18 EAKTHQUAKES ON THE PACIFIC COAST 



(3) A Duplex Pendulum Seismograph, to give independent rec- 

 ords of the horizontal motion on a fixed plate, the pencil being free 

 to move in all azimuths. 



(4) A Chronograph attachment, which is set in motion at the 

 beginning of a shock, and records the time of its occurrence. It 

 also marks the clock seconds upon the revolving plate of No. 1. 



" In the design of these seismographs the object has been kept 

 in view of making them easily capable of use by observers who 

 have not made seismometry a special study. They are entirely 

 self-recording, and require little attention during the long inter- 

 vals which must, in most situations, be expected to elapse between 

 one period of activity and the next. 



One group of instruments is arranged to give a complete record 

 of every particular of the movement, by resolving it into three rect- 

 angular components — one vertical and two horizontal — and reg- 

 istering these by three distinct pointers on a sheet of smoked glass 

 which is made to revolve uniformly by clockwork. A single earth- 

 quake always consists of many successive displacements of the 

 ground; hence the record traced by each pointer on the moving 

 plate is a line comprising many undulations, generally very irreg- 

 ular in character. The amplitude, period, and form of each of 

 these are easily measured, and by compounding the three we ob- 

 tain full information regarding the direction, extent, velocity and 

 rate of acceleration of the movement at any epoch in the disturb- 

 ance. 



This group of instruments is shown in Plate III. In the centre 

 is the plate of smoked glass, which gets its motion through a fric- 

 tion-roller from a clock* furnished with a centrifugal governor, 

 acting by fluid-friction, and balanced so that its speed is not sen- 

 sibly affected by the shaking of the ground. The clock is started 

 into motion by means of a Palmieri seismoscope, which appears in 

 the figure, behind the plate, on the right. This is a small common 

 pendulum, whose bob carries at the bottom a piece of stiff platinum 

 wire that projects into a recess in a cup of mercury below — the 

 recess being formed by an iron pin standing lower than the sur- 

 face of the surrounding mercury. On the slightest shaking of the 

 ground, contact with the edge of the mercury takes place, and this 

 closes a circuit which releases an electro-magnetic detent and starts 



*At the left-hand side of the cut. 



