43 



from tlie writer's orifjinal account of the instrunuiit, uiven in tin- Transactions 

 of the Scismolosjical Society of Japan, Vol. V, p. W. The reference letters apply 

 to Plate IX, fiif<. 2i( and 'M'), of the | ri'sent memoir, which tjivc two si'ctioual 

 elevations of the instnimenf, taken at right angles to each other. 



B^ is the bol) of the upper or cdiiimnii junduluin, ccmsisting of a hollow 

 cylinder of lead hung (with freedom to oscillate in any azimuth I liy two light 

 wooflen rods a a from a cross piece l>, which carries a steel pin r who.se point 

 rest.« in a conical cup of agate let into tiie u[)per fixed support S,. '9, is a i-igid 

 hraeket .standing out from the top of a post fixed in the earth. B,, also cylin- 

 drical, is the boh of the lower or inverted ]iendnhini. It is fixed to a stout 

 circular nxl d which is pivotted to a second fixed siip|„iit ,,i- hase 6'. l>y a some- 

 what peculiar joint. Two feet ee fixed to the lod cf tlie inverted i>endnhini 

 stand respectively in a conical hole and a V-slot on tlie upper surface of a steel 

 plate/, on the low<r surface of which there are another eunieal hole and V-slot 

 in a line at right angles t" the line of tho.se on the u])per surface. Into the 

 lower hole and \' a pair of inverted feet fixed to ^'o pres> u]i. The upper and 

 lower slots and holes are arranged so that th<'ir veitices ari' all in the .same hori- 

 zontal plane. This mode of support also gives freedom to oscillate in any 

 azimuth, and it is employed instead of the more -inijile method of pivotting a 

 .single foot in a single conical hole, in order that then' in;iy he no iVecdom on the 

 j)3rt of the lower pendulum to rotate about a veitieal axis. Tliere is therefore 

 no objection to using a prolongation of the lower I'eiuiulum a.- the indicating 

 |»ointer. The bobs /i, and B. are connected thus: -fiom a rigid l)rass bar 

 extending across the top of /Z, there depen<ls a rigid veitieal jirojecting ]iiece 

 ending with a spherical i<all which just fits in a cylindrieal hole in a tube fixetl 

 to B,. The pendulums therefore move freely together, this joint giving them 

 the nece.«.sarv power of vertical sliding relatively to caeh other through a small 

 distance. 



The spherical iiall on B, and the tube on B.^ are placed so that their inint 

 of contact is at the centre of percu.-sion of ixith ].endnlnms. This is the kiinlir 

 <-ondition which must be fulfilled in order that thi- jioint siionld be the " -teady 

 point" when a displacement of the earth occurs. The jn>int of eontaei i~. of 

 course-, a short distance below the centre of gravity of B^ and above that of li,. 



If for brevity we call 11', and 11'. the weights of the pendulums referred to 

 this point (that is, the OJ-tual weight of e:i(h multiplied by ilie di>tan<i' of its 

 centre of support from its centre of gravity an<^ divided by tlw distance of its 

 centre of support from the |Hiint of contact between the iiall and tnlie, then the 

 rtatir <-ondition which will give neutnd ci|uililitium in verv small ilisplaeemeiits 

 is that 



ir, /, = ir, /,. 



where /, and /, are the lengths of the pendulum^ nieasmed from their point ot 

 contact to their rcs|K'rtive |K>int« of sup]H.rt. In |>nictic-e a small margin of 

 stability must Iw given by making \\\ K somewhat greater than 11'. /, . 



