33 



0.001. \v(." find Itakiiiu' r = i inrli, as in Dr. Vt^rbeck's? apparuiii.-) tlio awi-lcni- 



tiiin of tlio block l)y r(illin<j; iVictidii tu lie ' -- • In a later cliaptor it will iio 



.<lii>\vn, as till' loult li' direct db.scrvations, that tlic lini'izoiila! acccliiatiun ul' a 

 |>aiticlc iin the cartliV .snii'acc iIucs nut reach this value exccjit in eaithi|nakos uf 

 c'>nsidcniblc intcn.sity, and even then falls short of it dnrin<r a gnat ]iait of each 

 (li.siilacement. Hcuco, apart from the friction of the writinij pointer, an instru- 

 ment of this kind !s likely to | ossc.-s so muk'Ii lesistance that it will fail altnucther 

 to n'<'ord small earthi|uakes, and give a verv imjicrfect record ol others which 

 aie gicut enough to leave some traces of their passage. A frictional acceleration 

 amounting to one-lbitieth of the value ni' gravity is altogether out ot' the (juestion 

 in a .soit^mometor, and it appear.s that even by increasing gr<atlythe ladius of 

 curvature, a rolling m:i.ss seismometer cannot easily, if at all, be brought to the 

 !=tate of etliciency which is reached without ditficulty by the horizontal pendiduni 

 and otlicr devices, and which is indisj)ensable to accuracy in the measurement of 

 small earth(juake.s. 



§ 3.'{. Olliir RdlHiui Sphere SeiinuKji-apIn. 



.\ single .-«olid .«pherieal IkiII has been used by Mr. (irav and is figured in 

 fig. '1\, Plate VII. The following ilcscription of it is, with the figure, taken 

 from the Philo.si^phieal Magazine lor Se|)tember, IS.SI : — 



" A sphere of lead,* iron, or any other heavy suiislancc rests on a Hat plate 

 li made truly plane and furnished with three levelling screws L. An arm .1, fi.xcd 

 to the base i>', is so formed that a eircular ring fixed to its end is held in a 

 horizontal position with it.s centre vertically above the highest point of the 

 sphere. Tliis ring carries a .«peeie.s of spring universal joint, <'onsisting of four 

 very light bent springs, J, arranged at right angles to one auolhei- and meelinii- 

 in a .•~mall round disc, A, at the centre. The lowci' end of the Icvi'r, /, passes 

 throirgh this ring />, and is fixed tu it at such a pcjinf that it- lower end, which 

 i< rounded, ju.st fits a small hole in the top of the >piicre >'. Ilelwi'cn .s' and 

 // a small sphere, x, is fixed to the lever I, and is so propoi tione<l that the lever/, 

 when |-.nshcd at /', t<'nds to rotate around a point a little above its lower eiul, 

 thus diminishing the |)nsli on the sjilicp,. ,S'. The s|irings / serve to allow the 

 lever / to turn in anv dirc-i'iiun, :ind are made .so light that lliey c;in oniv make 

 the ball roll with a very long period. When thus pi'u|iui tinned lliev .serve 

 the pnr[io.s<" 1)1" a nniversjil joint, ami at the same time give a little .-tability 

 lo the part.«, thus preventing the pl.tte /', if ii I«' pnl in motion, from causing 

 the ball to roll over. The lever / is a rod of bamboo wliirh is al the siuu' 

 liini' very li^hl and ligid ; at ihi' njiper end the rod is llattencd and hinged just 

 abuve the bend by :i piei-i- uf (uMgli .lapaiiesc | taper gilied tu its upper side 



■ l.iuil uriiilil III- nil IX IiiikI.v niiHuilatili' iii.ilcriul on lu'coiiiit. of ilK vinciisily, «liiili liy 



iilliiwiiiK ii 111 llulli'ii Olli ul (III' liiilloiii, i'»iip(iiillv uftiT !i luiij; tin I' ri-)il, woiilil k'^i" «rciil 



rvniiilaiiuo lo rolliiii; ami nil ul JMctiuimlilL- nliiliilil) ler »iiiiill ili»|iliicriiu-iil». (.I..\.K.) 



