INTRODUCTION. 



Any sudden nicch;i'ii<al disturbance oct-urrint; within nr nn thr ^iiifaic i>l' 

 the eartli sets up a state of elastic vibration in the .sulistaiiee of the crust, wiiidi 

 Is propagated witli diminishing intensity throughout the neiglihoui'hood of tlie 

 source, in the form of waves of compression, or distortion, or both. This motion 

 constitute'S an earth(juake. 



The disturbance may also produce, as a se'condary result, gravitation waves 

 in the water of seas or lakes. These admit of easy measurement by means of an 

 ordinaiy tide-gauge ; and in the present paper we have to do only with those 

 vibrations which owe their diffusion to the rigidity and compressibility of tiie 

 solid, or the compri'Ssil)ility of the fluid jiortions of (he earth's eiiist. 



The name earthijuake might lairly be applied to all elastic vibrations in sea 

 or land, without reference to their origin, whether tliat be the crumpling, tearing, 

 or slipping of strata, the eruption of a volcano, the collapse of a subterranean 

 eavitv, tin- explosion of a mine, the rMmI)ling of a cairiage, the tread of a loot, oi- 

 any one of the thousand other events which might be named as causes of a sudden 

 disturbance of e<juilibrium. A reasonable but not strictly detinabie limitation 

 restricts the use of the t«;rm to tiiose comparatively large motions which cxlmil 

 r)ver a considerabl(! area, and whose inimediatr cau.-es an^ natural and, in gcni'ral, 

 somewhat obs<'ure. 



The companitive suddenness of the movements to \vlii<ii Ün- name «arth- 

 ijuake is applied servi's to distinguish them from otiier nuieh more gradual ciaslic 

 ilisturl>aiic<'s which are continually taking jiku'i' in the caitb's crust. It may \>c 

 as^ertc'd with «•ontidcncc that tidal d< rortnations of tln' .-oUd substance nl' the 

 earth are ranscd bv the vai'ving altrai-tion of tlir mo.iri ami >nn, altliongli tii''ir 

 pn-scnce camiot be actuallv observed because (hev ar<! marked bv other and 

 irregular movements of the same or a higher ordi'r of magnitude. 'I'lie exi.stence 

 of tUi-H' ha.s Ikm-ii demonstrated by the remark:dile experiments wliieii .Mes«*rs. 

 ( ii'orge and Horace Itarwin have I'arried nut at the instance of a eiuiimittee 

 appointed by the llritish As-^oci.atiou for (he pnrpoM- of inves(iga(ing (lie lunar 

 di>(nrliance of gravity. Their observations show (hat even when (remors due (o 

 liK'jd tratlic are eliminat<-d, the .solid ground is nevi-r really at rest. An InercaM- 

 of air pre.-jsure over any district causes there a dimple or ela-(ic deprcsNinn ol' ilie 

 H)il, while ii relaxadnii o(" pressure allows (he eardis surface (<i bulge upward-. 

 The changing loacl due (o (lie heaping up and wididrawal of wa(er by tidal 

 action uiakoH the ground beneath sink an<l rise. (>tlier more su|H'rl!eial strains 



