354 Mr Fisher, On the transmission of 



On the transmission of Earthquake waves through the earth. 

 By Rev. O. Fisher, M.A., F.G.S., Hon. Fellow of Jesus College, 

 Cambridge, and of King's College, London. 



[Received 23 November 1903.] 



In Mr Milne's Report on Seismological Investigation at the 

 Southport meeting of the British Association, 1903, the conclusion 

 arrived at is that the high velocity of preliminary tremors of 10*5 

 to 12 kilometres per second suggests a high rigidity for the 

 earth ; while the approximate uniformity of speed within its core 

 indicates approximate uniformity in those properties which deter- 

 mine the rate at which it transmits vibrations. 



I have always doubted the doctrine of extreme rigidity of the 

 earth's interior, and have felt that the phenomena of instability 

 of the crust, and the enormous amount of steam emitted by 

 volcanoes point to the substratum of the crust being a liquid 

 magma holding water in solution at a temperature above the 

 critical. I have therefore made the following attempt to account 

 for the peculiarities of the transmission of earthquake waves on 

 this hypothesis, for if that can be done an objection to the theory 

 of liquidity will be removed. 



I shall assume that the disturbance originating a world- 

 shaking earthquake is caused by a relaxation of pressure of the 

 earth's crust upon the subjacent magma, which we may conceive 

 to be brought about, either by a fitful spasm of mountain elevation 

 due to their mass being lightened by denudation, or by ~ome other 

 cause. Hence on the subjacent portion of the magma the pressure 

 is slightly relieved, and a disturbance is produced. 



Henry's law of the absorption of gases by liquids asserts, 

 that the ratio of the volume of the gas which can be held in 

 solution by a given volume of the liquid is the same at all 

 pressures 1 . Thus if rV be the volume of gas which can be held 

 in solution by the volume V of the liquid, rV is the same at all 

 pressures. 



It apparently follows that if the liquid be in a state of com- 

 pression, so that when the pressure is relieved the volume V 

 expands to V + v, the volume of gas which it can then hold in 

 solution will be r(V + v). Consequently an additional volume of 

 gas rv, proportional to the increase of volume of the liquid, can 

 be held in solution, and no gas will be extruded in consequence 



1 Phil. Trans. 1803. 



