u 



The imilti|ilicatii)n i:;i\cn by :in iiLstriiiiKiit cif the ^m-t ruav be <letenninecl experi- 

 nu'iitallv: m- it may be ajiproxiuiatilv caleiilatud by takliii; a puiiit O at a 

 height ciiiial to seven-fifths ul' tile radius nf the Uirge sphere .S' as iisarly steady." 



In otii'T insti unicDts nscd liy the same observer a segment only of the 

 Killing- splieie has bieii eniphiyed, with an independent mass pivotted at the 

 iintre of ]icvenssion, which is arranged tu lie at sneli a hri^ht al)ovi' tlie centre 

 iif the sjilicrii'al segment tiiat tii«' cipiilibrium oi' tlie system is nearly neutral. 

 "This methcd cf constrncti'in allows the radius cf tiie sphere to be much in- 

 creased -, Init it introduces a dilticidty of adjustment, a <'oniplication of parts, and 

 jfcnerally a slight want of symmetry, which causes a little uncertainty in ihe 

 interpi'ctatinn oi' (lie recoi'ds.''* An instrument of this class is .-hown in tii;-. '2'2, 

 l^late VI 1. There the ]X)int P should l)e the ci'ntre of j crcussion of the rollinu 

 sc^incnt iS, til secure which the wciL^ht IT can l)e moved uji or down. ]'v varvin«; 

 the mass nf the pivotted rini; A' tlie centre of j;'ravity of the sy.stem can be brought 

 Im'Iovv ihe cente of curvature just enough to give a small amount of stability. 



A simpler arrangement, and one yielding nearly as much eHective inertia in 

 piojoition to tile ]ire.'sure on tlu' base, is a solid mass with as condensed a form 

 as possible, rigidly attached to a lii;lit spherical segnii nt which rolls on a lixcd 

 plane base. Tiie centre of gravity of the \vlu)le piece must be a very little 

 below its centre of curvature (unless u small aminnt of stability be given by 

 other means), and its centre of percussion, which lies only a little way above the 

 centre of gravitv, is of course to be taken as the stcaih' point. In the instniment 

 desc!ibcd above the pivotted ring is I'ljuivalcut to a | aiticle of the same ma.ss 

 situated at /': the arrangement now mentioned, which is nnich easier of construc- 

 tion ami a<ljustment, approximates closely to the former in pro])ortioii as the 

 dimcnsiiins ol' the lioi) (here iixed, insl'ad of being pivotted) are reduced, without 

 ri'duction of its mass. 



§ "'-J. Rtillilltl ('ilUllllrf ScKIIKir/lilpJi. 



The seismometers of § vj '_'II-.'!1 are intended to record horizontal motion in 

 any diri'ctioii. To icstriet the record to one azimuth it is only iiece.s.sary to use a 

 lulling surface without curvature, and therefore w itiiont freedom to roll, in a 

 line perpendicular to the direction of the motions which are to be registered. 

 1'hus a pair of circidar cylinders placed at right angles to each other on a plane 

 base may be u.scd to give separately two rectangular comjionents of earthijuake 

 motion perpendicular to their icspeetive axes. 



This plan has been adoptnl by Mr. (nay in an instrument described before 

 the Scismologicul Society of Japan (Trans., Vol. HI, p. 14."i) and again in the 

 I'hil. :Mag. for September 1,S81. Fig. '2:1, Pl.ate VII, shows the con.struetion. 

 The cylinders are marked e and <■'. \ bent arm .1, which is iixed to the base- 

 plate and pa.sses over oiu> cylinder, supplies fulcrums for a pair of multi])lving 

 pointers II.'. The cylinders are made hollow, and hence the steady line of each 



* Gray, loc. cit. 



