60 



ns ]);irtially to ob.sciire the rccoul. The iiuiltipücatioii was ! to 1 ; and one 

 revolution of the ))lato took 88 seeonds. 



Tlie eaith(|uake heti-an, as usual, with very sinill motions, at or near the 

 )ilaee marked n in the inner circle and «' in the outer. (The cireumferentia! 

 distance between the ends o{' the two pointers is shown hy the siiuultaneonsly 

 inscribed lines h and //|. After a few seconds, at r and the conteinjiorarv ]>oint 

 (■', tlu' motion bec;une snd<lenly violent, and, though the eartlujuake laste<l for a 

 lon<^ time, the dis])laeements which oecnircd at c and c' were not exceeded or 

 even ('(luallod diuin<>; the ri'inaindcr of the disturbance. 



The record cover.'; more tlian three revolutions, showint; that the eaithouake 

 continued for at least 4t; minutes. The greatest siuole displacement (at r) from 

 K. to AV. is 21 mm. on the recoid, or 5. '25 mm. of actual motion. C'ontemjiora- 

 neous with this is the ujreatest S. fo N. dis])lacement (at c'), which is IS mm. on 

 the record, or 4.5 nun. of actual motion, ('ombinintj these, we find that the 

 greatest single movement of the ground wa.-^ 7 mm., in the dii'cction N.W., 

 nearlv. This is the return which followed the first great displacement, whose 

 direction was nearly 8.K., and who.se am])litude was .somewhat less than that of 

 the return. 



At this ]ioint the period was about 0.7 .«ec. A simple harmonic oscillation 

 with this ]ieriod, and o.ö mm. amplitude, gives a maxinnun velocity of ol mm. 

 ])er second, and a maximum acceleration of 280 mm. ])er sec. ])er see., or ^j'^-of </. 



The perio<l of vibiation lengthens towards the end of the disturbance; and 

 at the beginning there are traces of short-] criod wa\cs ])ieceding as well as 

 superposed on the ])rincipal movements. 



j; 07. Kavthqxiikc r,f Manh I'-lth, IS.\..'. 



I'late XVI gives the record of another carth(|Uake, which came in the .same 

 month as the shocks described in the two preceding ]'aragraj>hs, and was regis- 

 tered by the seismograph of J^ 21. This was a less .severe, but very long 

 continued disturbance. The Kast-West record (the outer of the two) was some- 

 what obscured by a binadening of the circle traced ])rior to the eartlujuake. 

 The di.'^turbane(! extends o\-er just three com]tl(te revolutions of the jilate, and 

 i\\o cud overlaps the beginning in a way wiiich makes it almost impossibk' to 

 distinguish the beginning or the end. 



'J'he nudtijilying ratio w^as 4 to 1 : an<l one revolutit n of the |>latc oecu])ied 

 7S seconds. The greatest di.s|)laeemcnt is about 2 mm. The mean ])criod, 

 dtn-ing the early ])art of the di.stiirbanee, is somewhat less than 1 .see., but this 

 increases very nnich towards the clo.se. 



§ Ö8. Eniihqnah-c of Aiu/ii-f f^lli. JSS.?. 



The recoi'd of this somewhat sharp shock is .shown in Plate XYTF, in 

 which the multijilying ratio is 4 to 1. 1'he motion was |)rineipally E-\\', and 

 the jKiintei' registering that component haj)pens to ha\-e been set .somewhat 



