CHAPTER Vll. 



SUMMARY AND COMPAIllSON OK INSTRUMENTS AND MKTHOIiS. 



*; 75. U-ielcvine.is of all e.verpl Steady-pnint Seiiiiwmcterti. 



Eiionc;!! lias been said to show tluit, of all actual scii^mometcrs, those only 

 arc of \;iiuc wliicli aim at giving a steady line or a steady point diirinn; the 

 distnrl)anec — a steady line with respect to motion in one direction, or a steady 

 j)oiiit with res|K'ct to motion in two directions, or in all. Cruder devices, sncli 

 a.s standing c<j]ninn^, (ail hecause of the complexity of earthquidce motion. They 

 cannot lie trusted to yield even relative measurements of am])litiide or of destnio- 

 tivo power; and as to direction, it is iin]>ossi!)le to speak of the direction of the 

 ground's motion when we find dis])lacements vi' nearly cijual value occurring in 

 all possiI)le azimuths, duiiiig the continuance of a single earth(|uake. 



§ 7f). ClaHsifiCuiii»! iif Slc(('li/-priiiil Si i.^)liiiii)iliTs: 



AVc may classify these instruments with i-espi'ct to the component or eompo- 

 iieiits of motion they are designed to measure, as follows: — 



1. Tho.se pos.scssing one degree of hoiizontal IVeedom, and therefore capable 

 of measuring one component of the eaith's horizontal motion. They arc to he 

 used in pairs, giving two cora])onents, whi<li are most conveniently taken at 

 right angles to each other. To this ehiss belong the horizontal penilulnm in its 

 various forms (i;§ 2'')-2(S); the common vertical pendulum (or any of tlie modifi- 

 cations of" it given in ( 'hapter III ) when sus]iended, like tiic jicndulum of a I'locK', 

 on an axis, not on a point ; the I'olling cylinder (§ ■M\ with or witliout a iieavy 

 slab. A sphere I'olling in a fixed \'-groove might be added to llie bVt, also a 

 Hattened-out U-tube filled with mercury (§ (>!). 



TI. Tn.-<truments pos>es.«ing two degrees of horizontal freedom. This class 

 includes the common ])enduliim and its modifications (tlie dujilex jiendulum ( tc.) 

 when susjxiided by a Jioint (§§ Jiö et acq.); the horizontal pendulum witli 

 jointed supjwrting frame (§ 29); the rolling sphere, or spheres carrying a slal) 

 i§§ 31-3;3); a system of rolling cylinders at i-ight angles to each otiier, with a 

 slab between ( § .'i 1 ). 



III. Instruments jios.sessing vertical freedom onlv. The loaded horizontal 

 bar with fiexible or extensible sui)port, and its modifications (SS 40-4X1. It 

 would not be diflicult, were it at all desirable, to design instrurnents which v.ould 

 form two more classe.«, namely IV, in.struraents possessing vertical and also one 

 degree of horizontal freedom ; and Y, universal seismometers, having three 

 decrees of freedom. 



