May 20, 1887.] 



SCIENCE. 



491 



causing general alarm. Chimneys and bricks 

 were shaken down, and the oscillation of the 

 houses was strongly felt. In south-eastern Ohio 

 nearly every theatre, lodge, and prayer-meeting 

 was broken up in confusion. It does not appear 

 that the Appalachians offered any sensible barrier 

 to the progress of the deeper waves, but it does 

 appear that they affected in a conspicuous degree 

 the manner in which the energy of the waves was 

 dissipated at the surface. Another minimum area 

 was found in southern Indiana and Illinois, and 

 also in southern Alabama and Mississippi. There 

 is a curious circumstance connected with the 

 minimum area in Indiana and Illinois. On the 

 6th of last February an earthquake of notable 

 force occurred in just this locality. Circulars were 

 sent out at once, and, on plotting the isoseismals, 

 they showed a singular coincidence in almost 

 exactly filling the vacancy or defects of intensity 

 of the Charleston earthquake. At present there is 

 nothing to indicate whether this coincidence is 

 accidental, or whether there is some hidden rela- 

 tion. 



Where the waves passed into the newer delta 

 region of the lower Mississippi, the surface inten- 

 sity of the shocks rapidly declined. This is indi- 

 cated in the map by the compression of the iso- 

 seismals in those localities. We incline to the 

 opinion that this sudden diminution of the in- 

 tensity is due to the dissipation of the energy of 

 the waves in a very great thickness of feebly elas- 

 tic, imperfectly consolidated, superficial deposits. 

 It is a matter of common observation in all great 

 earthquakes, that the passage of the principal 

 shocks from rigid and firm rocks into gravels, 

 sands, and clays, is, under certain circumstances, 

 attended with a local increase in the amplitudes 

 of the oscillations and in the apparent local in- 

 tensity and destructiveness ; and the reason for it 

 is intelligible. But, where such looser materials 

 are of very great thickness and great horizontal 

 extent, the reverse should be expected : for, when 

 a wave passes from a solid and highly elastic me- 

 dium into a less solid and imperfectly elastic ©ne, 

 the amplitude may be suddenly increased at the 

 instant of entering ; but so rapid is the extinction, 

 that, if the new medium be very extensive, the 

 impulse is soon dissipated. 



Many reports throughout the central states in- 

 dicate localities of silence which are not expressed 

 upon the map. The reason for omitting them is, 

 that it has been impracticable to secure a sufficient 

 density of observation (i.e., a sufficient number of 

 reports per unit area) to enable us to mark out and 

 define these smaller areas with very great pre- 

 cision. To do this for the whole country would 

 require some tens of thousands of observations and 



the expenditure of tens of thousands of dollars to 

 systematize and discuss the data. A map shaded 

 to show the varying intensity by varying the depth 

 of the shading would have a mottled appearance, 

 in which the mottling would be most pronounced 

 in the areas of a little below the mean intensity, 

 say, between the isoseismals 3 and 5. This fact is 

 of great importance in the interpretation of the 

 isoseismals, for the omission to consider it results 

 in giving to the middle isoseismals too high a 

 value. In any isoseismal zone, what we should 

 like to ascertain is the mean intensity of the whole 

 area included within that zone. As a matter of 

 fact, the data we possess consist more largely of 

 maximum than of minimum or average intensi- 

 ties, and therefore tend to considerably augment 

 the mean derived intensity above the true mean. 

 This will become apparent by an inspection of the 

 map where the zones of 5, 6, and 7 intensity are 

 dit^proportionately broad, while those of 3 and 4 

 are disproportionately narrow. We have not at- 

 tempted to allow for this source of error, though 

 fully aware of it, because we had no means of de- 

 termining what allowance to make. We have 

 drawn the lines wholly upon the face of the re- 

 turns, and the investigators who may attempt to 

 utilize our results must grapple with the correc- 

 tions as best they may. 



Throughout the states of North Carolina, South 

 Carolina, Georgia, and north-eastern Florida, and, 

 in general, anywhere within about tvv^o hundred 

 and fifty miles of the centre, the energy of the 

 shocks was very great. At Columbia, Augusta, 

 Raleigh, Atlanta, and Savannah the consternation 

 of all people was universal. The negroes and 

 many of the poor whites were for a week or two, 

 not exactly ' demoralized,' but intensely moral- 

 ized, giving themselves to religious exercises of a 

 highly emotional character ; the stronger and 

 deeper natures among them being impressed with 

 a feeling of awe, the weaker natures with a feeluig 

 of terror. And this was general throughout the 

 large region just specified. In all of the large towns 

 within two hundred miles of Charleston, more or 

 less damage v\as suffered by houses and other 

 structures. Walls were cracked to such an extent 

 as to necessitate important repairs, dams were 

 broken, chimneys were overthrown, plastering 

 shaken from ceilings, lamps overturned, water 

 thrown out of tanks, cars set in motion on side- 

 tracks,animals filled with terror, fowls shaken from 

 their roosts, loose objects thrown from mantels, 

 chairs and beds moved horizontally upon the floor, 

 pictures banged against the walls, trees visibly 

 swayed and their leaves agitated and rustled as if 

 by a wind. These occurrences were general, and 

 were more strongly marked, until they became 



