July, 1887.] ELLIOTT SOCIETY. 167 



in ■which a wave of expansion, corresponding to the rarefied portion of an 

 aerial sound-wave, proceeds from the focus of distui'bance outwards, while the 

 motion of each successive particle is in the opposite direction, inwards. 



There is no known volcanic agency at work within hundreds of miles of us, 

 none nearer than Mexico or the West Indies, and if at work would, as is gene- 

 rally sup])osed, act outwardlj^ or explosively ; so that we may leave this agency 

 out of consideration for the present. The depth of 11 or 12 miles, would be 

 sufficiently great to give to pent up water and steam, a temperature of about 

 1200° F., but this powerful agency, if supposed to exist, would act outwardly, 

 and so would not meet the circumstances of our case. 



If we could suppose a cavity or cavities in the strata beneath us, compressed 

 by the strata above them, the release of this strain by the crumbling in of 

 their walls and the closing in of one or more cavities would produce a wave of 

 the character expressed above, and so far meet the circumstances of the case. 

 But the existence of such cavities at several places in the coast region of the 

 middle and southern states, implies corresponding variations in the average den- 

 sit}'" of the mass of strata beneath each corresponding station at the surface of 

 the earth ; which variations in density might be detected by observations along 

 that region. That there are considerable variations of density, in the beds and 

 strata along our Atlantic region appears to be certain, from the discrepancies 

 between the astronomical and geodetic latitudes of certain places, as given by 

 the Coast Survey, after all the corrections have been made for the dimensions 

 of the spheroid on which the triangulation has been developed ; and it is not 

 impossible, that a part at least of the observed discrepancy between the two 

 latitudes may arise from actual deficiency of matter, that is from an empty cav- 

 ity, but this at present cannot be proved. No indications have yet been given 

 where lie these regions of maximum and minimum density, nor is there any- 

 thing to shew that one of the last underlies our City or its neighborhood. The 

 southern stations whose latitudes have been compared and published by the 

 Coast Survey lie along a line at some distance from the coast, and near the foot 

 of the mountains, whose mass would tend to produce additional complication. 

 A comparison of the astronomical and geodetic latitudes of stations along our 

 coast line would be of interest in connection with our subject. 



I am unable to present any corresponding facts observed at Summerville, or 

 in the surrounding country. I have heard of no cases in which bodies have 

 been distinctly projected towards the S. or S. E., that is, totmrds tlie surface- 

 centre, such as I have observed in the cemeteries in this City. I can only await 

 the coming in of more facts than I was able to collect. 



[APPENDIX TO ABOVE EEMARKS. 



Depth of Focus, and Circle of Maximum Surface Disturbance. 



1. In figure, on page 168, let O be Focus or Origin of disturbance at dis- 

 tance KO below the surface of the earth KA. Suppose a wave of disturbance 

 to be propagated with velocity nearly uniform, along lines OK, OH, &c. , O A, it 



