Anearthquakecomeslikeathiefinthenight,withoutwarning.Itwasnecessary,therefore,toinvent

　　instrumentsthatneitherslumberednorslept.Somedeviceswerequitesimple.one,forinstance,consistedof

　　rodsofvariouslengthsandthicknesseswhichwouldstanduponendlikeninepins.whenashockcameit

　　shooktherigidtableuponwhichthesestood.Ifitweregentle,onlythemoreunstablerodsfell.Ifitwere

　　severe,theyallfell.Thustherodsbyfalling,andbythedirectioninwhichtheyfell,recordedforthe

　　slumberingscientistthestrengthofashockthatwastooweaktowakenhimandthedirectionfromwhichit

　　came.

　　Butinstrumentsfarmoredelicatethanthatwereneededifanyreallyseriousadvancewastobemade.The

　　idealtobeaimedatwastodeviseaninstrumentthatcouldrecordwithapenonpaperthemovements,ofthe

　　groundorofthetable,asthequakepassedby.WhileIwritemypenmoves,butthepaperkeepsstill.With

　　practice,nodoubt,Icouldintimelearntowritebyholdingthestillwhilethepapermoved.Thatsoundsasilly

　　suggestion,butthatwaspreciselytheideaadoptedinsomeoftheearlyinstruments(seismometers)for

　　recordingearthquakewaves.Butwhentable,penholderandpaperareallmovinghowisitpossibletowrite

　　legibly?Thekeytoasolutionofthatproblemlayinaneverydayobservation.Whydoesapersonstandingina

　　busortraintendtofallwhenasuddenstartismade?Itisbecausehisfeetmoveon,buthisheadstaysstill.A

　　simpleexperimentwillhelpusalittlefurther.Tieaheavyweightattheendofalongpieceofstring.Withthe

　　handheldhighintheairholdthestringssothattheweightnearlytouchestheground.Nowmovethehandto

　　andfroandaroundbutnotupanddown.Itwillbefoundthattheweightmovesbutslightlyornotatall.

　　Imagineapenattachedtotheweightinsuchawaythatitspointrestsuponapieceofpaperonthefloor.

　　Imagineanearthquakeshockshakingthefloor,thepaper,youandyourhand.Inthemidstofallthismovement

　　theweightandthepenwouldbestill.Butasthepapermovedfromsidetosideunderthepenpointits

　　movementwouldberecordedininkuponitssurface.Itwasuponthisprinciplethatthefirstinstrumentswere

　　made,butthepaperwaswrappedroundadrumwhichrotatedslowly.Aslongasallwasstillthependrewa

　　straightline,butwhilethedrumwasbeingshakenthelinethatthepenwasdrawingwriggledfromsidetoside.

　　Theapparatusthusdescribed,however,recordsonlythehorizontalcomponentofthewavemovement,which

　　is,infact,muchmorecomplicated.Ifwecouldactuallyseethepathdescribedbyaparticle,suchasasand

　　grainintherock,itwouldbemorelikethatofabluebottlebuzzingroundtheroom;itwouldbeupanddown,

　　toandfroandfromsidetoside.Instrumentshavebeendevisedandcanhesoplacedthatallthreeelementscan

　　berecordedindifferentgraphs.

　　Whentheinstrumentissituatedatmorethan700milesfromtheearthquakecentre,thegraphicrecord

　　showsthreewavesarrivingoneaftertheotheratshortintervals.Thefirstrecordsthearrivaloflongitudinal

　　vibrations.Thesecondmarksthearrivaloftransversevibrationswhichtravelmoreslowlyandarr