Volcanos and Earthquakes, 41 



internal agitation of the sea is 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 vapour would soon be so far cooled down as to 

 become liquid again, without the expansive force of the enor- 

 mous quantities of vapour formed at the volcanic focus being 

 thereby perceptibly diminished. In addition to this, the hy- 

 draulic resistance in the narrow channels, through which the 

 water is admitted, increases very considerably as its velocity 

 becomes greater. But the column of water, by which the 

 aqueous vapour 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 exttnt 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 hydrostatic pressure of the 

 column of confining water, yet this resistance may suffice, in 

 the manner just mentioned, to raise a column of lava, of even 

 a greater height than we have above reckoned, to the summit 

 of the volcano. If we may be allowed to make a comparison 

 with an analogous phenomenon, it may be remembered that the 

 touchhole of a camion, 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 observation, j 



and flames (?) accompanied with tremendous detonations, rise from the sea 

 near Lamerote, during tlie volcanic eruption on that island. Fish and pieces 

 of pumice were seen floating about. Several examples of this sort are 

 cited farther on. 



* But even when the touchhole becomes considerably widened by fre- 

 quent use, the cannon is still of service, although, indeed, its power is some- 

 Avhat diminished. Yet the force with which the powder projects the ball 

 is equal to about 2200 atmospheres, in which the loss occasioned in the ab- 

 solute expansive force of the powder by the touchhole, &c. is already 

 allowed for. Muncke in Gehlers Physikal. Worterbuch, new edition, v. 1. 

 p. 712. 



t Quarterly Journ. of Sc. July to Dec. 1827, p. 471, and Annal. de Chim. 

 et de Phys. xxxvi, p. 435. See also Muncke in Poggendoi-ff''s Ann. vol, xiii, 

 p. 244, and Buff^in the same, vol. xxv, p. 591. 



