guiding hypothesis, 1 ' was vague and 

 useless. 



At present the objects and aim of Seismology are of the highest 

 interest and importance to geology and terrestrial physics. It offers 

 to us the only path to discover the real constitution and condition of 

 the interior of our planet, and will become the key to open to us the 

 true nature, depth of origin, and source of volcanic heat. In these 

 respects one of the most primary objects of Scismometry is to arrive 

 at a knowledge of the depth beneath the earth's surface from which 

 earthquake shocks are delivered, i. e., the depth of the origin. 



The observer must form clear conceptions of the fundamental con- 

 ditions of propagation of seismic waves. Fig. 1 represents a vertical 

 section of part of the earth in the plane of a great circle, cutting the 

 surface at h'h, and passing through the origin of impulse at A — Ap, 

 being the prime vertical to that point whose depth beneath the surface 

 is BA. The wave starts from the origin (assuming the earth's mass 



