Volcanoes and Earthquakes 347 



It < learly cannot be the mere reduction of pressure. As long 

 as the pressure was equally distributed over the walls of the 

 vessel, we have seen that it was successfully resisted; so soon, 

 however, as it was suddenly relieved at one point, a great 

 uality in tin- tin-ion of the gas in the immediate vicinity 

 of the point would be the result, the gas immediately at the 

 opening assuming at once the atmospheric pressure, while 

 that at, say, the eighth of an inch from it is at the tension of 

 the gas in the tube. The practical effect of this sudden in- 

 equality of pressure would be to produce a tug on the mass of 

 elastic fluid, which would cause the walls momentarily to tend 

 to collapse, and this tendency to collapse would be transmitted 

 through the glass as a wave. This wave would to a certain 

 nt distort, and therefore weaken, the walls; and con- 

 nth, if the pressure inside were great enough, it would 

 hurst them ; if not, the only effect would be that a shock would 

 traverse the walls of the vessel, and the pressure would relieve 

 itself by the orifice. 



I.- t 11- suppose the vessel to be a subterraneous cavity 

 containing an atmosphere of very great tension, and that 

 suddenly the envelope gives way at one point, what will be 

 the result? Just as in the case of the glass tube, the sudden 

 i -lief of pressure will, in the way indicated above, cause the 



la to experience a momentary collapsing impulse, which 

 will be propagated as a wave until extinguished by the imperfect 



! -ity of the crust. The sudden outburst will be a \d< -mi. 



mption, and the consequent collapsing shock will be an 



iquake, whi h ith r will or will not be accompanied by 



"f tin- Tii>t, according to the strength of the walls and 



the greatness of the pressure. 



, however, not necessary that there should !> .1 visible 

 volcanic eruption. For, suppose two such subterraneous 

 cavit i, separated from one another by 



a wall weaker than that \vhi< h srparatrx nther of thorn : 

 the outside of the earth ; then, if the pressure in the one becomes 

 so great as t r between i ' M K -ult will 



be an earth|u.ii. ' -nnil.n in the two 



unit. ;nay go on m PMHIIL; until th-v IMUM into a 



