76
HYDRAULIC AND PNEUMATIC ENGINEERING
A
ship is a floating body and it displaces its own weight of
water. If,
for example, a ship weighs 10,000 tons it displaces 10,000
tons of
water. If 5000 tons of cargo are
added it floats deeper in the water and displaces 15,000
tons of water, and so on.
You
will now show that a floating body displaces its own weight of
water.
EXPERIMENT
No. 36
To
illustrate the law of Archimedes for bodies which float.
Use
the empty glass bottle as the
floating body. Close the lower hole
in the metal tank with a No. 1
stopper, put the large coupling in
the upper hole, fill the tank with
water until water runs out through
the coupling and stops. (l)Fig. 103.
Now place the bottle slowly in the
water and catch the water it displaces,
(2) Fig. 103.
HYDRAULIC
AND PNEUMATIC ENGINEERING 77
Now
make a spring balance with a bucket, (3), Fig. 103, as
follows: Find
a tin can around your home, punch two nail holes near the top,
and
attach the can to the elastic band by means of a cord, suspend
the
band from a nail driven in a piece of board.
Now
put the bottle in the can and mark the position of the bottom
of
the elastic band. Then take the bottle out and pour into the
can the
water displaced by the bottle, (4), Fig. 103. Do you find that
the
displaced
water weighs the same as the bottle, that is, that a floating
body
displaces its own weight of water?
You
have here illustrated the law of
Archimedes for floating
bodies.
EXPERIMENT
No. 37
To
illustrate the law of Archimedes for
bodies which
sink
in water.
Use
the bottle filled with water to
represent a body which sinks in
water, fill the metal tank, Fig. 104,
with water until it overflows through
the coupling and stops. Place the
bottle in the tank slowly and catch
the water it displaces.
Now
attach the full bottle to the bottom of the balance (2) and
mark the
position of the bottom of the rubber band. Submerge the bottle
in water
(3), mark the position of the bottom of the rubber band again,
then
pour the displaced water in the can, (4). Does the balance
descend to
the mark (2)? That is, is the bouyant effect on the bottle
equal to the
weight of water displaced by the bottle?
78
HYDRAULIC AND PNEUMATIC ENGINEERING
You
have here illustrated the law of Archimedes for bodies which
sink
in water.
RAISING
SUNKEN SHIPS
EXPERIMENT No. 38
To
show how sunken ships are raised by means of air.
Sunken
ships are raised by compressed air as illustrated in Figs. 105
and
106. Air is pumped into the ship until the ship and the air
displace a
weight of water slightly more than the weight of the ship; the
buoyant
force of the water then lifts the ship
to the surface.
HYDRAULIC
AND PNEUMATIC ENGINEERING 79
Illustrate
this with the apparatus shown in (1), Fig. 107. Fill the
bottle
with water to represent the sunken ship, submerge it in a pail
of
water, and blow air in through the hose. Does the ship float
to the
surface?
Sunken ships are also
raised
by means of large steel pontoons filled with
air as shown in Fig. 108.
Illustrate
this as shown in (2), Fig. 107. Use the bottle as the sunken
ship and two empty tin cans of the same size as the pontoons.
Punch
nail holes in the opposite sides of the top edge of each tin
can,
connect them as shown, force air into them a little at a time
in
equal amounts. Is the ship raised nearly to the surface?
80
HYDRAULIC AND PNEUMATIC ENGINEERING
HYDRAULIC
AND PNEUMATIC ENGINEERING 81
82
HYDRAULIC AND PNEUMATIC ENGINEERING
Note:
The ship would be floated into shallow sheltered water
in
this
way, then repaired by divers, and floated by compressed air as
described; or a coffer dam would be built around it and the
water
pumped
out; then the repaired ship would float when the water was
admitted
to the dam.
FLOATING
DRY DOCKS
The floating dry
dock, Fig. 109, is a huge steel or concrete trough shaped
structure with hollow sides and with large tanks along the
bottom.
It is open at both ends and when the tanks T.T.T., Fig. 110
are filled
with water it sinks to the water line L.L. The boat then sails
into
the dock and is securely braced, the water is pumped out of
the tanks
T.T.T., the dock rises until the water line is at W.W,, and
lifts the
ship above water.
The
dry
dock lifts its own weight and the weight of the
ship because
it displaces a weight of water equal to the combined weights.
HYDRAULIC
AND PNEUMATIC ENGINEERING 83
When
the ship has been repaired or when
the barnacles have been scraped from
its bottom and it is ready for sea,
water is again admitted to the
tanks, the dock sinks to
the water line LL, and the ship sails out.
You
will now make an experiment to illustrate the working of a
floating dry dock.
EXPERIMENT
No. 39
To make and operate
a floating dry dock.
Use
a flat cake pan to represent the dry
dock, and the bottle to represent the
ship.
Float the dock on
water in a sink or wash basin and
pour water into it until it floats
with the top about 1 in. above water.
This represents the real floating
dry dock, with its tanks full, ready
to receive the ship.
Float
the bottle on the water in the dock.
This represents the ship, in the dock
and ready to be raised.
Now
siphon the water out of the dry dock and over the edge of the
sink or wash basin. This represents the water being pumped out
of
the tanks of a real dry dock. Do you observe that both the
dock and
the ship are raised as the water is siphoned out? This shows
how
the dock and ship are raised when the water is pumped out of
the tanks
of a real dry dock.
Now
siphon water from the sink into the floating dry dock. Do you
observe that the dock and the ship sink as water enters the
dock?
This
represents how the real dock sinks when water is admitted
again to
the ballast tanks.
84
HYDRAULIC AND PNEUMATIC ENGINEERING
THE
GLASS SUBMARINE
EXPERIMENT
No. 40
To
make the glass
submarine submerge and rise in water.
You
will observe that the glass submarine (1), Fig. 112, is hollow
and that
it has a hole at the stern.
Place
it in a tumbler of water. Does it float?
Place
it, stern down, in the bottle full to overflowing with water,
close
the bottle, turn it on its side, and shove the stopper in
hard. Does
the submarine submerge? Withdraw the stopper slightly. Does
the
submarine rise and also move forward suddenly?
Repeat
this with the bottle between your eyes and a light and observe
the
air in the submarine. Is the air compressed when you shove the
stopper
in, and does it expand when you withdraw the stopper?
The
submarine floats in the tumbler because it is lighter than an
equal
volume of water. It sinks in the bottle when you force the
stopper
in because sufficient water is forced in to make it heavier
than an
equal volume of water. It rises when you release the stopper
because
the
air expands and forces sufficient water out to make it again
lighter
than
its own volume of water.
Water
is nearly incompressible but air is very compressible and when
you shove the stopper in you compress the air but not the
water.
HYDRAULIC
AND PNEUMATIC ENGINEERING 85
Find
a larger bottle and repeat these
experiments.
The
submarine
moves forward when you withdraw the
stopper because the expanding
air shoots a stream of water to the
rear through the stern and this
drives the submarine forward.
Illustrate
this with the apparatus Fig. 113.
Does the stream in one direction under
water force the nozzle in the other
and make it writhe like a snake?
RUNNING
WATER
FRICTION
As
soon as water starts to run in a pipe
it rubs against the inside of the
pipe and its velocity is decreased.
This rubbing is called friction and
it always decreases the flow of
water.
EXPERIMENT
No. 41
To
illustrate the effect of friction
on running water.
Use
the
apparatus, (1), Fig. 114.
Raise and lower the tank. Do
you find that the stream from the
nozzle never reaches the level of
the water surface in the tank?
86
HYDRAULIC AND PNEUMATIC ENGINEERING
It
does not do so because the friction in the tubes and
nozzle decrease its velocity.
Use
the apparatus (2), Fig. 114. Is the lower stream longer
than the upper, but do you find that it does not
reach as high as the upper stream? It does not, because
the velocity of the water in the lower tube and nozzle
is greater and therefore the friction is greater. Use
the apparatus, Fig. 115. Allow the water to run into the
tumbler for exactly 15 seconds and observe the amount,
then close the coupling above the tee, empty the water
back into the tank, transfer the elbow to the end coupling,
and allow the water to run into the tumbler from the
end for exactly 15 seconds. Is the flow of water less from
the end? It is less because the friction in the extra
pipes decreases its velocity.
It
is a matter of the greatest importance that friction be
taken into consideration in planning the piping for any
system of water supply or water power. The facts regarding
it may be stated briefly as follows :
The
friction of water in pipes:
(1)
Is greater in long pipes than in short pipes of the same size.
(2)
Is greater in rough pipes than in smooth pipes of the same
size.
(3)
Is
greater when the water is moving rapidly than when it is
moving
slowly .
(4) Is greater
in small
pipes than in large pipes of the same length.
HYDRAULIC
AND PNEUMATIC ENGINEERING 87
NOZZLES
When
you have
been watering the road or garden you have probably noticed
that the stream is longer when you use a nozzle than when you
simply
let the water flow from the end of the hose. Have you noticed,
however,
that you put less water on the road or garden in a given time
with a nozzle than without?
EXPERIMENT
No.
42
To
show why the
stream is longer with a nozzle than without.
Use
the apparatus (1), Fig. 117. Is the stream short and is the
pressure
low? Place a nozzle in the coupling (2), Fig. 117. Is the
stream long
and is the pressure high?
88
HYDRAULIC
AND PNEUMATIC ENGINEERING
You
have shown here that the stream from a nozzle is longer than
from
the hose because the pressure behind it is greater.
The
pressure at any point in a pipe carrying running water is
proportional
to: first, the height above the point of the water in the
tank;
and second, to the fraction of the total resistance the
running water
encounters beyond the point. The pressure behind the nozzle in
(2) is great because the resistance the water encounters in
the nozzle
is
great.
EXPERIMENT
No. 43
To
show that you put less water on a road in a given time with a
nozzle
than without.
Use
the
apparatus, Fig. 118, allow the water to run from the end of
the hose into the tumbler for exactly 15 seconds and observe
the
amount,
then insert the nozzle and repeat. Is the flow less with the
nozzle
than without?
HYDRAULIC
AND PNEUMATIC ENGINEERING 89
VELOCITY
OF FLOW
90
HYDRAULIC AND PNEUMATIC ENGINEERING
You
might think that the velocity of water from a nozzle would be
doubled
when you double the height of the water in the tank above the
nozzle. You will show, however, that you must make the height
four
times
as great to double the velocity.
EXPERIMENT
No. 44
To show that the
velocity of water is doubled when the head is made
four times as great.
Use
the apparatus, Fig. 119. Allow the water to flow into the
tumbler for
15 seconds with the head exactly one foot, observe the amount
carefully,
then repeat with the head exactly four feet. Is the amount
doubled?
The
head is the vertical distance the water surface in the tank is
above
the nozzle opening.
The
velocity of water in a pipe varies as the square root of the
head. That
is, if you start with a head of 1 foot, and increase the head
to 4 feet
the velocity is doubled, √4 = 2; if you increase the head to 9
feet the
velocity is trebbled √9
= 3, and so on.
AIR
LOCK
If
the pipe from your water tank to your
house runs up and down hill, it may
become stopped by an air lock as
shown in Fig. 120. In (1) the tank is
empty but water remains in the U part
to the level of the bathroom
faucet; above this is air. In (2) the
tank is again filled and the bathroom
faucet is open but the water does not
run. It does not because the air in
the pipe permits the 15
HYDRAULIC
AND PNEUMATIC ENGINEERING 91
foot
head at the tank to be balanced by
the 15 foot head below the bathroom
faucet. This is called an air lock.
The
air lock can be destroyed by opening
any faucet near the bottom of the U
because these let out the water and
then the air. It can be destroyed
here by opening the basement
faucet.
EXPERIMENT
No. 45
To
illustrate an air lock.
Use
the apparatus, Fig. 121. In (1) the
tank is empty and the U is half full
of water. In (2) the tank is filled but
the water does not run. It is air
locked because the air permits the 8
inch head in the U to balance the 8
inch head at the tank.
Open
the tee. Is the air let out? Close
the tee. Does the water flow, that
is, is the air lock destroyed.?
92
HYDRAULIC AND PNEUMATIC ENGINEERING
PNEUMATIC
ENGINEERING
Pneumatic
engineering is the engineering which deals with air
and other gases. You have already used two pneumatic
appliances
in the section on hydraulic engineering, namely, the siphon
and the pump; these are pneumatic and also hydraulic
appliances.
You have also made some experiments to show that
the atmosphere exerts pressure; you will begin your work in
pneumatic engineering by making further experiments along this
line.
ATMOSPHERIC
PRESSURE
EXPERIMENT
No. 46
To
show that the atmosphere exerts pressure.
The
Magdeburg hemispheres, (1) Fig. 122, are made of metal, are
hollow,
and are ground smooth around the edge so that they fit
together
air-tight. When the air is pumped out, through the handle on
one
side, they are hard to pull apart. The original hemispheres,
(2) Fig.
122, were 14 inches in diameter and required eight horses on
each side
to pull them apart. When the air is pumped out there is
nothing inside
the hemispheres to exert pressure outward and the pressure of
the atmosphere holds them together.
Show
this with (1), Fig. 123. Pull the handle up and there is very
little
air inside to exert pressure outward. Pull out the end
stopper. Does
the atmosphere make this rather difficult?
HYDRAULIC
AND PNEUMATIC ENGINEERING 93
Show
it also with (2). Fill the quart sealer one third full of hot
water,
put on the rubber ring and the cover but do not seal, place
the sealer
in a saucepan of salt water, heat until the water in sealer
has boiled
for one or two minutes, seal and stand aside until quite cold.
Unseal
and try to lift the cover. Is it difficult?
The
steam formed in the sealer drives out the air and when the
steam
condenses there is a vacuum above the water in the sealer.
There is
then no upward pressure under the cover and the atmospheric
pressure
on top makes it difficult to lift the cover.
94
HYDRAULIC AND PNEUMATIC ENGINEERING
When
the plunger is raised in the tube,
(1), Fig. 124, the atmospheric pressure
on the outside forces the sheet of
rubber in.
Illustrate
this
also by means of (2), Fig. 124. Suck
air out of the tube and close the
hose with a clip. Does the atmosphere
force the rubber in? Turn the rubber
in all directions. Is the pressure of
the atmosphere equal in all
directions.
HYDRAULIC
AND PNEUMATIC ENGINEERING 95
A
most striking method of showing
that the atmosphere exerts pressure
is shown in Fig. 125. A little water is
placed in an empty syrup can and
boiled until the steam comes out
for one or two minuts.
(Sic.)
The can is
then closed air tight and inverted
in a dish of cold water. In a short
time the can suddenly collapses.
The
reason for this is as follows: when the steam has driven out
the
air there is nothing left in the can but water and steam, and
when
96
HYDRAULIC AND PNEUMATIC ENGINEERING
the
steam condenses in the closed can, there is nothing in the
space above
the water, to exert pressure outward and the can must stand
the whole
pressure of the atmosphere. If it is not strong enough to do
this,
it collapses.
Beg or
buy a
gallon syrup can and try this experiment, it will certainly
surprise you. Be sure the opening is covered with water when
you
invert the can in cold water because the water will help to
make the
opening air-tight.
You
cannot make this experiment with a glass bottle because the
glass
is strong enough to support the atmosphere.
HOW
ATMOSPHERIC PRESSURE WAS FIRST
MEASURED
The
pressure of the atmosphere was first measured
by an Italian named Torricelli in 1643,
with apparatus similar to that shown in Fig.
126. His experiment was essentially as follows: A glass tube,
3 feet
long and closed at
one end, was completely filled with mercury (quicksilver)
to expel the air; the open end, closed
with the finger, was then inverted over a
dish of mercury, and the finger was removed
under mercury.
He
found that some of the mercury came out of the tube but that a
column remained to a height of about 30 inches above the
surface of
the mercury in the dish.
Since
no air enters the tube, the space above the mercury in the
tube
has nothing in it, that is, it is a vacuum. There is,
therefore, no
pressure
downward on the surface of the mercury in the tube, and the
pressure of the atmosphere downward on the surface of the
mercury
in the dish supports the column of mercury in the tube.
HYDRAULIC
AND PNEUMATIC ENGINEERING 97
HOW
THE PRESSURE OF THE ATMOSPHERE IS
MEASURED
If
this experiment is repeated with the tube shown
in Fig. 127, the top of the mercury in
the long closed tube is 30 inches above the
top of the mercury in the short open tube. Since,
as you will show shortly, this height is
independent of the area of cross section of
the tube, we can consider this to be just 1
square inch.
The
pressure
of the atmosphere on 1 square inch at
A, then, supports a column of mercury BC
which is 1 square inch in area and 30 inches
high, that is, it supports 30 cubic inches of
mercury.
Now 1 cubic inch
of mercury weighs .49 lbs. (nearly 1/2 lb.) and 30 cubic
inches of mercury weigh .49 x 30 = 14.7 lbs. The pressure of
the
atmosphere is therefore 14.7 lbs. per square inch, (nearly 15
lbs. per square
inch).
It is a very
astonishing fact that the atmosphere exerts 14.7 lbs. pressure
on each square inch of every thing at the surface of the
earth. It
is at first almost unbelieveable, but you have already made
experiments
which illustrate this pressure and you will make others as you
proceed.
EXPERIMENT
No. 47
To
measure the pressure of the atmosphere.
If
you have a spring balance you can measure the pressure of the
atmosphere directly with the apparatus, Fig. 128, as follows.
The
diameter of the plunger is a little over 5/8 inches and
therefore its
area is 3/10 square inch. If then the pressure of the
atmosphere is
15 lbs. on 1 square inch it is 15 x 3/10 = 4 1/2 lbs. on
3/10
square inch.
Soap the plunger well to
make it slippery, shove it about 3/4 way into
the tube, fill the remaining 1/4 of the tube with water, and
insert a
solid rubber stopper in this end, (1). Now turn the tube so
that the
plunger
handle points vertically upward, and pour a little water in
above
the plunger to make it air-tight, (2).
98
HYDRAULIC AND PNEUMATIC ENGINEERING
Now
to measure the pressure of the atmosphere,
attach the plunger handle to a spring
balance, hold the tube firmly against
the table, and ask your partner to
pull upward on the spring balance
while you observe the pull recorded
on the balance, (3).
Ask
him to lift the balance slowly until
the plunger is about two inches above
the water, then ask him to allow the
balance to go back slowly until the
plunger is only about 1 inch above
the water. While he is doing this you
must read the average pull on the
balance.
Do you find
this
average pull to be 72 ozs. or 4 1/2 lbs?
Note: While
your
partner is
raising the plunger, the friction of
the plunger against the sides of the
tube is working against the balance
and the pull will be over 4 l / 2
lbs; but while he is lowering the
plunger, the friction will be working
with the balance and the pull will be
less than 4 1/2 lbs. The average will be
about 4 1/2 lbs.
You
have shown here that the pressure
of the atmosphere is 4 1/2 lbs. on 3/10
sq. in. or 4 1/2 x 10/3 = 15 lbs. on 1
square inch.
THE BAROMETER
The
barometer, Fig. 129, is the chief instrument
used by the Weather Bureau in
forecasting the weather. It Is an
apparatus similar to that used by
Torricelli in his experiment. The pressure
of the atmosphere on the mercury in
the open tube or cup supports a
column of mercury about
HYDRAULIC
AND PNEUMATIC ENGINEERING 99
30
in. high in the long closed tube. The
pressure of the atmosphere varies from
hour to hour and the height of the
mercury column varies with it. Weather
forecasts are based on this variation.
It
has been found that when
the mercury falls much below 30 in.,
because the atmospheric pressure is
low, bad weather may be expected; and
when the mercury rises much above 30
inches, because the atmospheric pressure
is high, good weather may be
expected. The extreme variations are from about
29 in. to 31 in.
The
barometer (2) is the type used on ships, and
when a sailor says "the glass is falling" he
means that the mercury in the glass tube is
sinking below 30 in. and that bad weather is
to be expected; when he says "the glass is rising,"
he means that the mercury is rising above
30 in. and that fine weather is to be expected.
Another
type of barometer is
shown in Fig. 130. It is called an
aneroid barometer because it
contains
no liquid. It has a flat, round, air tight metal box from
which the air
is exhausted. The atmospheric pressure would force together
the top and
bottom of this box if they were not kept apart by the strong
spring
shown
100
HYDRAULIC AND PNEUMATIC ENGINEERING
above
the box. If the atmospheric pressure increases, the spring is
forced
down; if the pressure decreases, the spring rises. The
movements
are very small, but they are magnified by levers and are
communicated
to the pointer by means of a rack and pinion.
HOW
AIRMEN KNOW THEIR ALTITUDE
THE
ALTITUDE GAUGE
The
air zones of a modern battle are
illustrated in Fig. 131 and the
altitude guage by means of which the
airmen know their height is
shown in Fig. 132. This altitude
gauge is a recording aneroid barometer
called a barograph. It records the height
of the airplane in feet and is
suspended free of the airplane by
four elastic straps which protect it,
to some extent, from the vibration
of the machine.
The
construction of the barograph is as
follows. It has five or six flat
metal boxes, exhausted of air,
similar to the box in the ordinary
aneroid. These boxes are expanded by a
strong spring, as the height
increases, and this movement is communicated
to the long pointer. On the end of
the pointer there is a pen, with a supply of ink, which
bears against a sheet of paper on a drum revolved by
clockwork. The
pen makes a continuous record on the paper of the height in
feet.
The
Science Notebook
