The Seattle post-intelligencer. [volume] (Seattle, Wash. Terr. [Wash.]) 1888-1914, 1893-01-22

of the bowl commonly in such direction as
to indicate the course of the vibration.
Another primitive form is a series of solid
cylinders of uniform height but varying
diameters Get upright on a tablet con-
nected directly with the earth; when the
tablet Is shaken by the earth tremor the
cylinders, restrained by insertia, rock to
and fro until some are overturned, the in-
tensity OF the vibration being roughly
measured by the number overthrown and
the direction of propagation by the DO,
sitlons In which they lie. The modern
seismoscope commonly consists of a mass
OF metal, something like a clock weight,
suspended IN such manner as to swing
freely in all directions from an arm con-
nected with a pillar set in the earth:
the earth tremors are communicated TO
the pillar and tlencs TO the arm, while the
weight, tending TO remain stationary, ac-
CURES an apparent movement with re-
spect TO pillar and arm. Sometimes the
weight Is provided with an index, or pen
Gil, lightly touching a horizontal plate be-
low. IN which case the direction and am
plitude OF the apparent swinging of the
weight Tor actual swinging of the pillar
and army are inscribed on the plate: and
another weight may be suspended by
springs in such manner as to indicate the
vertical movement, which may also De in
scrioed on the plate by a suitable index.
TINS form OF apparatus Is called a seismom-
eter. Sometimes the seismometer is con-
nected with clock in such manner that
the clock is stopped by the apparent swing
OF the weight, when the apparatus gives
single time record of the earth tremor.
Again the clio may be provided with q
slowly rotating table so arranged as
10 be set in motion by the initial
tremor and fitted 10 receive marks from
pencils connected with the weights
in such manner that the various move
ments, both horizontal and vertical, with
the precise times OF their occurrence, are
successfully recorded; the apparatus then
being a seit-recording seismometer, or
seismograph, and the record being prop
eriy called seismogram. Now, it is evil
dent that II the SeiSmogram of an earth
quake shock is perfect, it will indicate the
exact horizon al and vertical movements
OF the earth in which the pillar is nxed, i.
e., practically it will record 1.-THE path of
the earth PARTICLE" and thus give a meas.
ure of the velocity and amplitude OF the
vibrations. Indeed the three rectangular
OBSERVATION-DISCLOSURES, north-south. up
down--of the motion recorded the seis-
mngran OF an earthqua Ike Sno K nave sac.
tua.iy neen combine ana converted into
tangible representation OF the vibration
by Prot. Sekiya, OF Japan. by means of a
long wire bent into such shape as to fol-
Whether SO slight as to be imperceptible
to the senses and recognized only by the
most deiica' e apparatus or SO violent as
to overthrow cities and fissure solid earth
or rock in the course OF each passing un-
dulation, the earthquake is simply an
elastic vibration IN the solid earth propa-
gated in all directions from a center Tor
focal tract) at 4 rate of about three miles
per second. The amplitude Of the vibra-
tluns Ii. e., the distance through which q
given particle OF earth matter vibrates)
IS probably In most cases only fraction
of an inch at depths below the surface;
but as the tremor approaches the surface,
and particularly when il passes into
houses, monuments and other structures,
and even into steep hills rising above the
general level. the amplitude NAV be many
times multiplied. Every object on the
earth's surface, each wall of house, each
element IN every artificial structure, has a
certain vibration period depending on its
size, form, elasticity and other conditions,
i. e., it is susceptible of rhythmic motion
at a certain rate depending on its mass
and constitution. This proposition is
illustrated by pendulum, which swings
easily at a rate varying with its length
1t is illustrated also by the rhythmic
swing OF bridge under the regular tread
OF marching army, phenomenon SO
we'll known that marching bodies of men
=
to avoid wrecking the structure.
Now when a succession of the rapid
vibrations constituting an earth tremor
passes into a single object, such as mon
ument or pillar, the object is at first shaken
violently but rapidly, as is the beam of
wood or iron under blow; certain of the
impulses, primary and secondary, agree
with the vibration period OF the pillar and
soon initiate penduium-like swing of the
object; and this swing is increased by
successive impulses, exactly as is the
rhythmic swing OF bridge under march
ing men, until the pillar may be over
thrown. When the impulses pass into a
compound structure, such as q house or q
church with q heavy spire at the cor
ner, the different elements tend to
swing each at Its own vibration period
until the structure may be torn asunder !;
and in like manner as the primary vibra-
tons approach the surface of the earth
they are broken up by interference and re-
Election and by passage into less solid and
therefore less elastic materials and are in
turn resolved into fluctuations OF cousid-
cradle amplitude affecting the ground it
self as we'll as objects resting upon it..
Thus the earth tremors, at first simple and
OF limited amplitude, are differentiated
and multiplied. The initial vibrations
correspond go nearly with those of sound
in rate that they are in part manifest to
roaring sounds. Commonly this is the
earliest manifestation of the great earth.
quake shock; then as the sound-switt vibra-
tons are resolved in part into the pendu-
lum-like swing of masses, windows rattle,
doors swing on their hinges, houses
shake. forest trees nod and tremhle and
the ground perhaps unduiates like a
gently swelling sea: still later. as the reso-
luton OF quick vibrations into SLOW pro-
ceeds. pictures are dashed from the walls.
doors are torn from their hinges, plaster
Is cracked and chimneys are overthrown,
walls tall apart, trees swing so violently
that their tops thrash together, the earth
Is torn and rent by waves veritabe as
those of the sea. and the sands are shaken
down until the ground-yater is forced out
In gushing fountains; and then as the
primary vibrations TAIL the long-period
oscillations TO which they give rise grad
ually die away. such Is the loWland
earthquake. In mountainous tracks
rocks may be dislodged. cliffs cleft asunder
and iandsiides started. and there is some
reason for supposing that fires have been
kindied by the crushing OF dry wood amid
the falling rocks.
While the character OF the primary and
consequent vibrations constituting the
earthquake is known, the causes of the
primary vibrations can not be stated in de-
tai, partly because they are diverse.
partly because of the presumption that
some remain TO be discovered. When the
volcanic vents through which the lava of
L i