218 



GEYSERS OF ICELAND. 



[Ch. XXXIII. 



projected to a greater height than the water. To leeward of 

 the vapour^ a lieavy shower of rain was seen to fall.^ 



Among the different theories proposed to acconnt for these 

 phenomena, I shall first mention one suggested bj Sir J. 

 Herschel. An imitation of these jets, he says, may be pro- 

 duced on a small scale, by heating red-hot the stem of a 

 tobacco x^ipe? filling the bowl with water, and so inclining 

 the pipe as to let the water run through the stem. Its 

 escape, instead of taking place in a continued stream, is then 

 performed by a succession of violent explosions, at first of 

 steam alone, then of water mixed with steam ; and, as the 

 pipe cools, almost wholly of water. At every such paroxysmal 

 escape of the Avater, a portion is driven back, accompanied 

 with steam, into the bowl. The intervals between the ex- 

 plosions depend on the heat, length, and inclination of the 



pipe ; their continuance, on its thickness and conducting 

 power. t The application of this experiment to the Gleysers 

 merely requires that a subterranean stream, flowing through 

 the pores and crevices of lava, should suddenly reach a fissure, 

 around which the rock is red-hot or nearly so. Steam would 

 immediately be formed, which, rushing up the fissure, might 

 force up water along with it to the surface, while, at tlie' 

 same time, part of the steam might drive back the water of 

 the supply for a certain distance towards its source. And 

 wlien, after the space of some minutes, the steam was all 

 condensed, the water would return, and a repetition of the 

 phenomena take place. 



There is, however, another mode of explaining- the action 

 of the Geyser, perhaps more probable than that above de- 

 scribed. Suppose water percolating from the surftice of the 

 earth to penetrate into the subterranean cavity AD (fig. 131) 

 by the fissures FF, while, at the same time, steam at an 

 extremely high temperature, such as is commonly given out 

 from the rents of lava currents during congelation, emanates 

 from the fissures C. A portion of the steam is at first con- 

 densed into wa,ter, while the temperature of the water is 

 raised by the latent heat thus evolved, till, at last, the lower 



* Mackenzie's Iceland. 



t MS. read to Geol. Soc. of Loudon, Feb. 29, 1832. 



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