24 



is accordingly arrested althougli the clock continues to run. One turn of the 

 plate takes from a miiuitc t" a miiuiti' and a half. 



In the (iguns the whole ai)iiaratu.s (except the clcK'k, which is sepaiatcly 

 suj)ported) is re|)resnited as mounted on a wooden patform on the top of a post 

 stuck in the eaith. Recently it has been found better to use a low solid stone tai)le, 

 as the waipini; ol' a wooden su|)],oit is a| t to jiroduce a radial creeping of the 

 pointers on the ])late, and conseq<iently a trra<lual widening of the lines they 

 trace as the jilato revolves. 



In practice the iiorizontal iieiidnlunis have their axes of support slightly 

 inclined forward.-;, to give a small degree of stability. This, indeed, must i)e 

 given in all seismometers, partly to j)revent the ('(piilibriuni from becoming 

 nn^tablc thiough any slight accidental change of adjustment, and jiartly to ))ie- 

 vent anv excessive displacement of the so-cal!cil steadv point by the a<'cniiinlalc(l 

 etli'cts of frictional and other di>turbing forces during a prolonged shaking. 



§ 25. llnrlzoiifiil Piii(lii/ii)ii Seis7»ngraj>li ivHIi Rwg Boh. 



Anotiicr I'oiin of horizontal ptMidulum seismoiiictcr, re ] »resenting a stage in 

 the development of the instrnment intermediate between the two forms already 

 de.«cril)ed, is shown in IMate IV, where ligs. 9 and 10 give a sectional elevation 

 and jilau rcsjicctively. The construction will lie evident from the drawing 

 without explanation. There are, as before, two similar levers set at right 

 angles to each other, whicii write their lecords on the same revolving smoked- 

 glass plate, a portion only of which appears in the drawing. Tiie distinguishing 

 jiecnliarity of this form is the shape of the hob, wliich is a hollow cylinder wide 

 enough to enclose the ui>right piece which affords a vertical axis of supjxirt. 

 By this arrangement the two axes may be brought near together without 

 necessitating the use of a small bob. The ring form has also the advantage of 

 having a relatively great moment of inertia about its own axis, which give.s it 

 much 1 ower to resist the tendency of friction at its joints to set it in rotation 

 during a displacement of the ground. Tiiis m<'rit, however, docs not counter- 

 balance othci' ]>ractical defects and inconveniences in this nioditlcation of the 

 instrument, Mliich is on tiic whole decidedly infcri<ir to the later form descriiied 

 in tile preceding ] aragraph. The nuiiti|ilying ratio of the instruiiient tiguied in 

 Plati' W is se\-cu to one. 



§ 2G. Jiclation affile Friction EiTor to the diiiianioii!!- of I lie frame 

 and to the suspeiidetl mas.9. 



It is interesting to eiKpiirc brietly low the i'lietinnul erior is lik-clv to be 

 affi'cted by ihangcs in the arrangement of the ]arts. F<ir tliis purjio.se we mav 

 conveniently scpaiate tlie whole frictional resistance into three parts— ('/) that due 

 to the marking j)ointer, (ß) that ilue 1 i the veitical component of the i^tatical 

 pressure ou the pivots, (;-) that due to the horizontal pressures on the jtivots, which 



