Yorkville enquirer. [volume] (Yorkville, S.C.) 1855-2006, 1910-11-29
HOW TO WEIGH THE EARTH.

|
HOW TO WEIGH THE EARTH.
With cane, Silver Wire, Leaden Globes,
| Pencil and Paper It Is Easy.
At the College of the City of New
York the other day a professor told
how he had picked up Old Mother
Earth, dumped her on the scales and
weighed her. AS a matter of fact,
weighing" is a misleading word to
apply to such a scientific achieve
ment, because the earth doesn't weigh
anything in the sense in which you
would say that your weight is 140
pounds or whatever it may be.
Your weight Is the force with which
the earth pulls you toward 1t, and, OF
course, the earth couldn't be said to
pull itself toward itself. What Is
really meant Is what all the rock and
earth and water in the globe would
weigh If they could be brought out
to the surface and measured.
For instance, we know the size Of
this planet, so we know how many
gallons of water could go into the
space 1t occupies. Well all the gal
600, ,000, ,000, 900, 100000 tons. One bill
lion billions Of tons! Now, the earth
isn't made Of water only, but Of rocks
and metals and sand and thousand
other things. How many times heavier
would II be when these are allowed
for. To use the scientific word, what
is the density Of the earth?
| Some scientists have tried to find
out by going very high up into the
mountains, where bodies don't weigh
the same as on the surface. Weight
being the force that pulls you to the
earth, some of the weight is due to
the part of the earth close to you and
some parts further away. By meas
uring on the top Of the mountain and
at the bottom the scientists could see
how much of the total was due to the
portion Of the planet close at hand
t. e., the mountain. Then by elaborate
times bigger the rest Of the globe must
be, and surveyors and geologists told
them how much the mountain itself
weighed.
Still another scheme is to hang a
plumb-bob up on one side Of noun
tain. The mountain will pull the
plumb-bob a little out Of the perpen-
dicular-a little toward itself. This
was the method in the Schichallion
experiment. 1t was found that
Schichallion, which is a mountain in
Scotland, pulled the plumb-bob IL
seconds out of plumb-12 seconds and
1900 part of degree, and you need
something better than the naked eye
to see DETECTION Of even one degree.
From this the scientist deduced that
If the earth was As as dense all
through the mountain 1t would weigh
4-5x2 1-2-4 1-2 times as much as
water. AS a matter Of fact, the enor-
nous pressure inside the earth makes
1t much denser in the middle. When
correction was made for this the figure
| 4 1-2 was raised to 1-3, giving five
thousand three hundred billion billion
tons, 100000, 600, 900, 900 600, 900.
| All the methods agree fairly well.
They all place the weight Of the ma
terial Of which the earth is composed
at between five and six times the
weight Of water. Closer than that the
scientists do not agree, and you can
take your pick, a trifle Of a couple
of hundred billion tons will never be
missed.
| The best way of making such a
measurement was invented in 1895 by
celebrated English savant Henry
Cavendish, and doesn't need a noun
tain or a mine or any similar trihe
that the average man doesn't carry
around with him as a rule. In fact
the apparatus can fit into an ordinary
room, though the rooms Of an or
binary New York fiat couldn't hold
very much else If the Cavendish ap-
paratus were crowded into any OF
them.
Cavendish's method was an indirect
one. The force that draws a body
to the earth-its weights mani
festation Of the power known as gravi-
tation. We know how to measure
this power, and everybody is familiar
with it's manifestations.
Everybody, on the earth or off, at
tracts every other body. The amount
of the attraction depends only on the
mass Of the two bodies that attract
each other and the distance between
them.
If you could take two baseballs
away from the earth, away from every
planet, put them out in space millions
of miles from any other body, and
| then leave them, inches or feet or
miles apart, they would proceed to
move toward each other. The only
the surface Of the earth is because
the earth is so much bigger than eth
er of them that they would rush to-
ward the earth-would fall, in other
words-before they had chance to
move toward each other.
Cavendish eliminated the earth as
disturbing factor by making his
.wo bodies attract each other in
horizontal direction. The earth's at
traction Is, of course, vertical. He
took a long thin stick, very light and
six feet in length, and suspended 1t
horizontally by silver wire fastened
to it's middle. To each end Of the rod
he attached little balls or knobs OF
lead.
The bodies that were to attract
these two knobs were two big balls or
I,to ftt1 Sn
that at a given signal he could bring
the large balls close up to the knobs
at the end Of the stick, one on one
side of the knob, one on the other
side of the knob at the other end.
He found that when he did this
each knob drew toward the big lead
ball near 1t, swinging the wooden ROD
round on its wire support. with
delicate instruments he measured
twisted out of place. Thus he could
tell what was the force Of attraction
the force of gravitation-pulling
the knobs toward the lead balls. He
knew how much the earth would at
tract his little knobs-that Is, he knew
their weight he knew how much ma
terial there was in the large lead
balls used, the ratio of the quantity
of material in the earth to the guam
tity in the big balls Of lead followed
from the simple laws established by
Isaac Newton. All the apparatus
used is thus comparatively simple
for a scientific experiment at any rate.
A silk or fiber thread can replace the
silver wire used by Cavendish, the
knobs and balls need not be Of lead;
they can be of any material, but the
heavier the better, only they must not
both be of magnetic material-iron.
for instance--otherwise the effects of
magnetism would oversnadow those
of gravity. What you have to do Is
to measure the force that attracts the
knobs to the heavy balls, and the dis
tance between the knobs and the
balls, then apply the simple formula
of Newton's laws of gravitation.
The apparatus is not extremely ac-
curate: you will not be able to trust
results too far; but when 1t Is con-
sidered that the closest guess of to-
day does not pretend to be within
hundred billion billion tons. and that
scientific observers differ by thou
II L.,