uiJia 



licsidc.« ]iosses5in(; tlic defoet just montioiipd, all static records are open to anotlior 

 important ohjection. If the cijuililniuiu of a seisinoiueter is very nearly neutral, 

 the so-called steady-point is liable to become shifted during an earth(juake to a 

 considei-able distance from its initial position; and even din-ing intervals of rest, 

 especially when these are long, a gradual creejiing of the pointer occurs, due to 

 slight accidental displacements of the line or point of sujujort, through changes 

 of teinpciaturc, \var|iiiig, and otlirr causes. Hence to use [1] or [2] we must 

 make the eijuilibrium tolerably stable. An()ther difficulty then presents itself, 

 namely the tendency which a stably hung body has to ac^juire, during a long 

 series of .shakes, an oscillation who.se amplitude niaj' be comparable to or even 

 much greater than the motions of the jxiint of .support. < U\\y one way remains 

 by which this objection may be overcome: the tendency to o.^cilIate may be 

 removed by introducing a considerable frictional resistance (§ 39). This, again, 

 gives rise to error by making the recorded displacements too small, especially 

 in slight eartlujuakes. It is, however, the only method V)y which records 

 obtained by plans [1] and \'2] can be prevented from being absolutely meaning- 

 le.^«. In [.'>] the ca.-^e is somewhat different. There the creej)ing of the steady 

 point, which octurs when the eijuilibrium is nearly neutral, is not so fatal an 

 ohjection as it is in [IJ and ["2] ; for it is often possible in the record to distin- 

 guish, more or less completely, any motion of this kind, especially when the 

 ]>rincipal earthquake motions are decidedly not rectilinear. The diagram in the 

 centre of Plate XIX may be referred to a.s an example of a static record which 

 h;Ls not been rendered unintelligible by the nearly a--tatic character of the seismo- 

 graph which gave it. 



§ 80. Records of dixplactmcid in conjtwcUim uith time. 



Records which show each successive disjilacement in conjunction with the 

 time are far more valualile than .«tatic records, Imth on account of the greater 

 fulness of the information they yield, and because they avoid to a great extent 

 tlie sources of doubtfulne.'s or error to w'hieh static methods of recording are 

 liable. When the movements are recorded on a continuously moving ])late, the 

 (•(piilibrium may .«afely be made much more nearly neutral than is piacticaV>le 

 with a statically recording instrument ; and the friction, instead of being arti- 

 fuially increa.sed, may advantageously be reduced almost without limit. Any 

 moderate degree of progres-sive disj)laccmcnt of the so-called steady point, by the 

 accumulated effect of successive earth waves, leads to no serious confusion ; and 

 any long-period swinging that may occur is easily distinguished from the legiti- 

 mate portions of the record. A small amoinit of .stability must be given to 

 prevent the fornur etl'eet from becoming excessive; but the equilibrium may bo so 

 nearly neutral that the slight friction which, in the most favourable conditions, 

 the pivots and the marking jiointer must produce, will suffice to make immode- 

 rate swinging im])ossible. When registering on a continuously moving plate, a 

 well constructed and well adju.sted seismograph should make one complete 



