In just one day and one night - August 24 to 25 - in 79 A.D. a sequence of deadly pyroclastic currents coming from Mount Vesuvius destroyed and buried the cities of Pompeii and Herculaneum. But this volcano, periodically active and despite his modest size considered one of the most dangerous volcanoes of the world, due its vicinity of the urban conglomerate that is the city of Naples, provided also important insights how a volcano "works".
Fig.1. F. A. Perret with an improvised "geophone," listening to subterranean noises at the Campi Flegrei (Italy) probably in 1906-1907 (the photo was published in 1907). As an able inventor, Perret used a microphone to amplify the rumors from inside the earth; a cable can be seen in front of his face connecting the geophone to a loudspeaker, positioned on his ear. Photo from "The Day's Work of a Volcanologist." The World's Work, V. 25, November, 1907 (image in public domain).
Frank Alvord Perret (1867-1943) was an American inventor and volcanologist, interested particularly in the volcanoes of the Canary Islands, Japan, Hawaii, Martinique -the infamous Pelee nearly killed him - and Italy. He studied physics at the Brooklyn Polytechnic Institute, but didn't graduate. As a gifted inventor he worked in the laboratories of Thomas Alva Edison, developing new motors, dynamos and batteries. In 1886 he became independent with his own "Elektron Manufacturing Company", which in the followings years experienced a notable success. His health began to fail in 1902 and a warmer climate - like on the Caribbean Islands - promised some relief. On the Island of Martinique he visited the ruins of the city of St Pierre, the desolation and destruction experienced there impressed him profoundly. In 1904, during a visit to Italy, Perret meet Raffaele V. Matteucci, director of the volcanological station of Mount Vesuvius. Matteucci got Perret interested even more in the young and emerging field of volcanology.
The deterioration of Perrets health continued and in 1906 he abandoned definitively his business to dedicate himself to a less strenuous and dangerous engagement: studying active volcanoes!
He monitored Vesuvius during its most recent phase of activity, lasting from 1906 to 1921, dedicating to the eruption of 1906 "the clearest and most complete report ever of a volcanic eruption and its aftermath" (as wrote Milderd Giblin in 1950). Mildred Giblin, from the Geophysical Laboratory of the Carnegie Institution of Washington, commented Perret's work by stating that the:
"scientific contributions of Mr. Perret are unique in that no other volcanologist had the time and opportunity to make so thorough and varied observations on so many types of active volcanoes. He was a daring and sagacious researcher, indefatigable in his quest for information. He was a proficient and discerning photographer, and his publications are freely illustrated with fine pictorial records."
Perret and Matteucci used the "Osservatorio Vesuviano", a hut build in 1841 on the north-eastern slope of Vesuvius, as permanent observation point. It was there, lying in his bed, that Perret one day noted a strange buzzing sound. Rising his head the sound disappeared, so Perret put an iron bar of the bed between his teeth - now he could feel a constant tremor coming from the underground. Perret had discovered the "Harmonic Tremors" - vibrations often preceding a volcanic eruption, generated probably by uprising magma inside the volcano.