Page:Popular Science Monthly Volume 31.djvu/586

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Report upon the Charleston Earthquake.—The United States Geological Survey, according to a communication from Messrs. Dutton and Harden in "Science," has received reports relating to the Charleston earthquake from more than sixteen hundred localities, giving a much larger amount of information than has ever before been collected concerning any one earth-quake. A considerable proportion of the reports were in answer to a printed list of questions which had been sent out, to direct attention to the most distinct and significant features of the phenomena. The first point to receive attention is the magnitude of the area affected by the shocks. The earth-quake was felt in Boston, near Lake George, and at two points in the Adirondacks, at several places in Ontario, Michigan, and Wisconsin (at La Crosse, nine hundred and sixty-seven miles from Charleston, the most remote point within the United States which has given a positive report), in some of the Florida Keys, in Cuba, and in Bermuda, a thousand miles from Charleston. The area within which the shakings were marked enough to attract considerable attention, would be somewhat more than circumscribed by a circle of a thousand miles radius. The movement might, however, have been detected, by instrumental observation, over a much greater area. There are some large tracts within the area which show comparatively feeble intensity. The most conspicuous of them is the Appalachian region. This fact is of interest in its bearing on the supposition that mountain-ranges serve as barriers to the propagation of earthquakes. Another minimum area was in Indiana and Illinois; and it nearly corresponded with the area in which a considerable earthquake occurred on the 6th of February. The coincidence is curious, if not significant. At nearly all places within about two hundred and fifty miles of the center the energy of the shock was very great. Coming nearer to the center, the intensity increased on all sides, with differences in kind as well as in degree. "The phenomena characteristic of the epicentral area cease with something like abruptness as we radiate away from the epicentrum. The central phenomena are those produced by shocks in which the principal component of the motion of the earth is vertical. Proceeding outward, 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 rapidity of these transitions, or the shape of the intensity-curve into which they may be translated, is supposed to be dependent upon the total energy and depth below the surface of the shock. The distance from the epicenter to the point where the rate of decline of the intensity is greatest, is simply proportional to the depth of the focus, and is the same whether the energy be greater or less. This gives a rule for estimating the depth of the focus. Applying the rule, we have a computed depth of twelve miles, with a probable error of one or two miles, for the focus of the principal shock at Charleston. There is reason for beliving that none of the great earthquakes of the last one hundred and fifty years have originated from a much greater, and few from as great, a depth. The city of Charleston was situated at from eight to ten miles outside of the area of maximum intensity. Had the seismic center been ten miles nearer to it, the calamity would have been incomparably greater than it was. The shocks were also probably made easier for the city by the loose nature of the soil and quicksands over which it is built. The time-data have not been fully worked out, but it is thought that they will give a speed of propagation exceeding three miles a second—a rate which "will probably prove unexpected to European seismologists."


Some Popular Errors about the Eskimos.—Mr. John Murdoch has exposed, in the "American Naturalist," a few popular errors in regard to the Eskimos, some of which have found their way into Hovelaque and Hervé's recent book on "Anthropology." Polyandry is not common among them, as is asserted there and by Bancroft, but is very rare, if it exists noticeably at all. Eskimo houses are seldom, if ever, holes dug in the earth, as the French authors say, but wooden, turf-covered lodges, built sometimes over an excavation of only moderate depth,