Mechanism of Stromboli. 499 



and the f imdus of the crater, to be one fourth of this, and the expansive 

 and repressive agencies in the nicely balanced equilibrium assumed, what 

 effect could any variation of barometric pressure, within the limits ever 

 experienced on any part of our globe, produce in disturbing such equi- 

 librium ? A rise or fall of the barometer at the rate of a tenth of an inch 

 per hour is known only to occur in connexion with the most violent hur- 

 ricanes. A fall of half an inch in the mercury within three or four hours 

 exceeds probably the utmost that occurs in connexion with the most vio- 

 lent Mediterranean storms. But let us suppose a fall of two inches in 

 the barometer to take place instantaneously, how far would that affect the 

 equilibrium supposed of such a column, however supported, and whether 

 free from aeriform matter or containing vesicles thereof ? Two inches of 

 mercury are equivalent to about T !g- of the usual pressure of the atmo- 

 sphere, or to less than one pound to the square inch at the sea-level. The 

 liquid lava supposed to fill the column may be allowed to have a specific 

 gravity of at least 2-000 ; a rise or fall, therefore, of a single foot in the 

 top surface of this column would equilibrate this exaggerated amount and 

 rapidity of barometric change. But the head of the column itself is de- 

 scribed by Hoffman as continually in oscillation upwards and downwards 

 through several feet. It is obvious, therefore, that changes of atmo- 

 spheric pressure have nothing whatever to do with the mechanism pro- 

 ducing the recurrent action of this volcano. 



Whatever reality there may be in the notion, long handed down, 

 of some connexion between the degree of activity of this volcano and 

 changes of weather appears to be merely superficial, and the true inter- 

 pretation will be referred to further on. In any case this notion of 

 equilibrium within the chimney of this particular volcano, and its dis- 

 turbance by changes of atmospheric pressure, would be equally applicable 

 to every volcanic vent in the world, and fails to throw any light upon the 

 special phenomena which characterize Stromboli, viz. the quasi regular 

 recurrence of its bursts forth. The geysers of Iceland belch forth water 

 and steam, and occasionally stones, and the order of recurrence is the same 

 which characterizes those of Stromboli. The latter does not send forth water 

 en masse, its ejecta being steam mixed with some gases, carrying up con- 

 siderable masses of solidified lava, chiefly in angular blocks, mixed occa- 

 sionally, but not always, with torn shreds and "lumps of half-solidified 

 lava, in a more or less plastic state, together with a preponderant volume 

 of dusty pulverulent matter. It is highly probable that water, not in the 

 state of steam, but in that of solid drops, is frequently blown from 

 Stromboli, and such may be felt falling to leeward after some of the bursts 

 forth, though not after all. It may be doubtful, however, whether or not 

 these drops may arise from steam condensed in the air. 



We thus have, to the same succession of phenomena as those of the 

 geyser, superadded in Stromboli some of those of a volcanic vent, of feeble 

 but long-continued activity. 



VOL. XXII. 2 Q 



