152 Prof. J. Prestwick 



S J L S 



I 



~FiG. 6. — Laya column in a state of rest. 

 S. Sedimentary strata ; a. impermeable ; b. permeable. L. Laya in duct. 



pressure may have been, the change from the static to the kinetic 

 immediately destroys the balance ; the lateral pressure of the lava, L, 

 in the duct is no longer equal to that of the superheated vapour in n, 

 which is therefore driven into the yielding mass of molten lava and so 

 ascends to the surface. 



Fig-. 7. — Laya column during flow. 



As the escape of this vapour of maximum tension relieves the 

 pressure on the water in the shaded part m, a portion of this water 

 is at once driven in to replace it ; and so long as the pressure of the 

 column of lava in the duct is less than that of the vapour or water in 

 the state in which it exists at n, so long will successive increments of 

 the vapour be driven into the lava and cause continuous explosions. 



The only limit to the depth at which the introduction of water into 

 the duct can be effected will be that at which the general underground 

 percolation of water is stayed in the manner before explained — that is, 

 by the increase of the temperature due to central heat — a depth far in 

 excess of that where we are supposing the introduction of the surface 

 waters to take place. 



With regard to the lesser depths and lesser general temperature 

 of the water in the strata surrounding the higher parts of the volcanic 

 duct — capillarity will there continue to force the water forward so long 



