28 



scre\v-t;i|)j)cd : the lower j.art is a >niootli cyliiider, and tlic elieeks a a jiass ju.<t 

 clear nf it on cither side, their distaiiee apart lieini;- adjnstahU' witliin small limits 

 bv tiif serew-liolt and nut J. Henee no horizontal translation of L can uceur, qnd 

 the t)nly freedom of motion |)ossessed hy tiie suspended rod and mass is freedom 

 to revolve about a nearly vertical line j lining the upper I'ud of T with the axis 

 of the |iin y). A ]air of long liandioo rods /', and J', serve to multiply the 

 motion, and record their dis|)!acenn'nts side by side on a revolving smoked-glai<s 

 plate G i)y means of the hinged |)oiuters ^| and (., . These are short jiieees of 

 straw tipped witii steel, and each is provided with a little lialanee-weight ('■, 

 beiiind the hinge, which lightens the pressure f)f tiie pointer on the ])late and so 

 reduces the friction. To I)ring tiie records parallel, the rod IL is set at right 

 angles to L,, and a counterpoise W serves to biing the centre of gravity of the 

 system bark into tiie line of /i^ . Tlie earth's nioticm is mnltiplicd seven times 

 in the record. The smoked-glass plate is similar in its arrangement of support 

 and driving gear to the one shown in Plate IT, but is of considerably larger 

 size in the present ease. 



§ 2!'. Horizrnitnl I'cnilii/iiin iilfh inn dcr/rees of frceilniii. 



In tiie horizontal ])endulum seismometers liitiierto described, eacii piece ha.s 

 lieen cajiable of showing motion in one azinuitii onlv — motion, namely, perpen- 

 dicular to tlie plane containing its axis of support and its steady line ; and a pair 

 of pieces has tiierefore been nec'cssary to determine horizontal movements in 

 geneial. j\lr. (Jray* lias adapted tiie horizontal iiendnlum to tlie determination 

 of motion in any azinintii by making the ])ivotted siip]drting frame in two pieces 

 placed in |ilanes at rigiit angles to each other, and jointed together in a vertical 

 axis. The arrangement will be clear by reference to Plate YI, figs. 19 and 20. 

 There, as in Plate IV, lh(> bob is a ring. The primaiy axis of suiijiort is an 

 fixed to the eartli. Tiu' pivotted fianie wliieli siiiijiorts tlie lioli consists of two 

 parts A and />, jointed to eacii otlicr with freedom of relative rotation about an 

 axis /; /;, wiiich stands to an in the relation of axis of instantaneous rotation for 

 the frame A. Tiie bob is jiivotted to cr, which bears a like relation to hb for the 

 frame B. Multiplication of the earth's motion is effected Iiy an independent 

 lever, arranged vertically. It pass(\s through a universal joint fixed to the base 

 ]>late and iias a small counterpoise near its upper end, wiiere it touches c below 

 the axis of tile i>ob. Obviously tiic l)oi) is not disturbed bv a small horizontal 

 movcnient in any direction. 



In tile next cha]it<'r otiier devices will be described, wiiich allow tiie hori- 

 zontal part of eartiii|Uake movements to be registered by atlbrding in some cases 

 a .steady line with respect to motion in one azimutli, in otiiers a steady poini 

 with respect to motions in all azimntiis. Of instruments giving a steady liin' 

 none is more successful and generally jiractieable than tlie sim])le horizontal 

 pendulum ; but of those which give a steady jioint it is prolialde that many yet to 

 bedescribed are preferable to the horizontal pendulum witii two degrees of freedom. 



* " On Stoiiiiy I'oinls &c." —Trans. Soisniologiual Soe. of Japan, Vol. Ill, p. 5. An eäsen- 

 tially similar in.slriiment is figured in the Philosophical Magazine for Sept. 1881. 



