July, 1887.] ELLIOTT SOCIETY. 1G5 



rotation to have taken place about a point not far from that angle, but -within 

 the area of the base, at some region of greatest adhesion or friction, as Mallet 

 supposes in his explanation. But frequently there has been motion of transla- 

 tion as well as of rotation, as though the obehsk had made a jump during the 

 rotation or after it, or had sHdden along the surface of the pedestal. In case 

 No. 3, we have a remarkable example, -where the pedestal has rotated through 

 25° on its plinth, and the obelisk through 25° more on the pedestal, the displace- 

 ment of the three centres from their original positions being very small. In 

 case No. 29, we have another remarkable example, a large slab rotated through 

 25°, around a point at one of its angles, resting on one of its six i^illars, three of 

 the others being thro-wn do-wn during the motion. 



If sufficient data were at hand, we would next endeavor to determine the re- 

 gion at the surface of the earth, immediately over the focus of distui'bance be- 

 low, and then the depth of that focus beneath the surface. The solution of 

 these problems requires that we first determine the position of three points or 

 regions of greatest disturbance on the surface of the earth. But sufficient data 

 for these determinations are not furnished either by the facts collected by the 

 members of the committee, or by the reports of the veiy few persons who have 

 com])hed with our request to coUect facts and send in to us their observations. 

 These observations relate almost entirely to injuries sustained by structures in 

 Charleston and Summerville, and in the absence of the requisite additional da- 

 ta, we can only make a first approximation, by taking a region about midway 

 between these two places as that which is nearly vertically over the focus of dis- 

 turbance beneath, and this region will he not far from the railroad station 

 known as 10 Mile Hill. The depth of the focus of disturbance, will then be 

 about 11 miles, on one of the hypotheses usually made, or about 15 on the 

 other ; depths much greater than those previously assigned to foci of earth- 

 quake disturbance. 



In each of these two cases, the distiu'bance is supposed to proceed in every 

 direction from a central region, or focus of disturbance, radiating along lines 

 approximately straight, but diminishing in intensity, or amplitude of disturb- 

 ance as it proceeds outwards from the centre, this diminution at each point of 

 the line traversed depending, according to some law, on the distance of that 

 point from the focus. At a point on the surface of the earth, vertically over 

 the deeper seated focus of distm-bance, the direction of the disturbance when 

 it reaches it, will be, approximately, vertically upwards, and the effects pro- 

 duced will be almost entirely iu that direction, with far less or but httle hori- 

 zontal disturbance. This point on the surface of the earth has been called 

 the epicentrum ; but we may call it simply tJie surface centre. Proceeding 

 outward every way from this surface centre, the Unes radiating from the focus 

 wiU meet the sm-f ace of the earth more and more obhquely, and effects will be 

 produced both in the upward and horizontal directions, and if we regard at 

 present this obhquity only, the intensity of the disturbance in the upward di- 

 rection will continually decrease, and that iu the horizontal du-ection -will in- 

 crease, each at a rate easily calculable. But, as we have said, the intensity of the 

 disturbance, along the lines radiating from the focus, continually diminishes 



