Evening star. [volume] (Washington, D.C.) 1854-1972, 1931-09-13

records waves in three dimensions at right
angles TO one another.
THE principle of our procedure is simple.
Small charges of dynamite fired at some
tsnceS from there by our portable seismc graphs.
vibrations of the ground are recorded by one
Of these instruments fast-movng photo
graphic film. A record is made of the vibra-
tiOns from each of the series Of explosions.
ON each record a vibrating tuning fork by
interrupting a beam Of light, places line every
hundredth of second BY an electric circuit
connected to the dynamite charge, a mark is
placed on the record at the very instant of ex-
plOSion. BY counting the timing lines from
that point, then, it is relatively simple matter
to determine to within n couple thcusandths of
a second the exact time which elapsed between
the explosion of the dynamite and the arrival
Of the first waves at the seiSmograph.
If all the waves followed the same path
say, slong the surface CT the ground. COM
psratively small number Of shots might suffice
for determining their velocity accurately. This.
however. is not the case. The first-arriviug
waves at successfully grea ter distances often
penetrate into the ground more and more
deeply before reaching the recording instru-
ment. As a result, it is necessary to make a
large number of observations in such cases and
subject them TO mathemat teal analysis in order
to determine the true velocity at various
depths
nR. LEFT goes to explain that the
vibrations travel through loose earth at the
surface. such as glacial till, at a far slower
speed than through the solid forms lions below
BY discharging the dynamite at successfully
greater distances, the first waves to reach the
instrument will for short distance travel
only through the low-speed material Beyond
certain point. however, depending upon the
thickness of the surface material the waves
will be able to make better time BY travelling
part Of the distance in the underlying high
speed solid rock
Thus they would arrive ahead of the schedule
they would follow the surface For exsm-
pie. the vibrations may travel through ordi-
nary earth at 1000 to 5000 feet per second
while they will travel in rock formations at
speeds of $4,000 feet or more a second There
fore. in taking set of observations along a
straight line. a profile Of the rock below the
surface will be revealed
One important question which the Harvard
experiments should open up is: How hot is it
in the very center Of the earth? We are told by
DIR. DAILY of Harvard: THE pressure in the
region of the earth's core ranges from s. OOO-
600 to SO. 900 900 pounds to the square inch.
Under such colossal pressure the earths core.
which IS probably made of iron, can be in a
fluid condition only if the temperature of the
earth is enormously high-at tens of thousands
of degrees Centgrade. Both pressure and tenn
perature down there are far beyond the range
of the expericceTca9 laboratory.
Tire physical state of the iron core cannot
as yet be described. Is it liquid. a gas or
iron in state unknown to physics? The con-
ditionS of the earth's core are starlike. From
their study can physicists of the future tell US
something more of the true nature of the
stars? If they can they will be pretty sure,
incidentally, to shed new light on the structure
and life story of the atoms for the secret OF
the star and the secret of the atom are proving
to be part of a single problem. the ultimate
nature of matter"