358 Mr Fisher, On the transmission of 



These two effects cannot be simultaneous because, as we have 

 already shown, so long as the liquid expands no gas will be 

 extruded. 



Consider then the effect of a diminution of pressure upon the 

 magma at the origin of disturbance. The relaxation of pressure, 

 although impulsive, cannot be instantaneous. The magma being 

 under the compression corresponding to saturation, the relief of 

 compression up to a certain point will in the first place cause 

 voluminal expansion, which, as already shown, will not be accom- 

 panied by extrusion of gas. This expansion will be propagated 



as an elastic wave with the velocity \ y. . As the relief of com- 

 pression continues, the expansion of the liquid magma will reach 

 the limit of voluminal expansion, and gas will begin to be ex- 

 truded. This stage of the disturbance will be propagated as a 



second, gaseous, wave with a velocity a / —=, , less than that of the 



elastic wave, with extrusion of gas but without further expansion 

 of the liquid element of the magma. Hence the disturbance due 

 to the elastic wave will reach a distant station first, and the dis- 

 turbance due to the gaseous wave will after an interval arrive. 



Similar changes will occur at every place which the distur- 

 bance reaches during its passage. Thus the first effect will be 

 voluminal expansion, so that an elastic wave will be continually 

 started in front of the gaseous wave. The consequence will be 

 that tremors arising from elasticity will continue without inter- 

 mission to arrive at a distant station up to the time of the arrival 

 of the gaseous wave. 



This sequence of events is similar to what is observed in the 

 case of a world-shaking earthquake, such as that of the great 

 Indian Earthquake of 12th June, 1897. Mr Oldham, in his 

 exhaustive memoir on this earthquake, divides the movement 

 propagated to distant observatories into 



(1) Waves of elastic compression ; 



(2) Wave of [supposed] distortion ; 



(3) Undulatory gravitational waves ; 



and states that the mean velocity of the second type is 5*29 km. 

 per second 1 . 



I use the term "supposed" distortion, because, as I understand 

 the question, this supposition has been made to account for the 

 two classes of waves having different velocities, not because the 

 type of disturbance specially indicates distortion. And it is because 

 these two sets of waves travelling with different velocities might 



1 Memoirs of the Geological Surveij of India, Vol. xxiv. p. 248, 



