Catoctin clarion. [volume] (Mechanicstown, Md.) 1871-1940, 1921-03-03

Catching earthquakes by means Of
the seiSmograph is II fascinating = sort.
writes j. J. Shaw. of the West EiMn-
wich SeiSmologicnl nbscrvat"r, in
The London Daily Mail
One must see that the clock mech.
aniSm and electric devices are nlHuyS
up to concert pitch. or after weeks ami
months of waiting fine record may
be lost One important Observatory
was caught mapping recently.
Earthquakes are ruptures IN the
earth's crust due to bending. cr1:nplng
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 sci ~ sut'i-
Client to shake down all structr res in
the vicinity; or, If sub-oceanic To SET
up tidal waves and shake the crock.
cry on board ships rs happens in u
house on land,
Sets Waves In Motion.
supCk SStS ull " IhNS III
waves, which rush outward ut differ
ent speeds.
There are three principal types
designated "P," IS" and "L."-"pri.
mary" secondary" and "lnrge." which
attain to maximum" (M). then die
away. 1t IL from the time that eapsrs
between the arrival of the primary
waves and the secondary waves that
the distance Is calculated. Thus:
lG6 seconds for 1000 miles, 3:4 seq
onds for 5000 miles. go secon Is for
5000 miles.
The secondary waves rarely survive
after travelling 5000 miles, when the
time between "P." and a." OF "i'."
and M" though much less reliable
has to be reSorted tn.
The only difficulty lies In det"rmin
Ing from the record which out of some
hundreds Of waves is the particular
one required
The chief se'smo9raphS IN us. are: :
Milne, British; Milne Shaw. British !;
GaltZin. Russian: Welchert German !:
Mainkn. German: Omori. Japan $50.
or these the Galitzn 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 glass fiber writing on
smoked paper.
Clockwork 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. Its direction can he
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 Ii will
serve as an example:
The IS .@lifference." as the In.
dlcatinG time is called, at West Brom-
witch was 555 seconds. representing
1885 miles.
Chicago. recently equipped with
MilneShaw seismographS, stated their
distance to be 5000 miles.
Circles drawn with radli of 1855
and 5000 miles from West Bromwich
and Chicago respectively intersect ut
two points. namely. just south Of
Alaska. l51 w. 81 N. and northwest
Brazil 63 W. 4 N.
A well placed third station with q
third circle Is necessary to determine
which of the two intersections is cor
rect No third station in Il suitable
position has so far reported. hnt The
fact that Aberdeen received the first
wave Is seconds earlier than West
Bromwlch and oxford | second later,
shows that the earthquake was in a
northerly direction, definitely deciding
If all data be cOrrect-thal the
earthquake was In the bed of the Pa
cific ocean, 300 miles south Of ALASKA
- md 900 ract nf British ('numliA