Volcanos and Earthquakes. 89 



and clouds of steam, just as Spallanzani,* Scrope,f and Hoff- 

 mann, j observed on StrovihoU. 



We have now to examine the circumstances under which 

 water might find its way to the origin of volcanic action. The 

 difficulties which present themselves when we suppose a direct 

 communication between the sea and the seat of the volcanos, 

 have already been discussed by Gay-Lussac. We shall make 

 an attempt to solve these difficulties. 



M we imagine the sea to have free access by means of fis- 

 sures to the seat of the volcanos, the depth of which, according 

 to the above calculation, may be taken at from 113505 to 

 1^*6829 feet, the elastic force of steam at that depth, where 

 t = 2282'' F,, will be = 5310 atmospheres. But the hydrostatic 

 pressure of these columns of water is only from 3547 to 3963 

 atmospheres. The expansive force of steam at that depth in 

 which the temperature is 2282° F. is, therefore, greater than 

 the hydrostatic pressure opposed to it, so that the latter can- 

 not resist it. But since, as the temperature decreases, this 

 expansive power diminishes more rapidly than the hydrostatic 

 pressure, there must be a certain depth and a corresponding 

 temperature in which thev will be in equilibrium. For a con- 

 stant increase of temperature of 1° F. in 51 ft., this point will be 

 at the depth of 88044 ft. below the surface of the sea, where 



* Voyag. t. ii. p. 21. 



t Considerations on Volcanos, &c. p. 54. A plienomenou obsarved by 

 Sci-ope during the nigbt in the crater of Stroniholi distinctly shows, that, by 

 the force of aqueous vapours alone, the column of lava is raised. The lava 

 once suddenly disappeared in the depth of the crater ; on the contrary, in- 

 numerable little columns of steam appeared at the edges of the mouth of 

 the crater, which arose with a hissing noise. This lasted for some minutes, 

 when the molted mass rose again from beneath, and the phenomenon 

 pursued again its ordinary course. Spallanzani remarks very justly with a 

 view to this, that the compressed vapours prevented from being discharged 

 by the sinking lava which had become tenacious on the surface, will now es- 

 cape laterally through the fissures in the walls of the edge of the crater, and 

 in this case the laVa cauuot be elevated by them. It is not until the lava has 

 been sufficiently heated and become again liquid, that the vapour can rise 

 again ^ith the lava, and that the phenomenon can be re-established. 



ij: Loco cit. p. 9. — D. Curbeto also observed that a dense smoke always 

 followed the streams of lava which were ejected on the 7th June 1731, 

 Von Buch in the Abhandl. d. Berliner Acad, of 1818-1819, p. 77. 



