In 1873 Bertelli, by means of microscopes fixed in several azimuths, made 5,500 observations on free pendulums. He also made observations on reflections from the surface of mercury.[143]
One result of these observations was to show that microseismic motions increased with a fall of the barometer. Similar observations were made at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near Rome. On January 14, 15, and February 25 these three observers at their respective stations simultaneously observed great disturbances.
Similar investigations were made at Nice by M. le Baron Prost.
Although doubt was cast upon Bertelli’s observations they appear to have been the origin of a series of microseismical observations, a distinguished leader in which is Professor Rossi, who, in 1874, found that large earthquakes were almost always preceded or accompanied with microseismical storms. In 1878 Professor Rossi worked upon these small disturbances with the assistance of the microphone 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 mètres high and eighteen mètres under the soil. Here over 6,000 observations were made by means of microscopes, on pendulums of different lengths, suspended in tubes cut in the solid rock.
Instruments employed in Italy.—It is impossible to describe in detail the various forms of apparatus which have been used by the Italian investigators. A description 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 movements have been arrived at.
The most important of these instruments is the Normal Tromometer of Bertelli and Rossi.
This consists of a pendulum 1½ 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 wire. This instrument is shown in the accompanying drawing.
Fig. 37.—Normal Tromometer.
b, bob of pendulum; p, prism; m, microscope; s, rim of scale.