45 



joint on tlie fixed bracket g, i.s connected to the bob at the centre of percussion 

 by a ball and tube joint. At its lower end a light horizontal pointer h is hinged, 

 and projects out horizontally, witii its .steel- tij)ped end resting on a revolving 

 smoked-gla.ss ])late, as in flic instruments already described. The supports s, 

 and S; are bracket.-' projecting IVom a pii>t ti.xed in the ground. 



*} 43. Fnrhf"' Iiiverttil Pnnlti/iiiii Sei.-^iiioinefer. 



The writer has only recently beci>me aware of how nearly, in its essential 

 idea, the instrunieiit (ic.scril)ed in § 4"2 is a return to a very old .seisniunieter, the 

 inventiiin of the late Principal Forljcs. The following account of it, as erected 

 for the oKservation of earthcjuakes at Conirie, is taken fioni tlio British 

 As.-^iX'iation Report for 1841, p. 47 : — 



■■ 77i(- InvKrteil FfnrUdum Scisninmctrr. — ( 1.) The smallest nf tlx' instru- 

 ment.- made on this principle has a pendulum thirty-nin«' inches inug, and is 

 fixed into a i)iass .«ieket at its lower cud. The connection bitwecii the pcudultuu 

 and the socket eon.'iists of a strong ela.stie wire, which, by means of a pinching 

 screw, can be either raised or dcprc-^-^ed in the socket, so as to increase or dimi- 

 ni?b the length and .■^ensiliility of the ])cn(lu]uni. Tiiere is a leaden !)al] near 

 the top of the pendulum from thicc to four pounds iu weight: it ha-^ a hole 

 through it< wntre .-o as to allow the pendulum rod to pa.ss freely through it, and 

 it can be fixed at any part of the rf)d by means of a pinching screw. At the 

 up|>er cxtrf-mity of the pendulum there is a .«oft lea<] jnncil, which rests on an 

 clastic wire contained in a i)i-a.'S tulie. The pencil is thus pre.<.sed against a 

 white surface of paper, forming the segment of a sphere, having a radius of 

 thirty-nine inches. The paj)er is pasted on a ])iece'of copper beaten into the 

 proper >hape. This copper segnu-nt rests on four upright iron rods which an- 

 fixed into the base of the instrument. The base consists of four currcsponding 

 flat iron bars, which cro.-s in the middle, and support at that point the socket 



al)ove <le.seribed, to which the elastic wire of the pendulum is fixed The 



instrument is fixed firmly to the floor of the room where it is set. By means of 

 three adjusting screws, whicli alli'ct llie -oiket, the ujiper extn niitv of tiie 

 |)endulum i- broiigiit to tiie centre of the segment to be marked i)v it. Anv 

 further <|escri|»tion of this in>tnHncm is rendered unnecessary in eon.-^e(|uence of 

 a I a per by I'rofes.sor Korbes, publi^hell iatelv in the Tran.sictions of the Royal 

 ."^K'iety of Kdinburgli, whi-re the nieehaiii'-m ami mathematical pro|)eities of it 

 are very < learly jxiinted out. {'2.) The oiher instiument constructed on this 

 principle has a iH'uduluni ten feet eight inej;es in hiigth. The spherical segment, 

 on wliieji the vibiation.'> of ii> jioint an' intended to be marked, i> util, as in the 

 iii^liiiincnt yiA de?4'ril>ed, supported on upright rods fixed to its la«', but is 

 susjH-nded over the |i4'n<lulum by a strong hold-fa-t of iron fixed into a wall. In 

 other rc.specis, the nieehauical con>ti'uction of this in>tiument is nuich the same 

 a- that of the former one." 



It ha- unfoi'tunntely been iin| o->ible to ol lain access, in .lapan, to ;i eopv of 



