>) 77. General principle of Sfeaihj-poiiil Seijuiwmetera. 



The steady line or steady point is? obtained I)y pivotting a mas=, or jjjronp of 

 niaK-^e.-;, with apiuopriato freedom, and in nearly neutral eqnilibriuni. Tlie axis 

 jibout which the i)ody or system spontaneonsly rotates in eonscipienee of the 

 comjwnent of earth displaoonient it is designed to measure, furnishes an approxi- 

 mate standard of rest. 



It is only approximate; first, l)eeause the earth's motions are not indefinitely 

 small; second, because the stability of the system causes some nrntinn to ensue 

 after every displacement : and third, iiccause of friction at the joints and at the 

 index which is useil to record the earth's disi>lacement with respect to the axis of 

 s|)ontaneous rotation. The first error may be rendered inappreciable by placing 

 the axis of spontaneous rotation sufficiently far from the supports, compared with 

 the extent i>f motion to be measured. The degree to which we may reduce the 

 second and third .sources of error depends mi tlu' metiioil adopted for registering 

 the motions. 



§ 78. Methods of regintering the moveme)it.< of llic r/roinid nhttirc 

 to the Steady Point. 



These arc four in number : — 



I 1.] Kv an index which .shows the greatest ;un;ilitnde of relative disjilace- 



iiient, without ."-howing it.s direction. 

 I 2. 1 ]5y indices which .show separately the greatest dis))laeiiiieMts in two 



or more directions. 

 [:^.l By a writing ])ointer which draws a diairrain of the motion on a 



fixed phitc. 

 [4.] l{y writing poiut^-rs whicii show tlie .successive motions, or compo- 

 nents of them, in conjunction with the time, on a plate oi- drum 

 which is kept always moving uniformly. 

 ['}.] 15y pointers writing on a plate or drum wliieii moves uniCormiv 

 after Iwing started into motion l)y the cartin|ual<c itself, but is 

 normally at rest. 

 In applying method [1| or |ö| to a (hiuiije-l'rcedom instrument (of Cl:u-s 

 II), we may use either a .single pointer to record the earth's complete horizontal 

 motions on the moving plate, or two pointers arranged so that each rwords only 

 one component. Date \''III shows i)oth plans, applied to a long jtenduluin 

 seismomet'T. 



■5 7!l. fihjectioiis to /Static Record«. 



Till- static iccoids which are obtained by methods f 1 |, (21, and [ ;> | give, 

 of eoiirs«', less complete- information than [4 J and |öj, in:tsnuich :ls they do not 

 show the ndation of displacement to time. The results obtained by method f.'{j 

 are of nnich greater interest than thos«- which ( 1 | and ( 2 | can yiclil. I'm 



