F. A. Ferret— The Flashing Arcs. 329 



Art. XXXII. — The Flashing Arcs: A Volcanic Phenome- 

 non^ by Frank A. Perret. 



On the afternoon of April 7, 1906, the present writer, in 

 company with Professor Matteucci, was skirting the southern 

 flank of Vesuvius on a trip to the main source of the lava at the 

 Bosco Cognoli. The volcano at this time was entering one of 

 those paroxysmal phases by which the eruption— already three 

 days old — worked progressively up to its great culmination, 

 which occurred, it will be remembered, between this and the 

 following day. The ejected detritus was of a mixed nature, 

 viz., the fresh lava, clear red in full daylight, being mingled 

 with old material from the upper portions of the cone, then in 

 process of rapid demolition. The frequency of the explosions 

 varied from approximately one every three or four seconds to 

 at least three per second. Although powerful, they were very 

 sharp and sudden in their nature, and at the instant of each — 

 but before it could be sensed by the eye or ear — a thin, lumi- 

 nous arc flashed upward and outward from the crater and dis- 

 appeared in space. Then came the sound of the explosion and 

 the projection of gas and detritus above the lip of the crater. 

 The motion of translation of the arcs, while very rapid in com- 

 parison with that of the detritus, was not above the limits of 

 easy observation and there could be no doubt as to the reality 

 of the phenomenon, which was rej)eated some hundreds of 

 times. 



The writer attempted photography but without success, the 

 failure being due in part, perhaps, to the velocity of the arcs 

 and their very moderate brightness, but also most certainly to 

 the extreme unlikelihood of the shutter being snapped at the 

 precise instant of the apparition. One of the photographs is 

 reproduced in fig. 1, and I have permitted myself to trace 

 upon the negative film with aniline two circles which print out 

 in the approximate appearance of the arcs, assuming these to 

 have been arrested instantaneous^ during their up- and out- 

 springing from the crater.* It should be stated that this illus- 

 tration, because of its crudeness and the lack of motion, con- 

 veys but a poor idea of the actual phenomenon, the beauty of 

 which lies in the delicate luminosity, the elegance and perfec- 

 tion of form, and the grace and vivacity of the arcs amid the 

 contrasting color and relatively sluggish movement of their 

 surroundings. 



During the extraordinary activity of Stroinboli in 1907 the 

 writer did not observe the flashing arcs although some of the 

 explosions appeared to have the qualities which should have 



* It may be that I have centered the circles too high above the crater. 



