.•?7 



resistance to prevent it tnun a«(uirinj;a swint;. If, howovor, \vc allow tlif motioas 

 to l>e traced on a (•initiiuioiislv moviiijj rcconl-recoiver, any mo<lerate swinjjinjj 

 of the j)en(iuluni. jirovided that is of ninoli lonirnr |M'rioil tluin the caitliipiake 

 waves, is not seriously ohieetional>le, and we may then reduce the friction us far 

 its possihle. The same remarks apply with c (iial force to all forms of seismu- 

 metor in which a so-callwl steady point is souifht after. In all such instnunents, 

 static records have n» nieaninir or value unlc.-s swinorintr is entirely prevented. 

 This can l>e done hv in(riHlucin<i cnouijh friction, hut to do it, especially when 

 tiie .stahility of the fn-e mass is considerable, involves a great sacrifice of accuracy 

 — a .sacrific«' which there is no need to make when a continously moving record- 

 receiver is used. 



<j SH. Actual Pendulum Seitniomeierf. 



Mr. R. Mallet, writinir in 185.S, refers to the common pendnluni, as "the 

 oldest, probably, of .seismometers, long set up in Italy and southern Europe. A 

 pendulum, free to move in any direction, carries below the bob a .style partly 

 immersed in a .stratum of fine drj' .sand, spread to a uniform thickness over the 

 concave surface of a circular dish placed beneath, marked to the cardinal jioints, 

 who.si' centre is beneath the |M)int of suspension of tlie pindulum when at rest, 

 and whose concavity is that of a spherical .segment of a radius equal to the 

 length of the [K-ndulum and style, plus rather more than the depth of the 

 stratum of .■^iiul. It was supposed that the style would mark a right line when 

 .st>en in a plan<' vertical to the .sand-bed, and in the direction of the shock.''* 



Pendulums provided with sliding pencils writing on paper fixed below, or 

 with some other contrivance fur giving .static records, have also been used at 

 Comrie, in S'otlandt. by a conimitt<'e of thi' Kritish Association ; in Japan, bv 

 Dr. Verbeck (in 1871 I and sul).s<Jnui'ntly by Wag« n -r, Kni[>ping, Milne, (irav, 

 and othei> ; at Uoni? by .S.vchi ; at Manila by Faura + : and no doubt by manv 

 ob.-icrvers in othi-r places. In one of ISIr. Milne's arrangement.s a long pendulum, 

 hung from the ro(if of a lumse, carried two vertical sliding .styles which rested on 

 the .surfat^- of two strips of smoked glass on a table iielow. The strips of gliU'S 

 were .startcti into motion by an eartluiuake disturbance (which released a catch) 

 and |>a.-.scd undf-r the styles, moving in liiifs at right angles to each other. This 

 gave a continuous re<'ord ol" tlie motion of the pendulum, without multiplication, 

 iK'ginning at an uncert;ün int Tval after the JK'ginning of the earthquake, and 

 lasting for only a few .seconds.** Dr. Wagener's apparatus (.sjicoe.'--.sfulIy used 

 liy Mr. Knipping in Tokio from IHTHjw.asa p.iidulinu •'{ feet long sujiporti il bv 

 a rigid frame, and provided with a multiplying hvcr by wliich the nlative 

 displacement of the csirth and thi- liob of thr [xiidulum w;i« magnifie<l 24 times. 



* Britiuli Annoc. Report, 1R68, p. 73. 



t Hriliith A>ii<H.-. Kcport, 1841, p. til. 



J Proc. Itoyal Socirly, Vol. .\X.\I, p, 4C(». 



** TrBiiii. uftbc Sc'iatiioloK'eal Sociely of Jupun, Vol. Ill, p. 12. 



