Page:Popular Science Monthly Volume 77.djvu/163

Rh however, clearly indicated by reports from which the intensity of the motion may be estimated, and from which isoseismal lines may be constructed.

The data contained in the press reports can be readily compared with the Rossi-Forel scale of intensities. The greatest intensity shown is in the falling of chimneys and in the cracking of walls, which barely approximates eight in the scale. It is not practical to separate these localities of greatest intensity from a more extended region where the earthquake had an intensity more nearly comparable with seven in the scale. Within this area furniture was overthrown, plaster fell from ceilings and from walls, and hanging pictures and other suspended ornaments were jerked loose from their fastenings. Outside of this most severely disturbed mesoseismal area there is a belt from ten to a hundred miles wide where the intensity approximates the next lower point in the scale. Here lighting fixtures, chandeliers and bookcases are reported to have swayed, dishes were broken, chairs rocked or were moved or overthrown, houses were rocked, chimneys cracked and clocks were stopped. Beyond this again is a zone where the evidence of the earthquake consisted in the more subdued motions described as shaking of houses and of furniture, rattling of dishes, bottles and tinware and swinging of suspended objects. This zone has a width of from fifty to a hundred and fifty miles and marks the location of the fifth isoseismal. Continuing the diminuendo, the earthquake next announced its rapid passage by the rattle of windows, the jarring and quivering of houses, and by gentle shaking and trembling of furniture. This is the fourth intensity, and characterizes a zone that merges imperceptibly into the next, where few people noticed the disturbance, and where it appeared as a merely perceptible jar, or a slight undulation, most frequently noted only in the upper stories of high buildings. In this gentle form it disappeared to human senses at a distance, in all directions, of some four hundred miles from the central region. How much farther did it speed, unseen, unheard and unfelt? You will remember that it left a record on the seismometer in Washington. This city is nearly four hundred miles beyond the zone where the waves ceased to be perceptible to the unaided human senses. From this record we may infer that in the brief span of two or three minutes the earthquake waves spread over a circular area about sixteen hundred miles in diameter.

A classified review of the little things that happened in the upper Mississippi Valley, when a block of the earth slipped in the northern part of Illinois may perhaps be of interest. The phenomena reported, affected at least five of the human senses, the senses of general well-being, of touch, of equilibrium, of hearing and of sight.

A rheumatic woman in the zone where the disturbance was very feeble "felt the vibrations keenly and told others of the earthquake,