512 



Mr. R. Mallet on the 



small fissures, permitting sea-water to percolate through the mass of con- 

 solidated and hardened volcanic rock, which, at this depth, constitutes the 

 mass of the island, and in which all three classes of ducts or channels, 

 B, D, and E, are formed, most probably between the partings of super- 

 imposed beds ; or a single large duct may bring in the supply of sea- 

 water, and have such a form, however irregular, as to preclude the 

 steam in the tube from blowing out into the sea, although having a 

 tension many times greater than the direct statical head of the sea-water 

 itself *. 



Diagram No. 4. 



Sett level 



All of the three ducts, B, C, and D, may be multiform, irregular, and 

 varied to almost any extent, provided only that they retain their relative 

 positions, and these only within certain large limits, as shown in the 

 Diagram No. 4. Supposing an outburst finished, and its fragmentary 

 matter partly returned to the bottom of the funnel of the crater at A, the 

 required conditions of whose form are only such that the fallen-back 



* The well-known phenomena of a tube re- 

 peatedly bent in a vertical plane filled with 

 water, but containing air or vapour at the 

 upper part of each of its bends, as at a, a, 

 Diagram 5, described in almost all large trea- 

 tises on hydraulics, may be referred' to as 

 sufficiently illustrating what has been above 

 stated. 



Diagram No. 5. 



