35 



oomo.« near the top. Ciilliiiir i", the oxtoinal, ami /,, tin intnuril railiiis, tlif 

 liciirlit of tlu' .-tcady lino ahovc tlii' baj-i' is 



-i i\' + v.: 



2 ,•; 



To give a small denjree of stability, Mr. Gray sui^jijests the uso of a small 

 cylinder rollinij: fVfcly inside each of tiie larger ones. A s])rinn', or a ligiit ihikIii- 

 Imn hung from ahove and gearing with the axis of the cylindei-, might lie used. 

 "An interesting niodilii'ation of this niaeliine might he mad' iiy ])laeing two 

 e(|nal eylindcrs on a horizontal j)late with their a.xes parallel, and placing on 

 them a .seeond horiz'mtal jilate so that its np])er suifaee should always lie in a 

 plane through their instantaneous axes. This e luhl l)e alone by causing the 

 plat:- ti> rest t>y means of arms on two jiairs of smaller cylindns of projiir 

 dimensions, st) disposed that each of the larger cylinders should hear at each end 

 one of the smaller cylinders coaxial with the larger cylinder and projecting trom 

 it. A third cylinder ]>laced on the ujiper jilat<- with its axis at ri'j;ht angles to 

 those of the first two would, for small motions of the earth, have a line in itself 

 which would remain approximately at rest." It must, however, he admitted ihat 

 this elalioratc plan for a seismometer with two degrees of lioiizontal I'recdom has 

 little to recommend it. 



For large motions Mr. ( Jrav proposes to arrange thi' nmltipKing levers m) 

 that when the dis|ilacement exceeds a •riven value they will go out of action, and 

 a direct writing appliance take th<'ir jilac ■. " \ verv simple method of writing 

 large motions would lie to attach a line point to the end of the e\lin(lei- at its 

 centre, and allow this point to wiile i>n a plate placed in frnnt of it and fixerl to 

 the liasc-plate." This would gi\-e a rci ord e;|ual to lather more than half the 

 actual motion. 



Compared with the rolling ^plu're. the rolling evlinder seismograjili lia~ llie 

 ailvantage of easier construction ; to turn a evlinder aceuralely lieing a matter of 

 far less diHIculty than to fntn a sphere. It is howc\-er, e(|Ually with the rolling 

 sphere, lialdc to the great piactical ohjec-tion that it:, fiictii.nal i( sistauce is so 

 great as to pievent it from propeiK- leeoidiuLT a -iowK- eliangiuL;' molion of the 

 ground. 



§ 3."». Till' C'liiiniioii J'liiiliiliiiii. 



It has been already pointed out that a ma-s hanging |>ivotted hy one point 

 to a lixid »upport, so that its centre of gravity lies vertically lii'low its point of 

 support, fails to act as an ahsoliite seismometer <in account of the slahilit\- of its 

 ei|nililiriiMii. Tlie arrangemc nt forms, of couis', a common |>endidnm. and 

 in lialili- to till- olijrftion thai during any prolonged ^llakin>J: of the >uppoii 

 the |M'iidulum acpiires a swing \\ linse amplitude mav he, and often ]-, iricalei' 

 than the motions cif the irroiind. 



If. however, we make the length of ihi' pendnhiin verv great, it- eipiilihrinni 

 will lie as nearly neutral as, in fact, it is ever de^iiaMe that the e,|uilil>iium of a 



