EARTHQUAKES ON THE PACIFIC COAST 19 



the clock. This occurs during the preliminr.ry tremors which are 

 usually found in advance of the main movements of an earthquake. 

 The same circuit starts another clock* (of the escapement type) 

 which fulfills two functions. It marks time on the revolving plate 

 during a part of the first revolution, and then continues to go as 

 an ordinary clock, so that, by inspecting its dial afterwards, the in- 

 terval which has elapsed since the occurrence of the earthquake is 

 known, and the date of the shock in hours and minutes is thus 

 determined with as much precision as the phenomenon admits of. 

 This part of the apparatus is omitted from the figure. The two 

 horizontal components of motion are recorded by a pair of hori- 

 zontal pendulums, set at right angles to each other, but with their 

 indices inclined so that they write side by side on one radius of the 

 plate. The pendulums are supported on a single stand, but with 

 independent adjustments for position and stability. Each has two 

 pivots, consisting of hard steel points, which turn in sapphire cen- 

 tres. At the pivots and at the tracing-points every effort has been 

 made to avoid friction. The indices are of aluminium, and a part 

 of their weight is taken by springs (not shown in the figure), so 

 that their pressure on the plate may be no greater than is necessary 

 to produce a trace on the sooty film. The vertical component of 

 motion is recorded by the instrument which appears behind the 

 clock. A massive bar, free to move vertically about a horizontal 

 axis, is held up by a pair of long spiral springs. Its equilibrium 

 is made nearly neutral by applying the pull of the springs at a suit- 

 able distance below the horizontal plane through the axis of sup- 

 port. A bell-crank lever with a jointed index gives a multiplied 

 trace of the apparent vertical oscillations of the bar, which corre- 

 spond to vertical displacements of the ground. In this instrument, 

 as in the others, sapphire centres are used to minimize friction. 



Eecords inscribed on the plate are preserved by varnishing the 

 plate and using it as a ' negative ' to print photographs. The 

 motion, as recorded, is magnified to an extent which experience 

 has shown to be desirable in dealing with disturbances ranging 

 from those which are just recognizable as earthquakes up to those 

 which are to some extent destructive.! 



*Not shown in the cut. 



tin the Lick Observatory instrument the horizontal components are multi- 

 plied 3.3 times and the vertical component is multiplied 1.6 times. The in- 

 dices are made of stout straws. 



