37 o THE POPULAR SCIENCE MONTHLY. 



to produce the greater number of these noises. The microphone is 

 especially sensitive to vertical movements of the soil, whereas the tro- 

 mometer 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 oscilla- 

 tion, he transported his microphone to Palmieri's Vesuvian observa- 

 tory, 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 Vesu- 

 vius 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 plum- 

 met, 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 relative- 

 ly 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 6aw 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 un- 

 derstood, 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. 



* "Report to the British Association on the Lunar Disturbances of Gravity," 1881. 



