The sun. [volume] (New York [N.Y.]) 1833-1916, 1899-04-09
OLD EARTH'S ATTRACTION.
SCIENTIFIC OBSERVATIONS MADE IN
THIS CITY RECENTLY.

Donau Measures the Attraction OF Gpny-
itRtion on Manhattan Isiand-vaIue of
hus Obtained So All Nn-
tions Interested in Such Experiments.
II a man had the misfortune to fall from =
balloon he would reach the earth much more
quickly II the balloon were near the North Pole
than IL IL were near tho equator. An Aus
trian man the Donau. which stopped
at New York recently. is making a tour OF i'm
vestigation for the purpose OF establishing tbs
accuracy Of this statement. One OF her Of
fcers. Lieut.. Rodler. has been testing the
force OF the earth's gravity ut every point at
which the vessel has .ouched. and the Captain
OF the Donau has been ordered BY his Govern
ment to eall at various points simply to give
Lieut.. Rodler an opportunity to make observa-
tions. While In New York Lieut.. Rodler ex-
perimented In a sub-basement OF Columbia
College. The United States survey officials
thought enough Of the matter to send a repre-
sentative to Columns with American instrn-
ments to verify the computations made by the
Austrian.
The observations OF Lieut.. RoIler'are part Of
q great international gravity survey which has
been agreed upon and Is being carried out by
the various Governments. 1t was made neces.
sary by a peculiar: existing condition. Al
though nearly every one knows that the earth
exerts q strong force OF gravity in all its parts.
yet IL Is not so we'll known that the force is va
riabls. 1t crows much stronger as either OF
the poles 10 approached. This 10 because the
earth Is nattened slightly 1t the North and
South Poles. The North and the South Poles
are much nearer tho exact centre OF the earth
than ia the equator. That Is to say. the dis
lance from the exact centre OF the earth to the
surface OF the tropic zone 10 much greater than
is the distance from the centre to the friRid
zone. Now. as the attraction OF Rravitation
grows stronger as the centre OF tho earth 10
approached. II Is plain that the force Of Crav-
ity OF the fririd zone must he stronger than
that OF the troric zone.
The earth as a whole exhibits this phenom-
enon. but there are certain spots which seem
to take on an individuality Of their own. These
spots affect testing instruments in such a wav
that til. general law might seem to be set ut
defiance. we're 1t rot that certain local condi-
tions serve as an explanation. The determi-
nation OF this varying force Of gravity Is not
accomplished. as might be supposed. bv drop-
ping a weight from a height and noting the
rate Of It's fall. This might be. and indeed has
been. accomplished. and chronograph records
have been obtained. But there is another
method OF obtaining records which is much
more simple. The trick is accomplished with
a pendulum. the rate OF the swing OF which en-
ables the observer to tell the exact force Of
gravity existing at any particular point. In
short. the nearertne pendulum is taken toward
the North or South Pole the faster 1t will
swing. because the attraction OF the earth be-
comes greater.
All Of the great scientific EXPEDITIONS carry
pendulum apparatus. Inasmuch as the pen-
dulum swings exactly In proportion TO its cis-
lance from the exact centre az the earth. the
observer can compute his distance awav from
the equator or from.the North Pole. other con-
Iifipna haintv npitnrr, uanna n ., oil ialanI
near japan was found TO be several thousand
Yards nearer the equator than had been sun.
posed before the observation was made. On
the other hand. when the exact latitude and
longitude OF a spot are known to acertaintv. anv
variation from the ordinary In the swing of
the pendulum will serve TO indicate the char
acter or the composition Of the earth directly
beneath the instrument. 1t Is a question Of
specific gravity.
Two sides Of a mountain may affect the PEN
dulum differently. By comparing The swing
on each side and the swing on top Of the noun
tain the observer can tell very accurately Of
what material the mountain is made. SINH
larry the volcanic nature Of some OF the South
Sea Islands has been determined ty n perdu-
lum. Ascension Island. for in stance. is really
nothing more than a great cinder heap. which
resulted from some volcanic upheaval in tbs
emote past. Its specific gravity was deter
mined BY the pendulum. ana It is known there-
from that even to a very remote de nth its Gin
der-like character is maintained. This YER
dict Of the pendulum is further borne out Ivy
the manner in which the island rocks under
the influence Of the very peculiar waves which
sometimes roll upon Ascension from the
northwest. These terrific combers are
periodic in appearance ard are often fifty feet
high. They follow one another in rand sue
cession and continue for several days. and
under their influence Ascension Island rocks
back and forth in a manner which 1t could not
do II made OF tirmer material than cinders.
Tho verdict Of the pendulum is thus borne out.
The mechanics Of this pendulum swinging
are highly interesting. In 'he sub-cellar Of
Columbia College Lieut.. Rodler swung the
pendulum and noted by means Of a chrome.
graph and a chron ometer Just how long. even
to the slightest fraction Of a second. II 70.
1uired for the weight to travel back and forth.
In order to be still more accurate, DR.. Davis 81
the astronomical department made calculations
Of the transirs OF certain stars. so that any de.
f~ciency Of the time calculations on the earth
nigh: be in a measure checked up. The PEN
dulum it.elf was n small affair. weighing but .
couple Of pounds. II was not more than ten
inches in length. The pendulum was hung
from a T-shaped bar. The under aide OF this
7 was ground down to a knife edge. This
edge rested on two small pieces Of agate.
Hence. as the pendulum swung il encountered
almost no friction and a slight touch or Inn
pulse given to it was suficient to set It going
for n whole day. In the top Of he pendulum
bar was a small square mirror. Nearby was
placed a candle, and as the pendulum swung
the light was reflected In the mirror and
flashed UP and down in the dark cellar. with
every pulsation OF the apparatus.
Lieut.. Rodler held his viGils all through the
night. when no disturbing influences were likely
10 interfere with the accuracy OF his observa-
lions He looked through a email telescope.
the large end Of which was closed except for =
small slit across the lens. As the pendulum
swung back and forth the light OF the candle
was flashed into the telescope at the end Of
every swing. The click OF the c~hronometer
apprtsed the observer Of the duration Of every
second or fraction OF a second as IL rassed.
and Lieut.. Rodler was enabled to calculate TO
o nicely the time required for the weight to
swing. The stronger the force OF gravity the
IL . Al. ,nuIu1,,na rvin