so 



witl) tat-h other. The j'late at each ~tation was to he moved Jiy an independent 

 eloek of the usual kind, it heins: needles.* that the plates should run at the same 

 speed. So far, the arrannrenient would give three (or four) ]iairs of indejiendent 

 reconls of anv earthquake. To eomplete the scheme, time must be simultaneous- 

 ly marked fiu all the i)lutes during: the disturbance. At one station a seismo- 

 s<"-ope was to be pla'rd, which, on the oci-urreuce of an earthquake, would start, 

 or throw into electrical connection with the station circuit, a clock which should 

 send lime signals every second to all the stations, to be recorded by clectro- 

 mairnetie point'-rs on all the plates. The signal-seiidin'j: clock should stoj». or be 

 automatically cut out of the circuit, before the plates had aceom]ilishfd quite one 

 revolution from the beginning of the time signals. 



The first signal, marked on all the plates, would serve to identify the same 

 instant of time in all the records, and the subsequent signals would determine 

 the velocity of the several plates. The scheme w<iuld work only if we could 

 recognise the same earth-wave on all. It would tlnni be easy to mea.sure with 

 much exactness the intervals lietween the instants ;it whidi the same phase of the 

 .same wave appeared at all the stations. 



The method gives so precise a means of determining small intervals of time 

 that we might use stations at no great distance from each other — say 500 metres, 

 which would probably give intervals of about one or two seconds. The.se, 

 however, woulil serve only to determine the azimuth of the epicKntywin. For 

 its position, four much more di.stant stations might perhaps be used ; or, better, 

 two independent groups; of three near statioa*, the groups being at a considerable 

 distance apart. Other plans might obviously be arranged by comliining three 

 stations near each oth<T with one distant station or more — all electrically 

 connected. 



But the practicabilit^v of any of those plans de|)ends on this fundamental con- 

 dition, that si">mc one wave in the di-^turbance is to be identified at all the stations. 

 The writer's observations havi- .»hown (what, indeed, was to be anticipated) that 

 different seismographs roconling the same earthquake at the same .«tation give 

 clfis<ly accordant results. It remains to be seen how far the character of the 

 motion will prove constant when observations arc made at stations whose dis- 

 tances from ea<'h other are compamiilc to or mueh greater than tlie wave-lengths 

 of the ctm.stituent vibrations. If the curved path in which a surface particle 

 moves be due to the simidtancous j)a.ssage of two or more systems of inde|)endent 

 wavc-s, travelling in different lines, no identification will be po.ssible. Hut the 

 large loops of Plate XX i-how that, at least in .-^mie c-a.^es, the jirincipal displace- 

 ments in two directions nccnr together — a thing very uidikrly to happen if they 

 are due to two independent sy.etems of waves. There appears, therefore, a fair 

 prospect tiiat, in favouraiile cases, the identification of .-'ome one prominent 

 motion at .station.s tolerably far a|>art may be pos.«ii)le ; and in that ciuse the 

 writer's metlxMl will serve to determiti"' th«' spcul ami dirertion of transit. 



