122 Scientific Proceedings, Royal Dublin Society. 



On the other hand, should such not have taken place, as the 

 amount of water absorbed increases, the tension of the fluid mass 

 will proportionally do so also. There will arrive a time when the 

 tension of the fluid mass will exceed the resistance of the surround- 

 ing rocks, or the superincumbent pressure, which will result in the 

 rending asunder of them and the extension of the fissure. Such 

 extension may be sufficient to make it reach the surface forming 

 the site of a volcano, or as it extends and gives place for expansion 

 the tension may proportionally so decrease until the balance is 

 restored before the surface is reached. The extension of such a 

 fissure will rather tend towards the surface, as least resistance 

 would be encountered in that direction. 



Such an extension of a fissure will give rise to two or more very 

 distinct series of vibrations : first, we shall have slow ones extend- 

 ing over a considerable length of time, due to the gradually in- 

 creasing compression around the expansible matter which, if 

 apparent at the surface, would assume the characters rather of tilt 

 than that of an earthquake. Local elevation of a small area such as 

 occurred at the Starza of Pozzuoli, pending some years before the 

 outburst and formation of Mount Nuovo, or the same thing at 

 Torre del Greco in the Yesuvian eruption of 1861. The actual 

 rending and enlargement of the fissure will give rise to a series of 

 vibrations of small amplitude, such as are first registered in an 

 earthquake. 1 These will be immediately followed by the sudden 

 arrest of expanding matter coming in contact with the walls of the 

 fissure, which space it injects immediately. The effect is well 

 imitated by allowing steam to escape from a boiler under high 

 pressure, and suddenly closing the opening. Other examples are 

 the sudden injection by water of a blind and collapsed hose, or the 

 rapid closing of a tap from which was flowing a stream of water 

 under pressure, conducted through a pipe of some length. This 

 impact of the fluid matter against the solid fissure walls is followed 

 by a series of diminishing oscillations or throbs. This group of 

 disturbances no doubt constitute the more powerful or destructive 

 portion of the earthquake, and the character of these vibrations, 



1 J. A. Ewiag, Earthquake Measurement, Mnn. Sci. Depart, Univ. To/do., No. 9, 

 p. 54, and following. 



