EAETH TKEMOES. 317 



seismical storms. In 1878 Professor Rossi worked upon 

 these small disturbances with the assistance of the micro- 

 phone and telephone, and his first results were published 

 by Professor Palmieri. 



Many of Professor Rossi's observations were made 

 in the grotto of Roca de Papa, 700 metres high and 

 eighteen metres under the soil. Here over 6,000 obser- 

 vations were made by means of microscopes, on pendu- 

 lums of different lengths, suspended in tubes cut in 

 the solid rock. 



Instmiments employed in Italy, — It is impossible to 

 describe in detail the various forms of apparatus which 

 have been used by the Italian investigators. A descrip- 

 tion of one or two of the more important instruments may 

 not, however, be out of place, inasmuch as they will assist 

 the reader to understand the manner in which the various 

 results respecting the laws governing microseismic move- 

 ments have been arrived at. 



The most important of these instruments is the 

 Normal Trom.om.eter of Bertelli and Rossi. 



This consists of a pendulum \^ metres long, carrying, 

 by means of a very fine wire, a weight of 100 grammes. 

 To the base of the bob a vertical stile is attached, and 

 the whole is enclosed in a tube terminated, at its base, 

 by a glass prism of such a form that when looked through 

 horizontally the motion of the stile can be seen in all 

 azimuths. In front of this prism a microscope is placed. 

 Inside the microscope there is a micromatic scale, so 

 arranged that it can be turned to coincide with the 

 apparent direction of oscillation of the point of the stile. 

 In this way not only can the amplitude of the motion of 

 the stile be measured, but also its azimuth. The extent 

 of vertical motion is measured by the up and down 

 motion of the stile due to the elasticity of the supporting 



