nias-;ive lilcick nf haiil woml wlinsc Wfiuht ui^.atly cxcix'iK'd tlint i<{ tiu' Imlls. 

 In tlic cciitn' iif tlii> a vertical tulmlar Imli' was lioicd, in wiiicli a woii;ht<'d 

 ]icncil, sliding IVurly, was I'ai'rird, its ]>nint lotint;' rm a ]iii'cc (if |iniir)' fixed to 

 tlio slal> lieli)\v. This gave a static record ol' the relative inotinn df" the Mori; and 

 the slab, witlimit ninltii)!i<atii>n.* 



A similar in^tniiniMU in \\hi<'h the l'a-e-|ilate is el' L;■]a^s, and tile reennl is 

 given liy a steel iieetUe writing en a snii>ke(l-glass phite. ha-^ recently hoc» 

 desirihed liy Mr. ( '. A. Stevi'nson tu the Hiival Seettish Societv of Arts, and has 

 lieen a|i]>lied iiy him to the nieusnrenient of earthquakes in Scotland. 



It a]»]iears to have lieen as^nined hv the?e oli.~crvers that (execjit for friction) 

 th«' liliiek resting on the halls wonld remain at rest dniing a horizontal disjrlace- 

 ment of the hase. A litÜe consideration, however, will show that when the hase- 

 plate moves in any dii'ection the hloek will in all cases move through a small 

 distance in tln' o|)]iosite direction. The centre of [lerenssion of a s]ihi/rieal hall 

 taken .«epaiately is at a height of seven-fifths of the radins from the bottom, and 

 hence if we wi.sh to keei) the block at rest it must be supported at that height 

 instead of at the top of the balls, I'laeed where it is, it has the ett'ect of raising 

 the centre of percussion or steady point of the .system considerably, but that 

 necessarily remains below the top of the balls. We may conveniently examine 

 the kinetit- cjnalitics of the system by con.sidering- the upper block as criiiivalout 

 to a number of particles, cue situated always at the top of each liall. When 

 three balls are u.sed the ma-s of each ol' thou particles is to be taken asone-third 

 the ma.ss of the block. Calling the ma.ss of the block '.IM, that of each ball 

 m, and the raditi.-; of ea<'h ball r, the .syistem is kinetically e:[tiivalent to three 

 j)icces standing n|)right, the moment of inertia of each about the base being 



and the height of the centre of gravity (jf each fioni the base is 



my + 2Mc 



Hence the centre of pereu.s-ion of each is at a height above the ba>c eijual to 



Imr- -f 4Mr- _ r{J;m -h 4M) 

 mr + 2^\r ~~ m + 21M 



This, whicli is > ^ )■ and < 2 i-, is the height of what we may here call the 

 sliaJ jj j)la»i; of the system ai)ove the ba.se, and this position should be taken as 

 the standard of" rest in reckoning the dis|daccm<'nt of the earth's surface. When 

 the l>ru«e moves throiigli a distance .», tiie top of c;i<'h ball i and tiicicforc the block I 

 moves oppositely through a liistaiwe 



Im -f 4M )' 



• Dr. Vorlicck 1<-II« th^! writer tlinl lir iliMiigncd, Ijiit iicvpr UHi'd, n lovor arraiigoinnit \>y 

 vliich a tniilti|>liu(l rocoril uriliu L-artli'ü iiiolioii wuulil liiivu licun fr!v(.-n. 



