20 



c'liiiiii'ctiiiii the ct'iitre (if ]^t_'U'u.<^i(lIl nf liotli }icc('s; in ntluT words, to make tlie 

 miiltiiilyinn; lever toiicli, at its own centre of jiereuFsion, the .'-Icady point or steady 

 line of tlie seih^monieter to whieh it is apjilied. If, however, we jjrefer to apply 

 the innltiplyinfi lever elsewhere than at the steady joint of the seismometer, we 

 may still fnllil tiie kinetic condition rigorously, ky arianjjing the j)oints of siip- 

 ]>oit and of contact so as to satisfy to tlie following ])ro]!oition : — Let F, as before, 

 he the joint of siijijiort and <i the steady jioiiit t>f the juincijial jiieoe or seismo- 

 meter jirojer; and let p he tiie jdiiit of attachment to the ground, and ^ the 

 corresjjonding centre of jjcrenssion of the niultijjlying lever, — then the jtoint of 

 contact l)etwecn them must occujiy the same lelative jiosition with rcsjiect to p 

 and q in the lever as it occujiies with resjicct to I' and (> in the jirinci])al jiiecc. 

 In i)iacti<'e, however, we may avoid the necessity of considering jirecisely the 

 static and kinetic (jualities of the mnltii>lying lever liy sinijily making it so light, 

 relatively to the jirincijial steadv mass, that its own weight and inertia do not 

 sensihly afi'cf't the result. A jjiacticaily far more inijiortant consideration in its 

 construction is the friction of the maiking jioint, whose had elT'ects increase with 

 tlie ratio of miiltii)licatiiin, and which, unless ju'ojier stojis are taken to reduce it, 

 inteiferes very seriously with the correctness of the indications. 



§ 22. Record of stioci'mve di.y)!iireim'iif-< vi eo)i/ miction nilli the time. 



To observe the numerous sucoes^^ive <lisj)lacements which make uj) an earth- 

 quake, it is oidy necessary to give the sin face on which the writing jiointers trace 

 their movements a contimious motion, the best direction for which is directly 

 away fioni the jiointer and at rigiit angles to its exjiicted disjilacemeut. In some 

 instruments the surface to be written on is set in motion by means of the earth- 

 ijuake itself — the distiubance being causeil to start a clock or other motor. This 

 jtlan is sidtject to the disadvantage that at the beginning of the eartlKjuake a 

 j.oi'tion of tlie distuvliauce, c-i' uncertain length, occuis witliont le;iviiig more than 

 what may be called a .slalif recoid. When the horizontal j>enduluui is used as a 

 Seismograph it is ijiiite ])iacti<'abli', and for manv rea>ons jireferable, to kecji the 

 surface moving under tiie jiointeis continuously and uniformly iiy clock-woi'k, in 

 exjiectation that a shock may begin at any moment. A sheet of smooth glass, 

 covered with a thin laver of soot by being held over the flame of a smoky lamji, 

 forms a convenient surface for the recejition of eartlniuake reccuds. By making 

 the I'lessure of the marking juiint on the ghus no greater than is needed to break 

 the film of lamji-iilaek, the fiiction between it and the jilate may be kept so 

 small as to introduce almost uo error; and, after an eaitluiuake, jieriuanent 

 cojiies of the record may easily be got bv varnishing the jilate and using it as a 

 "negative" from which to print jiliotograjihs. 



? 20. AftiKil lfori:oiifa/ Piiiilii/iim S( ixiiior/iuph. 



A seismograj)h consisting of a jair vi' Ix/ri/ontal jiiiidnluuis, which indicate 

 two com|)onents of the horizontal motion of the laith, was desigiK^d by the writer 



