59 



|K'riinl Wiivi's ;i]i|i(;iiliin' \iTV fMiiitlv at iiit< tvmI.--, and ilicsc apjH'ar fo iiim|iri>c 

 till' whole I't'ciiid of tlif liist ('aillii|iial<i'. 



Tin- si'cuiul ami relatively laij^i' .-li(«-l<, wliieli ueeiirnd alidiil twci lioiirs 

 latiM', also lietriiis in a tremor ol' .-lidit ))('iiii(l waves, wliicli lie<iiine \iiy pinmi- 

 nent at A, altliniiuli llnv "lav I»' detected lor some time liefore that point is 

 roachf'd. In a lew si'conds more these develop into lartre motions of mmh 

 I(inf];or jieriod. The i;i-eatcst displacement is at c, wlu'rc a Inreli from 10. to ^^'. 

 inoasiivi's '\'t mm. on the record, correspond inj;; to 5 mm. of aetnal earth motion. 

 A little later ail interxal of comparative rest occurs; Imt violent motions licijin 

 air:<in at (/. 'flie di-tnrliance continues irreirnlarly, and with Liiadiially diminisli- 

 injr amplitnde, for ni'arly 1? revolntions more. Tiio last trace of visilile motion 

 i.s at e on tin- inner I'irelc. The second record extends over fully L' revolntions 

 in all, .sliowlnjj tliat tin' <artlii|iiake lasted lor three minutes. 



The shoi't-pcriod waves with which the earthrpiake Ix'ijins are present, more 

 or les-i, din'ini; one minute. At tiist they exist alone. Then they may Retraced 

 sii|)erposwl on tiie jrrcat movemeiit> wliiili constitute the jirineipal -hock. I!ut 

 in the later parts of tite record, diirinj; the f;radual dying out of disturbance, 

 tliev do not ap])ear : the motion then consists entirely of Ioul;- {Ksriod waves. 



The .-hort-iK'riod waves at the lici;inninu; and durinj;; the eaily part of the 

 earthquake liavc a period of only ti.lT >t-r., an<l their amplitude is no more than 

 a very small fraction of a millimetre. The «greatest liack and foi-th motion, at i-, 

 o<'('U])ies aliont O.K sec. As the distiirliance ceases the averatje periotl of the 

 undulations a|i|K'ai-s to inerea.se. 



The record is, a> has heen said, ineom|.letc, inasmuch as it üive- onlv one 

 component of the horizontal displacement. lndc|iendent evidence showed, how- 

 ever, that tlie com|Kinent j^iven heri' was the principal part of the motion. l'"or 

 a roufijh estimate of maximum vehxity and acceleration in the Rn.st-AVest direction, 

 we may take the motion at r as a sim|>le harmonic displacement with ]ierioil (i.S 

 sec. and amplitude 2. Ö mm. This ijives a maxininm velocitv of I!l.(! nun. per 

 »'<•.; and a maxinmm ac<'eleration of 1")! mm. jier see. per sec, or ,.', of»/. 



;■ ')«. i:.iiii„iii<,/.r „f M,i,rii mil. /.sa.'.* 



'fen ilays later than the eartlnpiake just de.scrilied, anotlx'r occurred, of 

 sullicient violence to do a small amount rif daina<;e to liuil<lin<rs, l>v overturniu;^ 

 chimneys und crackiiiif walU. A record of this shock, ohtained l)\' a pair of 

 iiori/ontal pendulum< of tlx' coue-lioli form (ile-crihcd in ^ '2\), is trjven in I'lale 

 X\'. The inner |M)in|cr was placi-d .■^imewhat ol>li(|Ucly to the radius ol' the 

 plate, whicli accounts for the skew character of its markiu;rs. It n'^istertnl IvW 

 movement, and tli(' outer poin(<'r N-S niovemenl. Previous to the occunem'c of 

 the e;irthi|uaki', the cii-cular path tiaeed liv the oulir pointer li.'id Ix me so wide 



* ,S<-i- iilxi Triiii-. iil'llii' .S<-iKiiiiilii;;li'iil ,SiH-i4'r\ ulMiiimii, \iil. |\', |i. 7:!. In lliiil |i'1|m-i- iIh- 

 iiiiilli|>l)iiig rnliii •ii'>-<l in iilitniiiiiig llim n-cnrii huh iiiiii|v>Tlriitly );iveii no 'i lu I, iiisleiiil iif 

 4 1ul. 



