Natural History of Volcanos and Earthquakes. 257 



tures, by reason of their unequal specific gravities, seems to be 

 very mnch interrupted by the narrowness and sinuosity of the 

 passage. The intermitting springs of Iceland are probably 

 caused by the existence of caverns, in which the vapor is re- 

 tained by the pressure of the cohimn of water in the channel 

 which leads to the surface. Here this vapor collects, and presses 

 the water in the cavern downwards until its elastic force becomes 

 sufficiently great to effect a passage through the column of water 

 which confines it. The violent escape of the vapor causes the 

 thunder-like subterranean sound, and the trembling of the earth, 

 which precede each eruption. The vapors do not appear at the 

 surface till they have heated the water to their own tempera- 

 ture. When so much vapor has escaped that the expansive 

 force of that which remains has become less than the pressure 

 of the confining column of water, tranquillity is restored, and 

 this lasts until such a quantity of vapor is again collected as to 

 produce a fresh eruption. The spouting of the spring is, there- 

 fore, repeated at intervals, depending upon the capacity of the 

 cavern, the height of the column of water, and the heat gene- 

 rated below.* 



The two distinct classes of eruption in the Geyser, which we 

 have already mentioned, seem to be attributable to two different 

 cavities. A smaller cavity fills quicker, and, therefore, empties 

 itself more frequently ; a larger one fills slower, empties itself 

 seldomer, but with greater violence. But the playing of the 

 Geyser, the Strokr, and some others, is subject to very great va- 



* The eruptions of the Geyser and the Strokr, as obseived by Krug Von Nidda, 

 agree exactly with his explanation of the action of the intermitting springs of 

 Iceland. A thick column of smoke suddenly burst out of the latter, and rose to the 

 clouds. The water was hurled with terrific violence out of the crater, and mixed 

 like a fine mist with the rest of the colimin to a considerable height. From time 

 to time, thin streams of water were seen shooting in a vertical or oblique direc- 

 tion through the column of smoke, sometimes rising to a height of a hundred feet 

 and upwards. Large stones, which had been previously thrown in, were flung 

 almost out of sight, and many so perfectly vertically that they fell down again 

 into the crater, and were again thrown up into the air, like a juggler's ball. The 

 whole of the water was thrown out at the beginning ; and afterwards, the column 

 which ascended from the opening, was composed only of steam, which rushed out 

 with a whistling and hissing noise, and rose with incredible velocity into the 

 clouds. It continued for three quarters of an hour in this slate of activity. It 

 then again became qtiiescent, except that the water, deep in the tube, continued, 

 as usual, to boil violently. 



Vol. xxxvi. No. 2.— April-July, 1839. 33 



