Mechanism of Stromboli. 513 



debris shall have some sort of landing-place and support for the larger 

 portion of the mass, so as not all to drop into and permanently block the 

 tube. The lava oozing from the duct, or ducts, B, escaping amongst these 

 fragments, solders them more or less together ; and in proportion as its 

 rate of supply is greater or less, some of it may overflow and drop, in a 

 more or less liquid state, into the tube C and into whatever water it may 

 contain. The tube, however, is emptied at the outburst of nearly all 

 that it contains, and the tension therein being that of the atmosphere, or 

 little more, the sea-water again begins to fill it by the ducts D. This 

 water is already probably considerably warmed in the ducts D ; it receives 

 accessions of warmth from the sides of the tube and from the continual 

 blowing into it of superheated steam and vapours through the ducts E, 

 whose temperature is probably not far different from that of the lava at B. 

 The column of sea-water rises in the tube to a level, we will suppose S, by 

 which time the boiling-point has been reached at the lowest point of the 

 column, namely, that due to the statical head of water, and to all such 

 obstructions above the lip of the tube as tend to hinder the escape and 

 so increase the tension of the vapours and gases occupying the otherwise 

 empty upper part of the tube. At such an instant, the whole column 

 may be lifted through a few inches or feet vertically by the steam locally 

 generated at the bottom of the tube ; and as this incipient evolution of steam 

 escapes upwards the whole column of liquid will be suddenly dropped 

 back upon the bottom of the tube, to be again similarly lifted, and so on 

 until every portion of the column of water has acquired the full boiling- 

 point due to its depth, &c. This is the cause of the detonations heard 

 before the outburst. As soon as this has been reached, the whole mass 

 of water below S is driven violently upwards, and partly by its impulse, 

 but mainly by actual steam-tension, drives before it the mass of obstructing 

 matter filling the bottom of the funnel at A, and the whole is driven 

 forth together in a mingled cloud of dust, stones, shreds of half-melted 

 lava, steam, and pulverized water. When the tube is left empty, and 

 after the fall back of the fragments, the whole apparatus is ready for a 

 repetition of the process. It is obvious that the depth of the tube below 

 the level of the sea, and the temperature of its sides and that of the 

 steam entering at E, determine the force of the outburst, that the rate 

 of supply of w r ater and steam determine the intervals between the out- 

 bursts, and that the proportion between the volume of steam and that 

 of pulverized water, at each outburst, depends upon the capacity (that is, 

 the greater or less section) of the tube C. If that be narrow in proportion 

 to its total depth, as is probably the fact, then very little water will be 

 ejected in any state but that of steam. It is not necessary that the 

 temperature of the column of water in any part of the tube C should 

 ever reach the tension due to a temperature equal to that of the lava 

 escaping from B ; it only needs to be such as shall raise its own column 

 to the lip of the tube and overcome the obstructions there encountered 

 with a sufficient residual tension left to blow these a greater or less height 



