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

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.

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.

EXCAVATING THROUGH QUICKSAND

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.

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.

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