492 



SCIENCE. 



[Vot. IX., No 224 



terrifying and disastrous as the centre of the dis- 

 turbance was approached. At Augusta, 110 miles 

 distant from theepicentrum, the damage to build- 

 ings was considerable, and at the arsenal in that 

 place the commanding officer's residence was so 

 badly cracked and shattered as to necessitate prac- 

 tical reconstruction. In Columbia, 100 miles dis- 

 tant, the shock was very injurious to buildings, 

 and appalling to the people, but no sub-;tantial 

 structures were actually shaken down. In At- 

 lanta, 250 miles distant, there was no worse in- 

 jury than falling chimneys and some slight cracks 

 in the walls ; but the houses were instantly aban- 

 doned in great alarm and confusion by their oc- 

 cupants, and many preferred passing the night in 

 the streets to re-entering their dwellings. At 

 Asheville, N.C., 230 miles distant, and at Raleigh, 

 215 miles distant, the shocks were quite as vigor- 

 ous as at Atlanta. 



Coming nearer the seismic centre, we find the 

 intensity increasing on all sides. The region im- 

 mediately about the epicentrum in a great earth- 

 quake always discloses phenomena strikingly dif- 

 ferent from those at a distance from it ; and tiie 

 differences are not merely in degree, but also in 

 kind. The phenomena characteristic of the epi- 

 central area cease with something like abrujatness 

 as we radiate away from the epicentrum. The 

 central phenomena are those i^roduced by shocks 

 in which the principal component of the motion 

 of the earth is vertical. Proceeding outwards, 

 these predominating vertical motions pass, by a 

 very rapid transition, into movements of which 

 the horizontal component is the greater, and in 

 which the undulatory motion becomes pronounced. 

 The epicentrum, and the zone immediately sur- 

 rounding it, is the portion of the disturbed tract 

 which merits the closest attention ; for it is here 

 that we may find the greatest amount of informa- 

 tion concerning the origin and nature of the earth- 

 quake. To appreciate this, we will venture to 

 offer some theoretical considerations. 



Allusion has already been made to the indefinite 

 character of the data used for estimating the in- 

 tensity of the shock. There is no unit of inten- 

 sity which is at present available. In selecting 

 certain effects of an earthquake to characterize 

 varying degrees of intensity, the most that can be 

 hoped for is a means for discriminating whether 

 the relative energy of a shock is greater or less 

 in one locality than in another. But how much 

 greater and how much less — in conformity with 

 what law — is a jDroblem which remains to be 

 solved. An earthquake impulse, however, is a 

 form of energy transmitted as an elastic vvave 

 through the deeply seated rocks, and its propaga- 

 tion and varying intensity are subject to the laws 



of wave-motion. There must be, therefore, some 

 typical law governing the rate at which such a 

 wave diminishes the intensity of its effects as it 

 moves onward. To anticipate the objection that 

 this typical law would apply only to a medium 

 which is perfectly elastic, homogeneous, and iso- 

 tropic, while the rocks are far from being so, we 

 reply that we have investigated the objection, and 

 are satisfied, that, while it has some validity, the 

 effect of these inequalities is not great enough to 

 seriously impair the applicability of the law, nor 

 to vitiate greatly the results to be deduced from 

 it. The analysis we offer is a novel one. We at- 

 tach considerable importance to it, and the con- 

 sequences which flow from it are somewhat re- 

 markable. 



Let us suppose an elastic wave to originate at a 

 point C (fig. 1), situated at the depth q below the 



riG. 1. 



surface. Let the intensity of the shock (amount of 

 energy per unit area of wave-front) at the distance 

 unity from O be denoted by a. Since the intensity 

 is inversely proportional to the square of the dis- 

 tance, the intensity at the epicentrum would be 



-J- ■ Take any other point on the surface of the 

 earth at the distance x from the epicentre, and 

 connect it with C by the line Cx. The intensity 



at any such point will obviously be equal to -^* 

 If we denote the intensity by y, we shall then 

 have the equation, 



y 



This equation expresses a curve which will serve 

 as a graphic representation of the way in which 



