AIR _VS._ WATER
THE STEAM engine had its origin in a kitchen; the modern clock was
first conceived in a cathedral; our laws of motion were discovered
under an apple tree, but, strangest of all, pneumatic engineering had
its beginning in a _barber shop_. In each case it was an inquisitive
boy who played the leading rôle.
We do not hear much about the youthful father of pneumatic engineering,
possibly because he lived so very long ago, but his story is fully
as interesting as that of Watt or Galileo or Newton. Young Ctesibius
dwelt in the city of Alexandria, Egypt, 250 years before the birth of
Christ. His father kept a barber shop, and the young lad used to watch
his father practice his tonsorial art on the Greek and Egyptian dandies
of the time. No doubt Ctesibius’s father expected to make a first-rate
barber of his son, but history does not tell us much about his early
life.
The barbers of those days had their mirrors as do the barbers of the
present time. But a mirror in those days was a treasured possession.
It consisted of a brightly polished plate of metal in which a person
could see himself “darkly” or it might have been a plate of glass with
a black backing. These mirrors had to be carefully preserved from
injury and from moisture, and so, instead of having them mounted on the
wall as in a modern barber shop, they were stowed carefully away and
brought out only after the tonsorial artist had completed his operation
and was ready to exhibit his finished product to the customer.
THE BOY WHO DISCOVERED COMPRESSED AIR
Now in the tonsorial parlors of Ctesibius’s father the precious mirror
was suspended above the head of the customer and when the barber had
finished the shave or haircut he pulled down the mirror and let the
customer survey his remodeled countenance. After the customer viewed
himself to his heart’s content he merely released the mirror and it
was automatically drawn back into place by a counterweight that slid
in a case fastened to the wall. Young Ctesibius noticed that every
time the mirror was released a curious whistling sound came from
the case in which the counterweight moved. No doubt this phenomenon
had been recurring day after day for years, but it excited no more
curiosity than would a squeaky door, until, one day, the barber’s son
happened to notice it. Immediately he was all curiosity. No one knew
anything about air in those days and the boy could get no satisfactory
explanation of the phenomenon, so he started an investigation of his
own. He found that the counterweight fitted very closely in its case
and when it dropped it forced a stream of “wind” out of a crevice which
produced a whistling sound. That set Master Ctesibius to thinking. He
had discovered a method of harnessing the wind. No more barber shop for
him; he was launched upon a career of scientific research and invention.
The first use he made of his discovery was to build a pipe organ which
was driven by a water wheel and hence was known as a water organ. But
there were other and more lucrative fields for the newly discovered
power. Being invisible, compressed air proved a most valuable medium
for performing seemingly miraculous tricks with which the corrupt
priesthood of that time hoodwinked the public. Naturally such
air-controlled apparatus was kept secret, and we have only a meager
record of a few of the ingenious devices used in the temples. However,
Ctesibius made many scientific discoveries and became the most famous
mathematician and scientist of his age. We have read in Chapter IV
of his famous water clock adjusted to record hours of varying length
according to the season.
While Ctesibius is credited with being the first man to make scientific
use of compressed air, the use of free air or wind as a power far
antedates him. Sailing vessels were probably in use long before
windmills were invented. As pointed out elsewhere, windmills possessed
an advantage over water wheels in that they could be placed anywhere,
instead of being confined to the banks of streams. They were widely
used for pumping water, grinding grain, and sawing wood until the steam
engine arrived and displaced them.
Although wind power is but little used to-day, air is much more widely
used in modern machinery than is generally realized. A catalogue of the
various devices in which it plays a prominent part is likely to prove
astonishing. For instance, we employ it to bore through rock, to stop
the speeding express train, and even to quell ocean billows. Being
elastic and compressible, it makes an ideal spring for the storage of
power and a cushion for absorbing shocks. It is also used to fight
back water and permit men to work in the depths of the sea or in
water-bearing sand and earth.
Despite his extensive use of air Ctesibius knew nothing of the
atmospheric pressure. It was not until the seventeenth century that
atmospheric pressure was discovered and measured. It was then learned
that there is no such thing as suction.
RAISING WATER WITH ATMOSPHERIC PRESSURE
[Illustration: FIG. 40.--SECTIONAL VIEW OF A SUCTION PUMP]
A suction pump should be called an atmospheric pump. The operation
of such a pump is illustrated in Figure 40. A piston A slides in a
cylinder B. There is an opening in the bottom of the cylinder which
is normally closed by a clack valve C that opens upwardly. There is
a similar valve D in the piston A. When the piston is pushed down it
compresses the air in the cylinder, closing the valve C while valve
D opens, permitting the air to escape. Then on the upward stroke the
valve D closes and valve C opens permitting air to flow into the
cylinder. In this way the air in the pipe and cylinder are exhausted
after a few strokes. As the air is rarefied and the pressure is reduced
above the column of water in the pipe the greater pressure of the water
outside the pipe forces the column to rise until it eventually flows
through the valve in the piston and out of the spout of the pump.
The maximum height to which water can be raised by a suction pump
is thirty-four feet but in actual practice the lift seldom exceeds
twenty-six to twenty-eight feet because the fit between the cylinder
and piston is not perfect.
[Illustration: FIG. 41.--CONVENTIONALIZED SECTIONAL VIEW OF A FORCE
PUMP]
When water is raised to a greater height a force pump is used. Such a
pump is shown in Figure 41. The pump cylinder A has two valves B and C.
The valve B opens upwardly and valve C opens downwardly. The piston D
is a solid member containing no valve. When the piston rises the valve
C closes and valve B opens, letting water into the cylinder. On the
reverse stroke the valve B closes and the descending piston forces the
water out through valve C into pipe E. The height to which the water
may be lifted depends upon the power applied to the piston and not upon
atmospheric pressure. In a fire pump two force pumps are used which
alternately deliver water into an air chamber. The air is compressed
by the water that enters the chamber and exerts a steady pressure on
the water, forcing it out through the fire hose in a stream that is
practically steady.
It is a decided disadvantage to install a force pump in a deep well
because of the mechanical difficulty of operating the piston at a great
depth. Often the suction and force pump principles are combined. The
force pump piston and cylinder are located twenty-six and twenty-eight
feet above the level of the water in the well with a pipe leading down
into the water. The water is forced up into the cylinder by exhausting
the air in the cylinder and pipe and then is forced out of the cylinder
and up a pipe to the top of the well. This also has its disadvantages.
In deep wells the pump piston must still be located and operated at
considerable depth.
THE AIR LIFT
A much more simple pump for deep wells is the air lift in which there
are no valves and no piston. The pump consists of two pipes that are
let down into the water. (See Figure 42.) The larger pipe is open at
the top and bottom, and the smaller pipe discharges air into the bottom
of the larger one. This air being directed upward has a lifting effect,
and mingling with the water in the pipe produces a sort of froth which
is so much lighter than solid water that the atmospheric pressure on
the water in the well is sufficient to raise the froth. The water is
thoroughly aerated and to a large extent purified by this system of
pumping, so that the air lift commends itself particularly for city
waterworks. Sometimes a series of air lifts are used which lift water
by successive stages to a greater height than would be possible with a
single direct lift.
[Illustration: FIG. 42.--PUMPING WATER WITH COMPRESSED AIR]
EXCAVATING THROUGH QUICKSAND
[Illustration: FIG. 43.--SECTIONAL VIEW OF A PNEUMATIC CAISSON]
Centuries ago it was realized that it would be possible for men to
descend into the open sea if they were protected by a bell-shaped
chamber, for the air trapped in the chamber would furnish them with
oxygen requisite for breathing purposes, and would prevent the water
from drowning them out. This same idea of a diving bell is used on
land when sinking a shaft through quicksand or water-bearing strata. A
large box or caisson is used. (See Figure 43.) This box may be either
cylindrical or rectangular, and it is open at the top and bottom. The
lower edges of the box are shod with steel and form cutting edges that
will sink into the soil that is being excavated. At a height of about
seven feet above the bottom of the box there is a transverse diaphragm
known as the deck, and the space below is known as the working
chamber. This deck is very strongly constructed, as it has to support
the weight of the concrete shaft that is built above it. Laborers,
commonly known as “sand hogs,” enter the working chamber and dig out a
shallow pit in the floor of clay or sand. This pit is then extended to
the cutting edges of the caisson. The caisson thus undermined settles
down into the excavation and another pit is started. In this way, step
by step, the caisson is sunk into the ground. In order to overcome the
friction of the caisson against the sides of the excavation and to
insure its sinking, it is heavily weighted. When water-bearing sand is
reached, compressed air is admitted into the working chamber to force
the water out. When the air pressure is greater than the water pressure
it drives the water out of the sand in the working chamber, so that the
men can work in perfectly dry ground, even though the surrounding sand
may be so saturated with water as to form a quicksand.
Of course the farther the excavation proceeds below the water level the
greater the air pressure required, and the caisson would be blown up
out of the ground by this air pressure or would float on the water were
it not for the weights with which it is loaded. When an open shaft is
to be dug, pig iron is loaded on the caisson to force it down, but in
most work the object is merely to sink a concrete column down to rock
and so the caisson is filled with concrete above the deck. Sections
are added to the caisson as it sinks into the ground, and these
sections are filled with concrete. This method of building the column
facilitates the work of laying the concrete, and at the same time
provides the weight necessary to overcome the buoyancy of the caisson
and the skin friction on the side walls of the excavation.
After the caisson has been carried down to rock and a good seat has
been blasted out of solid rock, the working chamber is completely
filled with concrete and the concrete shaft is thus anchored to the
rock.
THE AIR LOCK
Access to the working chamber is had through a central shafting. In
order to hold the air pressure in the working chamber and yet provide
for the entrance of men and materials, an air lock is fitted to the
upper end of the shafting. This air lock, as shown in Figure 43,
consists of a chamber formed with a trapdoor at the top and the bottom.
Both doors open downwardly. To enter the caisson, the bottom door of
the chamber must first be closed and means are provided for doing this
from outside the air lock. The compressed air in the chamber is then
let out through a valve, and when the pressure drops almost to normal
the upper trapdoor falls open of its own weight, giving access to the
chamber. After entering, the upper door is closed while compressed air
is admitted into the chamber; the air pressure then serves to hold the
upper trapdoor closed. The bottom door, in the meantime, has been kept
closed by the air pressure below, which is greater than that above the
door. But after sufficient air has been introduced into the chamber to
equalize the pressure on both sides of the lower trapdoor, it falls
open of its own weight. The occupants of the chamber can then proceed
down a ladder to the working chamber. When leaving the caisson, the
action of the air lock is reversed. The bottom door is pushed up and
held closed for a moment while air is released from the chamber, when
the greater pressure beneath will hold the door closed, and after the
pressure within the air lock has been lowered practically to normal,
the upper door drops of its own weight, permitting the occupants of the
chamber to climb out. The same process must be undergone by buckets
loaded with sand or earth from the excavation and by empty buckets
returning to the working chamber.
More time is required for a man to pass through an air lock than for a
load of sand or any inanimate load. In some of the larger caissons a
separate small air lock is provided just for the use of the workmen.
In the air lock the pressure must be built up slowly so that it will
permeate a man’s whole system. When we realize that the pressure
that men have to support in caisson work may amount to from fifty
to one hundred tons on the whole body, it is difficult at first to
understand why the body is not flattened out like a pancake. It is only
by permitting the system to absorb the pressure so that there is as
much internal pressure as that outside that a man is able to enter a
compressed air chamber without harmful results.
THE CAISSON DISEASE
Compression is rather annoying to a man who has not experienced
the sensation, but the principal danger comes in decompression,
particularly after a person has been in the working chamber for a
long time. When breathing compressed air much larger volumes of
oxygen are taken into the lungs at each breath than in the ordinary
atmosphere, and one feels decidedly exhilarated by this unusual supply
of stimulating oxygen. But with the oxygen large volumes of nitrogen
are taken into the system as well. The nitrogen permeates the blood
while the oxygen is consumed and exhaled in the form of carbon dioxide.
The longer a man is exposed to the pressure the more nitrogen does he
absorb. When the pressure is released suddenly the nitrogen begins
to froth, just as a bottle of soda water does when the stopper is
removed. The nitrogen bubbles in the small veins stop the circulation
and produce the dreaded “caisson disease” which makes itself felt in
the form of severe cramps and excruciating pains. Not infrequently this
disease ends in death. By reducing the pressure gradually the nitrogen
is enabled to pass off completely without bubbling or frothing, just
as it is possible to let out the gas from a soda bottle without
frothing by permitting it to escape through a pin hole in the stopper.
The surest cure for a victim of the caisson disease is to put him
immediately into what is known as a “hospital tank” and build up the
pressure in the tank equal to that which he has just been subjected to
in the caisson, after which the air in the tank is let out so slowly
that there is plenty of time for the nitrogen to pass off without
forming bubbles.
A pressure of forty-five pounds per square inch above that of the
atmosphere is considered a severe pressure for excavation work.
But work has been carried on in pressures up to fifty-two pounds,
corresponding to a depth of 120 feet below water level.
Now that the cause of the caisson disease is understood formulas have
been worked out to insure proper decompression. On one occasion a diver
descended 306 feet into the ocean where the water pressure was 133
pounds on every square inch of his body. This meant that the air which
was pumped down to him had to be compressed to the same pressure. The
diver actually remained at the bottom only a very short time, but it
took two hours and three-quarters to bring him to the surface. He came
up half the distance very quickly and then had to rest on the bottom
rung of a Jacob’s ladder. The rungs on this ladder were ten feet apart
and he was instructed to rest on each rung a certain specified time.
When he was ten feet below the surface he had to wait three-quarters of
an hour for the last trace of nitrogen absorbed by his blood to pass
off.
BORING TUNNELS THROUGH RIVER BEDS
It is comparatively simple to sink a vertical shaft into water-bearing
soil, but a horizontal shaft involves serious difficulties. The action
of a diving bell is easily illustrated by inverting a tumbler and
pressing it down into a basin of water. The air trapped in the tumbler
will keep the upper part of the glass dry, and by inserting a tube in
the tumbler it is possible to fill the tumbler so full of compressed
air as to drive out all the water. This is virtually what is done in
the caisson; but when excavating a horizontal bore, the caisson must be
turned on its side. Turn the tumbler on its side and it is impossible
to keep the water out of it, no matter how much air we may blow into
it. The reason for this is that the pressure on the open end of the
tumbler is not uniform. At the bottom, where the water is deeper, it
will be greater than at the top. If air is pumped in to equalize the
water pressure at the upper edge of the glass, it will not prevent
water from flowing in at the bottom; and if it be equal to that at the
bottom, the water pressure at the top cannot hold the air in and keep
it from pouring out.
[Illustration: FIG. 44.--SECTIONAL VIEW OF A TUNNEL SHIELD]
Fortunately most of the soil through which a subaqueous tunnel is
driven is not very fluid. It is either sticky, as in clay, or sluggish
enough to prevent the water from flowing in rapidly. If there is enough
cover of silt or earth above the tunnel bore, it will help to hold
the air in the tunnel. When the bore comes very close to the surface
of the bed of the stream that is being tunneled, loads of clay are
dumped along the line of the tunnel to provide the requisite cover. In
tunnel boring a shield is used which is the equivalent of the caisson
in vertical boring. The shield is a cylindrical box with a diaphragm
across it corresponding to the deck of the caisson. (See Figure 44.)
In front of the diaphragm there is a small working chamber which is
protected above by an extension of the shield known as an apron. In
the diaphragm there are a number of doors at different levels, which
may be closed in case of danger. If work is proceeding near the top of
the shield, the upper doors are opened and the pressure is regulated
to equal the water pressure at that level. If the work is carried on
near the bottom of the shield, the upper doors are closed and only the
lower doors are open, and the pressure is increased to equal the water
pressure at that point. Sometimes the material is of such a nature
that the men can safely pass out of the doors into the working chamber
outside, but more often it is possible to work only within a limited
area immediately in front of the doors.
When the material is very soft it is often unnecessary to do any actual
excavation by hand in front of the diaphragm. The shield is merely
pushed forward through the mud or silt and the doors are opened to let
the material flow in through them. Workmen dig out this mud and it
is hauled out of the tunnel. Whenever bowlders are encountered it is
necessary for the men to work outside of the diaphragm to chip away the
rocks with compressed-air drills, or else bore them and blast them with
small charges of dynamite.
In quicksand the material is so fluid that it is unsafe to open the
ordinary doors of the caisson, and they are then provided with shutters
which are raised one at a time, to permit of operating on a very small
section of the head of the tunnel. After enough material has been
excavated from in front of the shield, the latter is pushed forward and
the excavating is renewed.
Unlike caisson work, the weight of the shield is of no assistance in
making it penetrate the soil, nor is it possible to move the entire
lining of the tunnel with the shield. The tunnel is lined with rings
of cast iron which are bolted together, the rings themselves being
made up of heavily ribbed curved plates. The shield is formed with a
“tail” which fits over the end of the tunnel line like a cap. When the
shield is to be pushed forward, a set of hydraulic jacks are fitted
between the end of the tunnel lining and the diaphragm of the shield,
and by means of these the shield is given a shove forward far enough
for a new section of the cast-iron lining to be added within the tail.
The tunnel-lining rings are usually sixteen inches wide so that it is
customary to move the shield ahead sixteen inches at a time or just far
enough for a new ring of lining to be installed. Of course, the tunnel
is fitted with air locks by which men can enter the working section
without permitting the compressed air therein to escape, and these
locks are just like those used in sinking a caisson, except that they
are horizontal instead of vertical.
QUELLING OCEAN BILLOWS WITH AIR
Air is also used in another and very novel way to battle against water.
In this case it is not quiet water pressure, but the tremendous power
of ocean storms that is combatted.
The influence of a shoal upon ocean waves has often been observed.
A sand barrier even when submerged to a depth of twenty or thirty
feet will break the waves of a heavy storm and leave an area of
comparatively quiet water behind it. The reason for this is that the
water in the waves does not travel with the waves, but undergoes
local oscillatory motion. This motion is in the form of circular or
elliptical currents which travel in a vertical plane. When a sand bar
is encountered it interferes with these local currents and breaks up
the waves.
Knowing this to be the case, it occurred to Mr. Philip Brasher that
some other means of disturbing the rhythmic movement of the water
might be found, and he determined to try the experiment of using
compressed air for this purpose. Accordingly he laid a perforated pipe
under water and connected it with an air compressor. Then when a storm
arose air was pumped into the pipe and it rose in bubbles through
the water. The effect of this upward movement of air was instantly
observable. It upset the rhythmic water currents and the waves which
struck this wall of air bubbles curled and broke as if they had
encountered a shoal of sand.
Several exposed piers on the Pacific coast have had a pneumatic
breakwater built around them so as to protect them or ships lying
alongside from being pounded by the waves. There is no expense attached
to the breakwater except in time of storm when the air pumps must be
kept going. One important advantage of the breakwater is that it does
not block navigation. A ship can sail right over the wall of bubbles
and find refuge behind it.