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Ch. XXXIIL] 



AGENCY OF STEAM IN VOLCANOS 



219 



part of tlie cavity is filled with boiling water and the upper 



under high pressure. The expansive force of the 



with steam under high pressure, 

 steam becomes^ at lengthy so great^ that the water is forced 

 up the fissure or pipe E B^ and runs over the rim of the basin. 

 When the pressure is thus diminished^ the steam in the upper 

 part of the cavity A expands^ until all the water D is driven 

 into the pipe ; and when this happens, the steam, being the 

 lio-hter of the two fluids, rushes up through the water with 

 great velocity. If the pipe be choked up artificially, even for 



Tier. 131 



a 



Supposed reservoir and pipe of a Geyser in Iceland."^ 



■ 



minutes, a pfreat increase of heat must take 



form in steam 



that the water is made to boil more violently, and this brings 



on an eruption. 



Professor Bunsen, before cited, adopts this theory to account 

 for the play of the ' Little Geyser/ but says it will not explain 

 the phenomena of the Great one. He considers this, like the 

 others, to be a thermal spring, having a narrow funnel-shaped 

 tube in the upper part of its course, where the walls of the 



become 



* From Sir George Mackenzie's Iceland. 



