Natural History of Volcanos and EarthquaTies. 245 



also a forerunner of almost all eruptions, especially" of those of 

 Vesuvius. 



But if a reaction should take place in the column of water, yet 

 the rising vapor would soon be so far cooled down as to become 

 liquid again, without the expansive force of the enormous quanti- 

 ties of vapor formed at the volcanic focus being thereby percepti- 

 bly diminished. In addition to this, the hydraulic resistance in 

 the narrow channels, through which the water is admitted, in- 

 creases very considerably as its velocity becomes greater. But 

 the column of water, by which the aqueous vapor is cut off from 

 communication with the surface, acquires very great velocity in 

 those narrow channels, from the enormously increased elastic 

 force of the steam, by which the resistance may very easily be 

 increased to the extent of much more than 1000 atmospheres. 

 So that, notwithstanding that the expansive force of steam whose 

 temperature exceeds 1754^ or 1881° F., is greater than the hy- 

 drostatic pressure of the column of confining water, yet this re- 

 sistance may suffice, in the manner just mentioned, to raise a col- 

 umn of lava, of even a greater height than we have above reck- 

 oned, to the summit of the volcano. If we may be allowed to 

 make a comparison with an analogous phenomenon, it may be re- 

 membered that the touchhole of a cannon, or of a bore-hole in a 

 mine, does not weaken much the action of the powder, although 

 the proportion of the diameter of the touchhole to that of the 

 mouth of the cannon is as 1 : 30.* If Perkins's well known ob- 

 servation,! that water and steam cannot be forced through nar- 

 row openings in the red-hot generator of a steam engine, is appli- 

 cable to the gigantic generator, which formed the volcanic focus : 

 this might be added to the causes already mentioned, which afford 

 resistance in the channels through which the waters are admitted. 



So long as the communication with the sea remained open, the 

 volcano could never come to a state of rest, although the forma- 



* But even when the touchhole hecomes considerably widened by frequent use, 

 the cannon is still of service, although, indeed, its power is somewhat dimin- 

 ished. Yet the force with which the powder projects the ball is equal to about 

 2200 atmospheres, in which the loss occasioned in the absolute expansive force of 

 the powder by the touchhole, &c., is already allowed for. Muncke in Gehlers 

 Physikal. Wortembuch, new edition, v. i, p. 712. 



t Quarterly Journ. ofSc. July to Dec. 1827, p. 471, and Annal.de Chim. et de 

 Phys. xxxvi, p. 435. See also Muncke in Poggendorff's Ann. vol. xiii, p. 244, and 

 Baffin the same, vol. xxv, p. 591. 



