The Seattle post-intelligencer. [volume] (Seattle, Wash. Terr. [Wash.]) 1888-1914, 1891-06-24
THE EARTH's FLATNEss.
Delicate Experiments Made in Seattle
to Determine 1t.


Professor T. C. Mendenhall, of Washing
ton City, who is at the head of the United
States coast and geodetic survey, is in the
city with one of his assistants, Mr.. Ere
mont Morse, of San Francisco, making ex-
periments to determine the exact figure of
the earth.
Every school boy is taught that the
earth is an oblate spheroid-a sphere
which centrifugal force has dattened at
the poles. But" said Professor Menden-
hall yesterday, ''the exact amount of this
fattening is not known, and we are now
trying to find this out. 1 subject is one
to which the coast survey has always
given more or less attention, and devoted
considerable time to it before / went into
that service.
We determine the contour Of the earth by
measuring the force of gravitation. 1t is well
known that the force of gravitation is stronger
the nearer we get to the center of the earth.
Since we get nearer this center as we go north
where the fattening is, it follows that if we can
discover the variation in the force OF gravity,
we can calculate from that the figure of the
earth, how much it has fiattened.
We discover this variation in the force of
gravity by careful observations of the time of
pendulum beats The swing of pendulum is
faster the stronger the gravitation, and con-
sequently our task comes down to finding the
times of vibration at various places.
We have with us new piece of apparatus
aud one which / believe to be simpler and bet
ter than any ever before made-just turned out
by the instrument division OF the survey. With
this apparatus we took observation in Wash
ington City and again in San Francisco Then
we went to Alaska, far toIthe north, where the
variation in gravitation was very noticeable.
In six or seven places there we took observations
After we have done here we shall go 10 Portland
and then back TO San Francisco, taking obser-
vations in each place.
The experiments, which will last until
Thursday night, are being made here in the
northwest corner of the basement of the
Central school,
The conditions here are better than we have
had, said Professor Mendenhsll, for we get an
almost invariable temperature. In Alaska we
had TO set up our instruments in tents and work
while the rain was pouring about US.
You can gain some idea of the difference in
the force of gravitation which we observe when
tell you that a gold brick weighing 80 pounds
in San Francisco would weigh five ounces moro
in sitka being there nearer the center of the
globe Were the gold weighed in a balance the
weight in the opposite pan would of course be
increased in like proportion and the difference
mentioned would not be apparent, but in q
noticeable.
The instruments used by Professor Men,
denhall are very delicate, and are SO exact
that he is able to determine the time of a
pendulum beat down to the 500,O00th part
of second. The pendulum itself. made of
aluminium and copper, is hung on agate
knife-b bearings in an air tight brass box.
The air is exhausted from this so that the
pendulum may have 10 overcome no re-
sistance. Experiments are made with
pendulums of three different lengths.
which of course beat in different time.
The temperature of the pendulums is also
carefully noted, so shat the change in their
length from expansion by heat. and the
consequent change in time of vibration
may be allowed for.
Each pendulum carries a mirror on
which the light is thrown through small
window in the brass box. There is a fash,
fash, flash as the pendulum swings. A
small telescope is used for watching the
flashes. In this telescope the field of vision
is opened and closed every second by a
shutter connected through an electric cir
cult with a chronometer. The pendulum
swings n little oftener than once a second,
and so, while the light from the swinging
mirror sometimes flashes into the telescope
at the instant the shutter is opened, the
opening of the shutter and the flashing of
the mirror do not generally coincide. The
time of vibration can be determined with
great exactness by noting the period of CO.
incidence.
The observations are being cont nued
night and day.