CHAPTER IV. 



INSTKU.MKNTS FuR OBSKiniNG VKHTICAL MOVKMKN T. 



5; 44. f.iKuhd Spira! Spring. 



In attiMnjttini; to ritri.-tcr tlu" vortiial iiniipniu-iu of oartlii(U;il<c iiiovomoiits 

 l>y the iiM' of a iiias,- who.si' iiiiTtia is to furnish a steady point, \vi' at oiux' nici't 

 witli tlu' iliffioiilty that gnivity acts in the direction in which freedom of motion 

 is bi- retained. Tlie weight must therefore he horue l)y some contiivance which, 

 l>y heing extensihlt-, or in some other way, will leave the mass freedom to 

 oscillate ill ;i vertical line. The simplest exaniph' is afforded liy a mas,s hung 

 liy a long spiral spring from a fixed sujiport. By limiting its freedom to one 

 line we may restrict this apparatus to the mea.<uremcnt of vertical movement.s, or 

 by leaving it free to oe<ill:iti' horizontally, like a pendulum, as well as vertically, 

 we may obtain a universal seismometer, which, if furnished with .suitable indices, 

 may simultaneously record three rectangular comijoiieiits of the motion. It will 

 generally be found convenient, however, to deal with the vertical component 

 sepanitely, and therefore to deprive the instrument intended to record it of 

 any but vertical frewlom. 



To make the ecjiiilibrium of the hanging mass nearly enough ne\itral, the 

 spring would need to be of great length, since, when hung directly by a .stretched 

 spring, the period ol' vertical oscillation of the ma.ss will be only that of a 

 simple |>endulnm wh(wc length i- e<|ual to the di.-tanee by which the .sjiring is 

 elongated. Thi.s, in fact, makes a (lir(<t!y loaded spiral spring almost ini]>rac- 

 ticable :ls a vertieal seismometer. 



■t 4Ö. ffiirizoiil'i/ tilt- with flexible support and loaded end. 



A <-<,niiiiitte<' of the Hriti.sh Association appiinted in 1841 " for registering 

 >hiH-ks of earthi|Uakes in Great Hrit.ain " <lescribe in their report* a seismometer 

 for verficid motion, consi.sting of " a horizontal l>ar, fi.xed to a .solid wall by means 

 of a strong flat watch-spring, an«! loaded at the opposite end. If the wall 

 suddenly rises or sinks, the loaded end of this horizontal rod remains, from its 

 vis inertia, nearly at rest, and thus can inf)ve any light .«ub.stancc (as paper or a 

 .-ti-aw) brought against it by the vertieal movement of tiie grouiul, and which 

 light hiibstano' is xi adjusted iL» to stick wherever the ro<l leaves it." Tiie 

 re|M>rt go<-s on to .»ay that an instrument of this kind, set up at ( 'oniric, gave on 

 one <Mtit-ii>n a re<t>ril of vertical movement to the extent of half an inch. Nothing 

 is told a.» to the |)criod of vertieal o.^ciilatioii of the bar, whose e(juililii iniii was 

 piobaiily too Htabie to make it act well a» a .s<'ismoiiiet<'r. 



• Brilidli Aniocialion K<-|iort for 1842, p. 94. 



