SHIPS THAT SAIL THE SKIES
Shortly after the Civil War broke out, Thaddeus S. C. Lowe, an
enthusiastic American aëronaut, conceived the idea of sending up scout
balloons to reconnoiter the position of the enemy. These balloons
were to be connected by telegraph wires with the ground, so that they
could direct the artillery fire. The idea was so novel to the military
authorities of that day that it was not received with favor. Balloons
were looked upon as freak inventions, entirely impracticable for the
stern realities of war; and as for telegraphing from a balloon, no one
had ever done that before.
[Illustration: (C) Underwood & Underwood
A big German Zeppelin that was forced to come down on French soil]
But this enthusiast was not to be daunted, and he made a direct appeal
to President Lincoln, offering to prove the practicability of this
means of scouting. So he took his balloon to Washington and made an
ascent from the grounds of the Smithsonian Institution, while the
President came out on the lawn south of the White House to watch the
demonstration. In order to test him, Mr. Lincoln took off his hat,
waved his handkerchief, and made other signals. Lowe observed each
act through his field-glasses and reported it to the President by
telegraph. Mr. Lincoln was so impressed by the demonstration that he
ordered the army to use the observation balloon, and so with some
reluctance the gas-bag was introduced into military service, Professor
Lowe being made chief aëronautic engineer. Under Lowe's direction the
observation balloons played an important part in the operations of the
Union Army.
[Illustration: Courtesy of "Scientific American"
Observation Car lowered from a Zeppelin sailing above the clouds]
On one occasion a young German military attaché begged the privilege
of making an ascent in the balloon. Permission was given and when
the German officer returned to earth he was wildly enthusiastic in
praise of this aërial observation post. He had had a splendid view
of the enemy and could watch operations through his field-glasses
which were of utmost importance. Realizing the military value of the
aircraft, he returned to Germany and urged military authorities to
provide themselves with captive balloons. This young officer was Count
Ferdinand von Zeppelin, who was destined later to become the most
famous aëronautic authority in the world and who lived to see Germany
equipped with a fleet of balloons which were self-propelling and could
travel over land and sea to spread German frightfulness into England.
He also lived to see the virtual failure of this type of war-machine
in the recent great conflict, and it was possibly because of his deep
disappointment at having his huge expensive airships bested by cheap
little airplanes that Count von Zeppelin died in March, 1917. However,
he was spared the humiliation of seeing a fleet of Zeppelins lose their
way in a fog and fall into France, one of them being captured before
it could be destroyed, so that all its secrets of construction were
learned by the French.
THE WEIGHT OF HYDROGEN
Before we describe the Zeppelin airships and the means by which they
were eventually overcome, we must know something about the principles
of balloons. Every one knows that balloons are kept up in the air by
means of a very light gas, but somehow the general public fails to
understand why the gas should hold it up. Some people have a notion
that there is something mysterious about hydrogen gas which makes it
resist the pull of gravity, and that the more hydrogen you crowd into
the balloon the more weight it will lift. But hydrogen has weight and
feels the pull of gravity just as air does, or water, or lead. The only
reason the balloon rises is because it weighs less than the air it
displaces. It is hard to think of air as having weight, but if we weigh
air, hydrogen, coal-gas, or any other gas, in a vacuum, it will tip
the scales just as a solid would. A thousand cubic feet of air weighs
80 pounds. In other words, the air in a room ten feet square with a
ceiling ten feet high, weighs just about 80 pounds. The same amount of
coal-gas weighed in a vacuum would register only 40 pounds; while an
equal volume of hydrogen would weigh only 5-1/2 pounds. But when we
speak of volumes of gas we must remember that gas, unlike a liquid or
a solid, can be compressed or expanded to almost any dimensions. For
instance, we could easily fill our room with a ton of air if the walls
would stand the pressure; or we could pump out the air, until there
were but a few ounces of air left. But in one case the air would be so
highly compressed that it would exert a pressure of about 375 pounds
on every square inch of the wall of the room, while in the other case
its pressure would be almost infinitesimal. But 80 pounds of air in
a room of a thousand cubic feet would exert the same pressure as the
atmosphere, or 15 pounds on every square inch. And when we say that a
thousand cubic feet of hydrogen weighs only a little over 5 pounds, we
are talking about hydrogen at the same pressure as the atmosphere.
Since the hydrogen is sixteen times lighter than air, naturally it
will float in the air, just as a piece of wood will float in water
because it is lighter than the same volume of water. If we surrounded
the thousand cubic feet of hydrogen with a bag so that the gas will
not diffuse into the air and mix with it, we shall have a balloon
which would float in air provided the bag and the hydrogen it contains
do not weigh more than eighty pounds. As we rise from the surface of
the earth, the air becomes less and less dense, or, in other words,
it becomes lighter, and the balloon will keep on rising through the
atmosphere until it reaches a point at which its weight, gas-bag and
all, is exactly the same as that of an equal volume of air.
But there are many conditions that affect the height to which the
balloon will ascend. The higher we rise, the colder it is apt to
become, and cold has a tendency to compress the hydrogen, collapsing
the balloon and making it relatively heavier. When the sun beats upon a
balloon, it heats the hydrogen, expanding it and making it relatively
lighter, and if there is no room for this expansion to take place
in the bag, the bag will burst. For this reason, a big safety-valve
must be provided and the ordinary round balloon is open at the bottom
so that the hydrogen can escape when it expands too much and the
balloonist carries ballast in the form of sand which he can throw over
to lighten the balloon when the gas is contracted by a sudden draft of
cold air.
Although a round balloon carries no engine and no propeller, it can be
guided through the air to some degree. When an aëronaut wishes to go
in any particular direction, he sends up his balloon by throwing out
ballast or lowers it by letting out a certain amount of gas, until
he reaches a level at which he finds a breeze blowing in the desired
direction. Such was the airship of Civil War times, but for military
purposes it was not advisable to use free balloons, because of the
difficulty of controlling them. They were too liable to fall into the
hands of the enemy. All that was needed was a high observation post
from which the enemy could be watched, and from which observations
could be reported by telegraph. The balloon was not looked upon as a
fighting-machine.
ZEPPELIN'S FAILURES AND SUCCESSES
But Count Zeppelin was a man of vision. He dreamed of a real ship
of the air--a machine that would sail wherever the helmsman chose,
regardless of wind and weather. Many years elapsed before he actually
began to work out his dreams, and then he met with failure after
failure. He believed in big machines and the loss of one of his
airships meant the waste of a large sum of money, but he persisted,
even though he spent all his fortune, and had to go heavily in debt.
Every one thought him a crank until he built his third airship and
proved its worth by making a trip of 270 miles. At once the German
Government was interested and saw wonderful military possibilities in
the new craft. The Zeppelin was purchased by the government and money
was given the inventor to further his experiments.
That was not the end of his failures. Before the war broke out,
thirteen Zeppelins had been destroyed by one accident or another.
Evidently the building of Zeppelin airships was not a paying
undertaking, although they were used to carry passengers on short
aërial voyages. But the government made up money losses and Zeppelin
went on developing his airships.
Of course, he was not the only one to build airships, nor even the
first to build a dirigible. The French built some large dirigibles, but
they failed to see any great military advantage in ships that could
sail through the air, particularly after the airplane was invented,
and so it happened that when the war started the French were devoting
virtually all their energies to the construction of speedy, powerful
airplanes. As for the British, they did not pay much attention to
airships. The idea that their isles might be attacked from the sky
seemed an exceedingly remote possibility.
RIGID, SEMI-RIGID, AND FLEXIBLE BALLOONS
Count Zeppelin always held that the dirigible balloons must be rigid,
so that they could be driven through the air readily and would hold
their shape despite variations in the pressure of the hydrogen. The
French, on the other hand, used a semi-rigid airship; that is, one
in which a flexible balloon is attached to a rigid keel or body. The
British clung to the idea of an entirely flexible balloon and they
suspended their car from the gas-bag without any rigid framework to
hold the gas-bag in shape. In every case, the balloons were kept taut
or distended by means of air-bags or ballonets. These air-bags were
placed inside the gas-bags and as the hydrogen expanded it would force
the air out through valves, but the hydrogen itself would not escape.
When the hydrogen contracted, the air-bags were pumped full of air so
as to maintain the balloon in its fully distended condition. Additional
supplies of compressed hydrogen were kept in metal tanks.
[Illustration: (C) Underwood & Underwood
Giant British Dirigible built along the lines of a Zeppelin]
[Illustration: (C) Underwood & Underwood
One of the engine cars or "power eggs" of a British Dirigible]
In the Zeppelin balloon, however, the gas was contained in separate
bags which were placed in a framework of aluminum covered over with
fabric. Count Zeppelin did not believe in placing all his eggs in one
basket. If one of these balloons burst or was injured in any way,
there was enough buoyancy in the rest of the gas-bags to hold up the
airship. As the Zeppelins were enormous structures, the framework had
to be made strong and light, and it was built up of a latticework of
aluminum alloy. Aluminum itself was not strong enough for the purpose,
but a mixture of aluminum and zinc and later another alloy known
as duralumin, consisting of aluminum with three per cent of copper
and one per cent of nickel, provided a very rigid framework that
was exceedingly light. Duralumin is four or five times as strong as
aluminum and yet weighs but little more.
[Illustration: Photograph by International Film Service
Crew of the C-5 (American Coastal Dirigible) starting for Newfoundland
to make a Transatlantic Flight]
The body of the Zeppelin is not a perfect circle in section, but is
made up in the form of a polygon with sixteen sides, and the largest of
the Zeppelins used during the war contained sixteen compartments, in
each of which was placed a large hydrogen gas-bag. A super-Zeppelin,
as the latest type is called, was about seventy-five feet in diameter
and seven hundred and sixty feet long, or almost as long as three New
York street blocks. In its gas-bags it carried two million cubic feet
of hydrogen and although the whole machine with its fuel, stores, and
passengers weighed close to fifty tons, it was so much lighter than the
air it displaced that it had a reserve buoyancy of over ten tons.
KEEPING ENGINES CLEAR OF THE INFLAMMABLE HYDROGEN
As hydrogen is a very inflammable gas, it is extremely dangerous to
have an internal-combustion engine operating very near the gas-bags. In
the super-Zeppelins the engines were placed in four cars suspended from
the balloon. There was one of these cars forward, and one at the stern,
while near the center were two cars side by side. In the rear car there
were two engines, either of which could be used to drive the propeller.
By means of large steering rudders and horizontal rudders, the machine
could be forced to dive or rise or turn in either direction laterally.
The pilot of the Zeppelin had an elaborate operating-compartment
from which he could control the rudders, and he also had control
of the valves in the ballonets so that by the touch of a button he
could regulate the pressure of gas in any part of the dirigible.
There were nineteen men in the crew of the Zeppelin--two in the
operating-compartment, and two in each of the cars containing engines,
except for the one at the stern in which there were three men. The
other men were placed in what was known as the "cat walk" or passageway
running inside the framework under the gas-bags. These men were given
various tasks and were supposed to get as much sleep as they could, so
as to be ready to replace the other men at need.
The engine cars at each side of the balloon were known as power eggs
because of their general egg shape. At the center of the Zeppelin the
bombs were stored, and there were electro-magnetic releasing-devices
operated from the pilot's room by which the pilot could drop the bombs
whenever he chose. The Zeppelin also carried machine-guns to fight
off airplanes. Gasolene was stored in tanks which were placed in
various parts of the machine, any one of which could feed one or all
of the engines, and they were so arranged that they could be thrown
overboard when the gasolene was used up, so as to lighten the load of
the Zeppelin. Water ballast was used instead of sand, and alcohol was
mixed with the water to keep it from freezing. The machine which came
down in French territory and was captured before it could be destroyed
by the pilot, found itself unable to rise because in the intense cold
of the upper air the water ballast had frozen, and it could not be let
out to lighten the load of the Zeppelin.
[Illustration: Photograph from Kadel & Herbert
The Curious Tail of a Kite Balloon]
[Illustration: British Official Photograph from Kadel & Herbert
Observers in the Basket of an Observation-Balloon]
THE ZEPPELIN'S TINY ANTAGONISTS
The one thing above all others that the Zeppelin commander feared
was the attack of airplanes. In the early stages of the war, it
was considered unsafe for airplanes to fly by night because of the
difficulty of making a landing in the dark. Later this difficulty
was overcome by the use of search-lights at the landing-fields. The
airplane would signal its desire to land and the search-lights would
point out the proper landing-field for it. So that after the first few
months of the war Zeppelins were subjected to the danger of airplane
attack. Of course, on a dark night it was very difficult for an
airplane to locate a Zeppelin, because the huge machine could not be
seen and the throb of its engines was drowned out by the engines of the
airplane itself. Nevertheless, Zeppelins were occasionally located
and destroyed by airplanes.
[Illustration: Photograph by Kadel & Herbert
Enormous Range-finders mounted on a Gun Turret of an American Warship]
The danger of the Zeppelin lay in the fact that it was supported by
an enormous volume of very inflammable gas and the airplane needed
but to set fire to this gas to cause the destruction of the giant of
the air. And so the machine-guns carried by airplanes were provided
with explosive, flaming bullets. A burst of flame within the gas-bag
would not set the gas on fire, because there would be no air inside to
feed the fire, but surrounding the gas-bag there was always a certain
leakage of hydrogen which would mix with the air in the compartment and
this would produce an explosive mixture which needed but the touch of
fire to set it off. The Zeppelin was provided with a ventilating-system
to carry off these explosive gases, but they could never be disposed
of very effectively, and, as a consequence, a number of Zeppelins were
destroyed by the tiny antagonists that were sent up by the British and
the French. To fight off these assailants the Germans provided their
Zeppelins with guns which would fire shrapnel shell. It is difficult
for a Zeppelin to use machine-guns against an airplane because the
latter would merely climb above the Zeppelin and would be shielded by
the balloon itself. And so the Germans put a gun emplacement on top of
the balloon both forward and aft. There was a deck extending along the
top of the balloon which was reached by a ladder running up through
the center of the airship. But it was impossible to ward off the fleet
little antagonists, once the dirigible was discovered. True, a Zeppelin
could make as much as seventy miles per hour, but the fastest airplanes
could travel twice as fast as that.
SUSPENDING AN OBSERVER BELOW THE ZEPPELIN
One ingenious scheme that was tried was to suspend an observation car
under the Zeppelin. The car was about fourteen feet long and five feet
in diameter, fitted with a tail to keep it headed in the direction it
was towed. It had glass windows forward and there was plenty of room
in it for a man to lie at full length and make observations of things
below. The car with its observer could be lowered a few thousand feet
below the Zeppelin, so that the observer could watch proceedings below,
while the airship remained hidden among the clouds. The observer was
connected by telephone with the chart-room of the Zeppelin and could
report his discoveries or even act as a pilot to direct the course of
the ship.
But despite everything that could be done, the Zeppelin eventually
proved a failure as a war-vessel because it was so very costly to
construct and operate and could so easily be destroyed, and the Germans
began to build huge airplanes with which bombing-raids could be
continued.
Strange to say, however, although the Germans were ready to admit the
failure of their big airship, when the war stopped the Allies were
actually building machines patterned after the Zeppelin, but even
larger, and expected to use them for bombing-excursions over Germany.
This astonishing turn of the tables was due to the fact that America
had made a contribution to aëronautics that solved the one chief
drawback of the Zeppelin.
A BALLOON GAS THAT WILL NOT BURN
When we entered the war against Germany, our allies placed before
us all their problems and among them was this one of the highly
inflammable airship. Could we not furnish a substitute for hydrogen
that would not burn? It was suggested to us that helium would do if we
could produce that gas cheaply and in sufficient quantity. Now, helium
has a history of its own that is exceedingly interesting.
Every now and then the moon bobs its head into our light and we have a
solar eclipse. But our satellite is not big enough to cut off all the
light of the big luminary and the fiery atmosphere of the sun shows
us a brilliant halo all around the black disk of the moon. Long ago,
astronomers analyzed this flaming atmosphere with the spectroscope,
and by the different bands of light that appeared they were able to
determine what gases were present in the sun's atmosphere. But there
was one band of bright yellow which they could not identify. Evidently
this was produced by a gas unknown on earth, and they called it
"helium" or "sun" gas.
For a quarter of a century this sun gas remained a mystery; then one
day, in 1895, Sir William Ramsay discovered the same band of light
when studying the spectrum of the mineral cleveite. The fact that
astronomers had been able to single out an element on the sun ninety
million miles away before our chemists could find it right here on
earth, produced a mild sensation, but the general public attached no
special importance to the gas itself. It proved to be a very light
substance, next to hydrogen the lightest of gases, and for years it
resisted all attempts at liquefaction. Only when Onnes, the Dutch
scientist, succeeded in getting it down to a temperature of 450 degrees
below zero, Fahrenheit, did the gas yield to the chill and condense
into a liquid. The gas would not burn; it would not combine with any
other elements, and apparently it had no use on earth, and it might
have remained indefinitely a lazy member of the chemical fraternity had
not the great world conflict stirred us into frenzied activity in all
branches of science in our effort to beat the Hun.
Because the gas had no commercial value, there was only a small amount
of helium to be found in the whole world. Not a single laboratory in
the United States had more than five cubic feet of it and its price
ranged from $1,500 to $6,000 per cubic foot. At the lowest price it
would cost $3,000,000,000 to provide gas enough for one airship of
Zeppelin dimensions and it seemed absurd even to think of a helium
airship.
AMERICAN CHEMISTS TO THE RESCUE
Just before the war it was discovered that there is a considerable
amount of helium in the natural gas of Oklahoma, Texas, and Kansas,
and Sir William Ramsey suggested that our chemists might study some
method of getting helium from this source. The only way of separating
it out was to liquefy the gases by subjecting them to extreme cold. All
gases turn to liquid if they are cooled sufficiently, and then further
cold will freeze them solid. But helium can stand more cold than any
other and this fact gave the clue to its recovery from natural gas.
The latter was frozen and one after another the different elements
condensed into liquid, until finally only helium was left. This sounds
simple, but it is a difficult matter to get such low temperature
as that on a large scale and do it economically. To be of any real
service in aëronautics helium would have to be reduced in cost from
fifteen hundred dollars to less than ten cents per cubic foot. Several
different kinds of refrigerating-machinery were tried and finally just
before the war was brought to a close by the armistice we had succeeded
in producing helium at the rate of eight cents per cubic foot, with
the prospect of reducing its cost still further. A large plant for
recovering helium was being built. The plant will have been completed
before this book is published, and it will be turning out helium for
peaceful instead of military airships.
The reduction in the cost of helium is really one of the most
important developments of this war. By removing the fire risk from
airships we can safely use these craft for aërial cruises or for quick
long-distance travel over land and sea. For, even in time of peace,
sailing under millions of cubic feet of hydrogen is a serious matter.
Although no incendiary bullets are to be feared, there is always the
danger of setting fire to the gas within the exhaust of the engines.
Engines have had to be hung in cars well below the balloon proper.
But with helium in the gas-bags the engines can be placed inside the
balloon envelop and the propellers can operate on the center line of
the car.
In the case of one Zeppelin, the hydrogen was set on fire by an
electric spark produced by friction on the fabric of one of the
gas-bags, and so even with the engine exhausts properly screened there
is danger. The helium airship, however, would be perfectly safe from
fire and passengers could smoke on deck or in their cabins within the
balloon itself without any more fear of fire than they would have on
shipboard. Wonderful possibilities have been opened by the production
of helium on a large and economical scale, and the airship seems
destined to play an important part in transportation very soon. As this
book is going to press, we learn of enormous dirigibles about to be
built in England for passenger service, which will have half again as
great a lifting-power as the largest Zeppelins. The final chapter of
the story of dirigibles is yet to be written, but in concluding this
chapter it is interesting to note that the world's greatest aëronautic
expert got his first inspiration from America and finally that America
has now furnished the one element which was lacking to make the
dirigible balloon a real success.