100 Krug von Nidda on the Mineral Springs of Iceland. 



of water to the atmosphere, and carry the water upwards along 

 with them. The violent bursting forth of the masses of steam 

 gives rise to the subterranean thundering noise, and the shak- 

 ing of the ground, which precede every eruption. The first 

 portions of the steam do not penetrate to the surface, but are 

 condensed in the cooler layers of water through which they 

 must pass ; the latter thus acquire a temperature sufficient to 

 permit the passage of the succeeding masses of steam. The 

 column of water, once agitated, does not now, as previously, 

 offer such opposition to the pressure of the steam, and this op- 

 position becomes always feebler, as more of the resisting water 

 is projected from the mouth of the spring by the escaping 

 masses of steam. When the reservoirs of steam are so much 

 emptied, that the expansive power yields to the equilibrium 

 established with the column of water in the passage, the latter 

 shuts up the communicating opening, and the previous tran- 

 quillity is restored, until enough of new steam is accumulated 

 to produce another emptying of the cavity. 



The spouting of the spring hence occurs at intervals depend- 

 ing on the capacity of the reservoir of steam, the pressure of 

 the column of water, and the evolution of heat beneath. 



The geyser exhibited, at the time I observed it, two distinct 

 kinds of eruptions. The smaller were repeated regularly at 

 periods of two hours, and the water spouted to a height of only 

 15-20 feet. The larger occurred at intervals of 24-30 hours ; 

 and the masses of steam then rose to the clouds, while the jets of 

 water were elevated to the height of 90 feet. These two sorts 

 of eruptions are to be referred to two different caverns. A 

 smaller cavern is more rapidly filled, and is consequently more 

 frequently emptied ; a larger one is more slowly filled, and is 

 more rarely emptied, but with so much the greater force. 



The Strokr whose eruptions almost surpass in sublimity 

 those of the great Geyser, has this peculiarity in its structure, 

 that it is both a permanent and an intermitting thermal spring. 

 It is recognised as a permanent thermal spring, by its uninter- 

 rupted violent agitation and boiling ; and as an intermitting 

 thermal spring, by its gigantic eruptions, which seem to occur 

 at intervals of two or three days. A part of the steam gene- 

 rated beneath, passes through the column of water, and meet- 

 ing with no obstacle, escapes into the atmosphere. Hence the 



