cal Magazine ftir Si'])tenilK'r 1881. There each bob coiLsist-* i)f' two masses MM 

 fixed to the ends of a cniss liar h. whieii is suspended Ity a wire t i'roin an adjus- 

 tabk' i>r<ijectinu; arm c clamiu'd to the j)ost 1^. One of the liars h is bent to 

 make it to |':l<s clear of tlis' other. The stmts <( n are short eoiii| and with the 

 len<;tli of the tie, and con^'ciuentlv bear only a small fraction of the weight. 

 The niultiplving arms II are inelineil to the directions of displacement and 

 I arallel to each other, so that their records may ire traced side by side — an arrange- 

 ment \vlii<-l) ailo\v~ th<' two (■ iiii|'i iirats of tiie iu"tinii to be more readily com- 

 pared aiul (■ im|oundcd, 



S 2^. Horhiiiilii/ Ptnihiluin nlflriKf joint :. 



The method of suspcn- ion just described leaves om- joint at which there is 

 rolling contact and eonscin-'utly friction, namely tiie s(^i-ket of the strut : but 

 there is no reason why we ^Imuid utit u.-c a tiexiiile jiece instead of a joint there 

 as well a:- at the other end of the axis of supjuut. To do this it ismnly necessary 

 to fork the strut, let its end pniject beyond the axis of support, and tie it back to 

 that axis by an ela.^tic wire, or preferably by a thin ribbon of tempered Steel, 

 Irlaced with it.- flat ?ide ve:tical. A lixed vertical [lin standing in the axis of 

 su]i|uit and caialle of sliding freely in a hoiizont.il .-lot in the strut, is added to 

 jirevent any bodily tianslation of the stiut to one side or the other during an 

 eaitlv[uakc, .■^) that the hanging ma.-^s may have no freedom to move otherwise 

 than by rotating about a nearly veitieal line joining the upper and lower points 

 of attachment. 



This idea has been jiractically carried out in the instrument shown in Plate 

 y. l*"ig. l.'I is an elevation showing one of the two pendulums, and Fig. 14 is a 

 .<ectional plan. The j:ost P, iirmly stuck in the earth, carries two horizontal 

 le\i-is /,, and /,, set at right angles to each other, to ii<<.rd two rectangular 

 components of horizontal motion. The bobs J/, J/, are lixed to the rods i, L.^ and 

 tied to a | air of snuil vices at the top of the jio.st by fine steel wires TT. Fig. 

 I'} is a plan of the toji of the jrost, and shows the arrangement of the vices, w hich 

 are ti.xcd in a manner which give them two (horizontal) degrees of freedom of 

 adju.-lmi'iil. ,\t till' back cikI of" each rod L is a fork (shown on a larger .'^cale in 

 fi;;-. I'i, 17. and IS) whi<'h eon-i-ts of tw<» parallel clucks of bras-; itn termina- 

 ting in a vice l> which i- cl;impc<l by the screw-bolt and nut r. A split iipiigiit 

 |>in p is fixed to the post, and a short thin flat band of very flexible steel is 

 clamjied belwr'cn it and l>, the end of /^. 'I'his is kept in tension by the thrust 

 exerted liy A, ami when the horizontal pendulum r-wings the spring benrh at or 



rlo.-e to a veitieal line in the centre of the pill. .\ -ector of the pill, lacing 



towards it, is cut out to give the spring loom to bend aiioiit the axis of the pin 

 (we figs, n; and 1S|. 'J'he split sirles of the pin /) arc pre-^'il together by the 

 nut u aiKJ so can."*'"«! to hold the spring chunpc«! between them. In (ig. I.H oiii' 

 I'I the cheeks II is removed ami the pin j, -howii in \'itic:il -•(•tjon: (lie (lut 

 .-pi ing appears there and i- le|ter..d .«. ( )iilv the upper portion of the pin /, i.s 



