81 



When the jjlatc is to he set in niotiim Iiy an earthquake, a light ilrivin» 

 «•lc)ck is clcsiral)lc, whose sjK'ed will tjiiickly attain its steady value. The same 

 form of govi'rnur as is usctl in the oontinuuusly running eloek may ajipropriately 

 i)e a))]>lii(l, and the same means of coniniiniicating motion to the plate. 



Fi<rs. 41 and 4ö, I'late XXI, give a seetion and plan of a driving clock 

 which is started i>y the release of the armature in fig. 40. The cord a pa.ssc.s 

 over a ))ullev on the ceiling, and carries a driving weight of about half a kilo. 

 h is a drum on which the cord is wound : its arhor jirnjects on lioth sides and is 

 jointed, as in fig. 8, Plate III, to two spindles which revolve in slot-guides cc 

 and carry dis<-s dtl, which roll on two glass plates near their edges, and so cause 

 them to revolve. These form two record-receivers; and a third is also driven by 

 the same clock, by means of an independent cord on b, whicli, wlien the clock 

 runs, drags along a carriage on a straight pair of rails, the carriage having a long 

 strip of smoked glass fixed to it. The axles and wheels of the carriage, and the 

 rails, are arranged on strict kinematical principles, to avoid the possibility of 

 any transverse shaking; and the cord which drags the carriage is kept tight 

 enough to prevent longitudinal shaking. 



§ 82. JlequircmciiU of a ScUmnlogical Obfcrvatorij. 



I'"i>r the registration of oidinary cartlujuakes the ci|uipiiicnt, t > lie I'nirly 

 com])let<', .-hould comprise : — 



[.\|. A pair of single-freedom horizontal seismiij^raphs writing on a 

 jilate which is kept always revolving. 



[B]. A I air of single-freedom horizontal seismograj)hs writing on a plate 

 which is started into motion by an earthtpiakc. 



(('|. \ double-freedom hori/.ont:d seismograph writing on a plate whicli 

 may either be kept always revolving, or be slarted into motion by an 

 cartlu|iiakc. 



fl)|. .\ doidilcfn cdoni horizontal seismograph writing on a fixed i>lale. 



|K|. .V Vertical motion (single-freedom) seismograph writing on a plate 

 which is started into motion by an earthquake. 



[F|. For tlii-sc one continuous-motion driving clock with a governor will 

 be needed — like that shown in I'lale II. It may drive two plates, 

 if ri'ipiired, by having the arbor finm whicli tlicv'laki' their motion 

 jtrolonged outwards on both .«ides. 



\(i\. \ scf'oiid smaller cliN-k, also with a goviinoi', and «ifh a magnetic 

 .••farting appendage, will be requircil for \\\\ and [K|. 



|II|. One or, iielter, .M-vcral cireuit-clo>ing s<'ism')s<'opes (§§ (i.S and 71 I 

 shouM be I u( in connection with (his clock. 



|I|. The same cirenit-<'loser >hoidd actuate a time-taker, to jjive a gene- 

 ral idea of the timr- of iM-currenee of the disturbance. 



|.I|. .\ useful .addition is a set <if ele<'tr"magnctic time lii-kers (in one cir- 

 cuit with a «lock), whidi mark seconds .-inuiltaneouslv on all the 



