Page:Popular Science Monthly Volume 31.djvu/384

370 to produce the greater number of these noises. The microphone is especially sensitive to vertical movements of the soil, whereas the tromometer fails to reveal them. Nevertheless, there was more or less concordance between the agitations of the two instruments. In order, then, to determine the noises corresponding to various kinds of oscillation, he transported his microphone to Palmieri's Vesuvian observatory, where mild earthquakes are almost incessant; here he discovered that each class of shock had its characteristic noise. The vertical shocks gave the volleys of musketry and the undulatory shocks the roarings. By a survey with his microphone he concluded that the mountain is divided by lines of approximate stillness into regions where the agitation is great. If a metal plate dusted over with sand is set into vibration by a violin-bow rubbing on its edge, all the sand congregates into lines which mark out a pattern on the plate: these lines are nodes, or lines of stillness. It appears, then, that, when Vesuvius trembles with earthquake-shocks, its method of vibration is such that there are nodes of stillness.

At the Solfatara of Pozzuoli the sounds were extraordinarily loud; and the prevailing noise could be imitated by placing the microphone on the lid of a boiling kettle. Similar experiments have since been made by Milne in Japan with similar results.

Some years ago my brother Horace and I made some experiments at Cambridge with a pendulum, so arranged as to betray the minutest displacements. It was then but few years since Bertelli and Rossi had begun to observe; we had read no account of their work, and earth-tremors were quite unsuspected by us. Indeed, the object of our experiment, the measurement of the moon's attraction on a plummet, was altogether frustrated by these disturbances. The pendulum was successfully shielded from the shaking caused by traffic in the town, so that there was no perceptible difference in its behavior in the middle of the night on Sunday, and in the day-time during the week. We were then much surprised to find that the dance of the pendulum (for it was not a regular oscillation) was absolutely incessant. The agitation was more marked at some times than at others; the relatively large swinging, though absolutely very small, would continue for many days together, and this would be succeeded by a few days of comparative calm. In fact, we saw the seismic storms and calm of the Italians. As the instrument was designed for another purpose, and was not quite appropriate for microseismic observation, we did not continue to note it after a month or two. But the substantial identity of the microseisms of England and Italy seems fairly well established.

The cause of these interesting vibrations are as yet but little understood, and it may be hoped that the subject will receive further attention. It seems probable that they are in part true microscopic earthquakes, produced by the seismic forces in the neighborhood.