536 O. Poulett Scrope — The Mechanism of StromhoU. 



within the vent was one of the causes of the extrusion of the lava 

 from the same orifice.^ 



The sluggish character of the eruptive action of Stromboli, as of 

 other volcanos in a similar phase, is owing no doubt to the slow and 

 equable rise of intensely heated lava within the vent ; so slow as to 

 admit of but one steam-bubble at a time being generated and pushed 

 up ; the lava about it losing much heat carried off in a latent form 

 by the steam-bubble as it explodes, and requiring a further addition 

 of heat to bring about, after an interval of more or less duration, the 

 expansion of another steam bubble. During this interval, the surface 

 of the lava remaining in the vent freezes, and by increasing the 

 resistance to the expansive force below, further checks the develop- 

 ment of the following steam-bubble. In cases where the increments 

 of lava are forced up more rapidly, the formation of bubbles within 

 the vent is proportionately accelerated, as well as their rise and 

 escape from the mouth, till perhaps the explosions become so rapid 

 and continuous as not to be easily counted. This is the case in every 

 violent eruption ; the puffs of steam from the vent being as rapid and 

 the intervals between them as short as the very similar puffs from 

 the chimney of a locomotive engine in full work. The first phase, 

 that of sluggish activity, in a volcano, may be compared to the slow 

 ebullition of a kettle on a hob ; the phase of greater violence to one 

 boiHng over upon a brisk fire. 



The volumes of steam which are seen to burst from the exposed 

 lava surface of a volcanic vent in eruption, whether singly and at 

 intervals, or in rapid succession and with tumultuous violence, are 

 generated in the lava itself, and not in any imaginary fountain or 

 pipe of hot water beneath it. The existence of interstitial water, 

 either in a liquid, or a gaseous form in lava, both as it rises in and 

 issues from a volcanic vent, has so long been recognized among 

 geologists as an established fact, that it seems like going back to the 

 scepticism of the Wernerian age to find a different motive force and 

 mode of action brought forward to account for the phenomena of a 

 volcano. 



Had Mr. Mallet contented himself with the view expressed by Sir 

 C. Lyell in a passage which I am inclined to believe suggested his 

 theory, — since he refers to the context ('Principles,' 10th edition, 

 vol. ii. p. 220), namely, that the phenomena of geysers " have no small 

 interest as bearing on the probable mechanism of ordinary volcanic 

 eruptions, namely, that the tube itself is the main seat or focus of 

 mechanical force," — he would have committed no eiTor. But that 

 would have been no novel idea, and he would not have written his 

 paper and astonished the Eoyal Society with the assertion that the 

 mechanism of Stromboli has not merely some similarity with that of 

 a geyser, but that the volcano actually contains a geyser in its inside, 

 which is the true and only source of the phenomena exhibited there. 



^ I learn from Prof. Owen that on the night of the 3rd Octoher, 1845, he watched 

 the explosions of Vesuvius from the deck of a vessel in the Bay of Naples ; and 

 timing them hy his watch, he found the intervals to vary between the maximum of 

 6| minutes and the minummi 2 to 2^ seconds : from 3 to 4 minutes was about the 

 average time between the explosions. 



