CHAPTER II. 



INSTUTMEXTS roi! OliSEU VINT, IIoüIZoNTAl. MuVKMENT 

 THE UolllZüNTAL I'KNIH 1.1 .M SKISMOGRAPII. 



S 14. MeU'iircrnrni af rarthqiiakr iimfioiis hi/ vtjtriiicc to a Stcndi/ Point. 



rii-tniiiiciits (Itsiuiicd top till' nicasurcini'iit of raillKiuakc iiidtinns aim, witli 

 i'cw aii<l imimipoitaiit ('Xc<'|itioii.-;, at j^iviiiL;' a point wliii'ii doc.-^ not innvc' (liirinij 

 tlic disturbiint'o, and wliirli will tliorcforp .serve as a datum witli respect to wliicli 

 tlie movements of till- caitli inav lie determined. No aetnal instrument skives a 

 lijjoronsly steady jioint tlironiiliout a proloniicd distnrlianec, luit certain devices 

 wliicli will i\n\\ lie dcsciihed acliieve tile desired olijeef nearly cnoiiuli i'nr ]irae- 

 tieal pur]i()ses, and it is liy tliem that oiir exact kno\vli(lt:;e of eartlii|nake motinn, 

 sj far as we lia\(' any txaet knowledjje, lias been stained. 



I; 1Ö. Kim tic condition yirlillii;/ u Sti'iidi/ J'oinf. 

 If any one jioint of a iii;id body suffers a disjilaeement in tlielinc joinini;- it 

 with the centre of inertia of the bodv, ilic \\holc mass will .-hare the same motion, 

 which is then one of 1 lire tian.-lation : hut if the point sutlers a small displace- 

 ment in any direction at lijjjiit anpries to tiiis line, tlie motion of tiie body will 

 consist of lotaJion ahciit an axis (called the In.stantaneous Axis) which intersects 



the ]irolontration of the line joiiiin<r the displaced 

 ^^ ^^ ]ioint with the (■entre of inertia, and is |H'r])eii- 



diculai- to the ];lane containini;- that line and the 

 direction of dis))laeonient. Points situated in the 

 in.stantaneous axi.« remain at rest, and all other 

 iiaiticlcs of the Imdv move thn Uirh distances i>ro- 

 ]>oitiiiiial to their distance from the instantaneous 

 axis. 'i1ins, in the fitrnre, let the mass sketche<l, 

 whose centre of ineitia is at Cf, he j'ivotted at 

 P. AVlien /-• is moved in the direction GP or 

 pfr, all ]ioints of the mass are eipially disjilaced: 

 hut when P moves thioiiffh a small di.stance in 

 any direetiun PX perj eiidi( ular to Ptr, the mass 

 will revolve about a certain axis //, which is the 

 instantaneous axis for the assunu'd (infinitesimally 

 small) displacement. // is in the plane (jPY, 

 PY heine drawn perjiendicular to i^A' and PG, 



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