Omaha daily bee., 1916-05-23
Why Air Makes
Bodies Lighter
Garrett p. ssrviss.

Garrett p, ssrviss
A IR presses upon all bodies immersed in it
n equally in every drection-upward, down
ward and sideways. This is property common
10 all fluids whether the fluid be liquid, like
water, or a gas or mixture Of gases, like the at
mosphere Solids have not this property, but
only press downward in obedience 70 gravity.
But fluids as we'll as solids, are affected by gray-
ity which draws them all towards the center Of
the earth thus giving them weight in proportion
10 their density
In fluid the effect OF the weight Is distrlb-
uted in every direction while in a solid as ! have
just said, the effect is perceived only IN down
ward direction Consequently, when solid is
immersed in a fluid the upward pressure Of the
latter counteracts certain proportion Of the
downward pressure OF the solid and thus dlmn-
ishcs lis weight
The amount by which the weight is thus
diminished depends upon the relative density
Of the solid and that Of the fluid surrounding H.
and is measured by subtraeting from the total
weight Of the solid the weight Of volume Of
the fluid exactly equalling the volume Of the
solid Any increase IN the density Of the fluid
will increase its pressure, and any decrease will
decrease H.
A gas viii act In this way upon a liquid as
we'll as upon solid, although both are fluids
To prove this il is only necessary TO confine the
liquid 50 that II cannot escape This brings Us
10 the principle Of the barometer.
Mercury Is metal, which in ordinary cir
cumstanees Is liquid instead Of solid II the mer.
curry be confined in a tube IN such a way that
the effect 81 the pressure Of the air can be made
evident. the air will tend TO push il upward.
To obtain this effect, however, il is necessary
70 remove the air from above the mass Of mer-
curry that is being experimental with. This Is
done by creating vacuum IN the upper part Of
the tube containing the mercury Then there
is no downward pressure Of the atmosphere on
the mercury, but only pressure tending TO lift
IT, II will be lifted 70 height IN the tube de.
pending upon the relative weight Of mercury and
air.
In barometer there is tank, or reservoir,
Of mercury at the bottom OF the apparatus, which
Is exposed 10 the pressure Of the atmosphere
Dipping into this reservoir Is vertical tube
open AZ The bottom, SO that the mercury can
enter, but closed at the top, and previously ex-
hausted Of ar, so that above the mercury in the
tube no atmosphere pressure exists Thereupon
the pressure Of the air in the reservoir causes the
mercury 10 rise ~ the tube up 70 a point where
its weight becomes equal 10 the air pressure
Experiment has shown that the height 10
which The mercury rises is near sea-eye and
IN ordinary states Of the atmosphere, about sev.
cnty.siS centimeters, OF say thirty inches Al
that height the weight OF the mercury is bal.
anecd against The pressure Of the atmosphere
But, as the latter varies TO slight extent
through changes OF density, due 10 variations OF
temperature, moisture etc, the height OF the
mercury iluetuates little up and down, accord.
ing 10 circumstances. When the barometer Is |
carried to the top OF a mountain, or up in bal
loon, the mercury falls, in proportion TO the Gle.
vation, because the density OF the atmosphere
decreases with increase Of height from the earth
In similar way the air presses upward a |
column OF water, confined in a tube, with vac. |
num above il, but since mercury is, bulk for bulk.
rather more than thirteen times heavier than
water the air pressure is able 10 lift column OF
water about thirty-threc feet instead Of thirty
inches II Is upon this fact that the action OF 4
suction pump depends.
The valve OF the pump docs not really Suck
The water upward, but il removes the air from
the top OF the column OF water in the tube. and
The pressure Of the air in the well or other reser
yor forces the water up the tube to height
where IT'S weight becomes equal 10 the atmos
pheric pressure
So too, the common syringe is moi filled bv
suction when the valve is drawn backward. but
BY air pressure, Suction as ordinarily used IA
a word based upon false conception You do
not suck lemonade through a straw: the air nres.
sure drives il through. What you suck out i The
ar in the straw