The Columbus dispatch. (Columbus, Miss.) 19??-1922, 1921-05-11


tween Arrival Of Primary and SEQ
ondary Waves.
Catching earthquakes by means Of
the seismograph is a fascinating sport,
writes j. j. Shaw, Of the West Brom-
witch Seismological Observatory in
The London Daily Mail.
One must see that the clock mech.
anism and electric devices are always
up to concert pitch, or after weeks and
months Of waiting a fine record may
be lost. One important observatory
was caught mapping recently.
Earthquakes are rnptures In the
earth's crust due to bending, crumplng
and thrusts, stresses which are COM
mon at the foot of mountain chains.
The stresses accumulate, the strain
becomes too great, finally the rock
strata fractures like an over-bent stick,
and huge vibration is set up sufIl-
cient to shake down all structures in
the vicinity; or, If sub-oceanic, to set
up tidal waves and shake the crock
cry on board ships as happens in a
house on land.
Sets Waves in Motion.
A shock sets up different types of
waves, which rush outward at differ.
ent speeds.
There are three principal types
designated "P," IS" and "L."-"pri-
mary" secondary" and large" which
attain to a maximum" (M.), then die
away. 1t is from the time that elapses
between the arrival of the primary
waves and the secondary waves that
the distance is calculated. Thus:
166 seconds for 1000 miles, 1894 sec
onds for 5000 miles, 500 seconds for
5000 miles
The secondary waves rarely survive
after travelling 6,000 miles, when the
time between "P." and IL" or "P."
and "M.," though much less reliable.
has to be resorted to.
The only difficulty lies In determln
ing from the record which out of some
hundreds of waves is the particular
one required.
The chief seismographs in use are +:
Milne, British; Milne-Shaw. British
Galitzin, Russian; Welchert, German
Mainka, German; Omori, Japanese.
OF these the Galitzin and the Milne
Shaw are the most sensitive. The first
three record by ray of light being
photographed on sensitized paper; the
last three by a glass fiber writing on
smoked paper.
Cloclwork keeps the recording sur
face always moving, and the time is
marked on electrically every minute.
Locating the place of origin is rath-
er more difficult. It's direction can be
sometimes determined from one sta.
ton when the first impulse is well de.
fined, but much more reliable is the
"three station" method.
How They Are Located.
The big. shock of December 10 will
serve as an example:
The "S.-P. difference" as the in.
dlcating time is called, at West Brom.
witch was 555 seconds, representing
4SS5 miles.
Chicago, recently equipped with
Milne-Shaw selsmographs, stated their
distance to be 5000 miles.
Circles drawn with radll of 1855
and 5000 miles from West Bromwich
and Chicago respectively intersect at
two points, namely, just south OF
Alaska, 151 w. "a n, and northwest
Brazil, GO w. 4 N.
A we'll placed third station with a
third circle is necessary to determine
which Of the two intersections is cor.
rect. No third station I'm a suitable
position has so far reported, but the
fact that Aberdeen received the first
wave 10 seconds earlier than West
Bromwich and oxford 1 second later,
shows that the earthquake was in q
northerly direction, definitely deciding
If all data be correct-that the
earthquake was in the bed Of the Pa.
clfc ocean, 300 miles south OF Alaska
and 1000 west Of British Columbia.