EARTHQUAKES. 369 



eters, and the minuteness of the movements to be observed no doubt 

 entails especial difficulties. The " normal tromometer " of Bertelli 

 and Rossi is a simple pendulum, about six feet long, with an arrange- 

 ment for observing the dance of the pendulum-bob with a microscope. 

 With this and other instruments it has been established that the soil 

 of Italy trembles incessantly. The agitation of the pendulum is usu- 

 ally relatively considerable for about ten days at a time ; toward the 

 middle of the period it increases in intensity, when there generally 

 ensues an earthquake which can be perceived without instruments ; 

 the agitation then subsides. This has been called by Rossi a seismic 

 period or seismic storm. After such a storm there ensues a period of 

 a few days of relative quiescence. 



The vibration of the pendulum in these storms is in general par- 

 allel to neighboring valleys or chains of mountains, and its intensity 

 seems to be independent of wind, rain, and temperature. Care is of 

 course taken not to mistake the tremors due to carts and carriages for 

 microseismic agitation, and it has been found easy to effect this sep- 

 aration. The positions of the sun and moon exercise some influence 

 on these tremors, but the most important concomitance which has 

 been established is that they are especially apt to be intense when the 

 barometer is low. 



Microseismic storms are not strictly simultaneous at different places 

 in Italy ; but if a curve be constructed to represent the average inten- 

 sity of agitation during each month, it is found on comparison of the 

 curves for a year for, say, Rome, Florence, and Leghorn that there 

 is a very close agreement between them. 



Rossi has also made some interesting experiments with the micro- 

 phone on microseisms. In this instrument one electrical conductor is 

 arranged to rest on another at a single point say, a nail resting on 

 its point on a shilling. One copper wire is attached to the nail and 

 another to the shilling, and an electric current, with an ordinary tele- 

 phone receiver in the circuit, is then passed through the system. As 

 long as the microphone is still, nothing is heard ; but on the occurrence 

 of the very slightest tremor, a noise is audible in the telephone. The 

 instrument can be made so sensitive that a fly may be heard to walk 

 near the microphone with a loud tramp, and a touch with a hair to 

 the nail or to the shilling would sound like the grating of a harsh saw. 



Rossi placed his microphone on the ground in a cavern sixty feet 

 below the surface, on a lonely part of Rocca di Papa, an extinct vol- 

 cano not far from Rome, while he listened with his telephone at the 

 surface of the earth. He then heard the most extraordinary noises, 

 which, as he says, revealed " natural telluric phenomena." 



The sounds he describes as " roarings, explosions occurring isolated 



or in volleys, and metallic or bell-like sounds." They all occurred 



mixed together, and rose and fell in intensity at irregular intervals. 



He found it impossible by any artificial disturbance to a microphone 



vol. xxxi. 24 



