122 EARTHQUAKES. 



The origin and phenomena of earthquakes were treated of by Kev. John Mitchell, A. M.,* as 

 far hack as 1760 ; and he appears to have been the first to appreciate and advocate the wave 

 motion of the earth at such times. He conceives the impulse given by the sudden production 

 or condensation of aqueous vapor, under the agency of volcanic heat beneath the bed of the 

 ocean, and that the motion of the earth is due to a wave propagated along its surface from a 

 point where it was produced by this impulse. On the supposition that the crust, filled with 

 cavities and fissures, floats on molten matter, he goes on to say: "As a small quantity of va- 

 por, almost instantly generated at some considerable depth below the surface of the earth, will 

 produce a vibratory motion, so a very large quantity (whether it be generated almost instantly 

 or in any small portion of time) will produce a wave-like motion. The manner in which this 

 wave-like motion will be propagated may in some measure be represented by the following 

 experiment. Suppose a large cloth or carpet, spread on the floor, to be raised at one edge, and 

 then suddenly brought down again to the floor, the air under it, being by this means propelled, 

 will pass along till it escapes at the opposite side, raising the cloth in a wave all the way as it goes. 

 In like manner a large quantity of vapor may be conceived to raise the earth in a wave as it passes 

 along between the strata, which it may easily separate in a horizontal direction, there being 

 little or no cohesion between one stratum and another. The part of the earth that is first 

 raised, being bent from its natural form, will endeavor to restore itself by its elasticity ; and 

 the parts next to it, beginning to have their weight supported by the vapor which will insinu- 

 ate itself under them, will be raised in their turn, till it either finds some vent, or is again 

 condensed by the cold into water, f and by that means prevented from proceeding any further." 

 Eecognising the sea-waves following earthquakes as due to the undulations given to the ocean 

 water at the point directly over that at which the primary terrestrial wave originates, he sug- 

 gests the determination of the centre of disturbance by observations of the direction and times 

 at which several waves reach different stations. 



In 1846 Mr. Mallet, then president of the Geological Society of Ireland, collated the mass of 

 facts furnished in narratives of earthquakes, and educed from them a theory of motions more 

 consonant with the known laws of mechanics. A paper " On the Dynamics of Earthquakes," 

 replete with interest, may be found in volume XXI of Transactions of the Eoyal Irish 

 Academy, directions for scientific inquiry in the Admiralty Manual of 1849, and several sub- 

 sequent investigations and elaborations of the same subject by him in the Reports of the British 

 Association. In one of the last-mentioned volumes viz: that for 1847 there is also a detailed 

 report on the "Geological theories of elevation and earthquakes," by Wm. Hopkins, Esq., 

 in the second section of which he discusses, with much detail, the vibratory motions of the 

 earth's crust produced by subterranean forces. 



Mallet conceives that Mitchell wholly mistook the nature of the earthquake, the mechanism 

 advanced by the latter to account for the origin of the wave and its propagation through the 

 floating crust being inconsistent with the conditions essential to that order of wave which the 

 ascertained phenomena of earthquakes show to be the true one. He divides the phenomena 

 into two classes, viz : those which properly belong to the transit of the wave or waves through 

 the solid or watery crust of the earth, the air, &c., and those which are only the effects of this 

 transit. Both must be kept distinct from co-existent forces, as volcanic eruption, permanent 

 elevation and depression of land, which form no true part of the earthquake, however clearly 

 they may be connected with originating its impulse. Of whatever nature that impulse, the 

 phenomena that present themselves will differ according as its origin is inland or under the 

 sea. If inland, we have, first, the great earth-wave, or true shock, a real roll or undulation of 

 the surface, which travels with immense velocity outwards in every direction from the centre of 

 impulse, and is finally spent and lost in the ocean ; second, the forced sea-wave, carried into 

 deep water on the back of the earth-wave. If the beach be very sloping and the water still, 



* Philosophical Transactions, Vol. LI. t See also Prof. Bischoff, Edinburgh new Phil. Jour., Vol. XXVI 



