"DEVIL'S EGGS"
In modern warfare a duel between fixed forts and floating forts is
almost certain to end in a draw. Because the former are fixed they make
good targets, while the war-ship, being able to move about, can dodge
the shell that are fired against it. On the other hand, a fort on land
can stand a great deal of pounding and each of its guns must be put out
of action individually, before it is subdued, while the fort that is
afloat runs the risk of being sunk with a few well-directed shots.
But fortifications alone will not protect a harbor from a determined
enemy. They cannot prevent hostile ships from creeping by them under
cover of darkness or a heavy fog. To prevent this, the harbor must be
mined, and this must be done in such a way that friendly shipping can
be piloted through the mine-field, while hostile craft will be sure to
strike the mines and be destroyed.
The mines may be arranged to be fired by electricity from shore
stations, in which case they are anchored at such a depth that ships
can sail over them without touching them. If a hostile vessel tried to
dash into the harbor, the touch of a button on shore would sink it when
it passed over one of the mines. But the success of electrically fired
mines would depend upon the "seeing." In a heavy fog they would prove
no protection.
Another way of using electric mines is to have telltale devices which
a ship would strike and which would indicate to the operator on shore
that a vessel was riding over the mines and would also let him know
over which particular mines it was at the moment passing. No friendly
vessel would undertake to enter the harbor in a fog or after dark and
the operator would not hesitate to blow up the invader even if he could
not see him.
However, the ordinary method of mining a harbor is to lay fields
of anchored mines across the channels and entrances to the
harbor--sensitive mines that will blow up at the slightest touch of
a ship's hull--and leave tortuous passages through the fields for
friendly shipping. Of course pilots have to guide the ships through the
passages and lest enemy spies learn just where the openings are the
mine-fields must be shifted now and then.
The mines are, therefore, made so that they can be taken up by friendly
mine-sweepers who know just how to handle them, and planted elsewhere.
These are defensive mines, but there are other mines that are not
intended to be moved. They are planted in front of enemy harbors to
block enemy shipping and they are made so sensitive or of such design
that they will surely explode if tampered with.
THE MINE THAT DOES ITS OWN SOUNDING
A favorite type of mine used during the war was one which automatically
adjusted itself to sink to the desired depth. Submerged mines are more
dangerous to the enemy because they cannot be seen and avoided. They
should float far enough under the surface to remain hidden and yet not
so deep that a shallow-draft ship can pass over them without hitting
them. As the sea bottom may be very irregular, it is impossible to
tell how long the anchor cable should be without sounding the depth of
the water at every point at which a mine is planted. But the automatic
anchor takes care of this. Very ingeniously it does its own sounding
and holds the mine down to the depth for which it is set. The mine
cable is wound up on a reel in the anchor and the mine is held fast to
the anchor by a latch. The anchor is of box-shape or cylindrical form,
with perforations in it. At first it sinks comparatively slowly, but as
it fills with water it goes down faster. Attached to the anchor is a
plummet or weight, connected by a cord to the latch. The length of this
cord determines the depth at which the mine will float.
[Illustration: Courtesy of the "Scientific American"
FIG. 22. How the mine automatically adjusts itself to various depths of
water]
The operation of the mine is shown in Fig. 22. When it is thrown
overboard (1) it immediately turns over so that the buoyant mine _A_
floats on the surface (2). While the anchor is slowly filling and
sinking, the plummet _B_ runs out (3). If the mines are to float at a
depth of, say, ten feet, this cord must be ten feet long. As soon as it
runs out to its full length (4) it springs a latch, _C_, releasing the
mine _A_. Then the mine cable _D_ pays out, as the anchor _E_ sinks,
until the plummet _B_ strikes bottom (5). As soon as the plummet cord
slackens a spring-pressed pawl is released and locks the mine-cable
reel, so that as the anchor continues to sink it draws the mine down
with it, until it touches bottom (6), and as the anchor was ten feet
from the bottom when the plummet touched bottom and locked the reel,
the mine must necessarily be dragged down to a depth of ten feet below
the surface.
The mine itself, or the "devil's egg" as it is called, is usually a
big buoyant sphere of metal filled with TNT or some other powerful
explosive; and projecting from it are a number of very fragile prongs
which if broken or even cracked will set off the mine. There is a
safety-lever or pin that makes the mine harmless when it is being
handled, and this must be withdrawn just before the mine is to be
launched. In some mines the prongs are little plungers that are
withdrawn into the mine-shell and held by a cement which softens after
the mine is submerged and lets the plungers spring out. When the
plungers are broken, water enters and, coming in contact with certain
chemicals, produces enough heat to set off a cartridge which fires the
mine.
PICKING INFERNAL MACHINES OUT OF THE SEA
The enemy mine-fields were often located by seaplanes and then
mine-sweepers had to undertake the extremely hazardous task of raising
the mines or destroying them. If they were of the offensive type, it
was much better to destroy them. But occasionally, when conditions
permitted, mine-sweepers undertook to raise the mines and reclaim them
for future use against the enemy. The work of seizing a mine and making
it fast to the hoisting-cable of the mine-sweeper was usually done from
a small rowboat. Raising the first mine was always the most perilous
undertaking, because no one knew just what type of mine it was and how
to handle it with safety, or whether there was any way in which it
_could_ be made harmless. There were some mines, for instance, that
contained within them a small vial partly filled with sulphuric acid.
The mine carried no prongs, but if it were tilted more than twenty
degrees the acid would spill out and blow up the mine. Such a mine
would be exceedingly difficult if not impossible to handle from a boat
that was rocked about by the waves.
After the first mine of the field was raised and its safety-mechanism
studied, the task of raising the rest was not so dangerous. A water
telescope was used to locate the mine and to aid in hooking the
hoisting-cable into the shackle on the mine. The hook was screwed
to the end of a pole and after the mine was hooked, the pole was
unscrewed and the cable hauled in, bringing up the "devil's egg"
bristling with death. Care had to be taken to keep the bobbing boat
from touching the delicate prongs until the safety-device could be set.
However, this painstaking and careful method of raising mines was
not often employed. Shallow-draft mine-sweepers would run over the
mine-field, dragging a cable between them. The cable would be kept down
by means of hydrovanes or "water kites" deep enough to foul the anchor
cables of the mines. The "water kites" were V-shaped structures that
were connected to the cable in such a way that they would nose down
as they were dragged through the water and carry the cable under. The
action is just the reverse of a kite, which is set to nose up into the
wind and carry the kite up when it is dragged through the air. By means
of the cable the anchor chain of the mine was caught and then the mine
with its anchor was dragged up. If the mine broke loose from its anchor
it could be exploded with a rifle-shot if it did not automatically
explode on fouling the cable.
FLOATING MINES
When England entered the war she mined her harbors because, although
she had the mastery of the sea, she had to guard against raids of enemy
ships carried out in foggy and dark weather. But the mines were no
protection against submarines. They would creep along the bottom under
the mines. Then cable nets were stretched across the harbor channels
to bar the submarines, but the U-boats were fitted with cutters which
would tear through the nets, and it became necessary to use mines set
at lower depths so that the submarines could not pass under them; and
nets were furnished with bombs which would explode when fouled by
submarines. In fact, mines were set adrift with nets stretched between
them, to trap submarines. Floating mines were also used by the Germans
for the destruction of surface vessels and these were usually set
adrift in pairs, with a long cable connecting them, so that if a vessel
ran into the cable the mines would be dragged in against its hull and
blow it up.
The laws of war require that floating mines be of such a design that
they will become inoperative in a few hours; otherwise they might
drift about for weeks or months or years and be a constant menace to
shipping. Sometimes anchored mines break away from their moorings and
are carried around by ocean currents or are blown about by the winds.
A year after the Russo-Japanese War a ship was blown up by striking a
mine that had been torn from its anchorage and had drifted far from the
field in which it was planted. No doubt there are hundreds of mines
afloat in the Atlantic Ocean which for many years to come will hold out
the threat of sudden destruction to ocean vessels; for the Germans knew
no laws of war and had no scruples against setting adrift mines that
would remain alive until they were eaten up with rust.
[Illustration: Courtesy of the "Scientific American"
FIG. 23. Ocean currents of the North Atlantic showing the probable path
of drifting mines]
The chart on the next page shows the course of ocean currents in the
North Atlantic as plotted out by the Prince of Monaco, from which it
may be seen that German mines will probably make a complete circuit of
the North Atlantic, drifting down the western coast of Europe, across
the Atlantic, around the Azores, and into the Gulf Stream, which will
carry them back to the North Sea, only to start all over. (See Fig.
23.) Some of them will run up into the Arctic Ocean, where they will be
blown up by striking icebergs and many will be trapped in the mass of
floating seaweed in the Sargasso Sea. But many years will pass before
all danger of mines will be removed. In the meantime, the war has left
a tremendous amount of work to be done in raising anchored mines and
destroying them.
EGG-LAYING SUBMARINES
Early in the war the British were astonished to find enemy mine-fields
in their own waters, far from any German ports. They could not have
been planted by surface mine-layers, unless these had managed to
creep up disguised as peaceful trawlers. This seemed hardly likely,
because these fields appeared in places that were well guarded. Then
it was discovered that German U-boats were doing this work. Special
mine-laying U-boats had been built and one of them was captured with
its cargo of "devil's eggs."
A sectional view of the mine-laying U-boat is shown opposite page 272.
In the after part of the boat were mine-chutes in each of which three
mines were stored. A mine-laying submarine would carry about a score of
mines. These could be released one at a time. The mine with its anchor
would drop to the bottom. As soon as it struck, anchor-arms would be
tripped and spread out to catch in the sand or mud, while the mine
cable would be released and the mine would rise as far as the cable
would allow it. The U-boat commander would have to know the depth
of water in which the mines were to be laid and adjust the cables to
this depth in advance. This could not be done while the U-boat was
submerged. With the mines all set for the depth at a certain spot,
the U-boat commander had to find that very spot to lay his "eggs,"
otherwise they would either lie too deep to do any harm to shipping, or
else they would reach up to the surface, where they might be discovered
by the Allied patrols. As he had to do his navigating blindly, by
dead-reckoning, it was very difficult for him to locate his mine-fields
properly.
But the Germans did not have a monopoly on submarine mine-laying.
The British also laid mines by submarine within German harbors and
channels, right under the guns of Heligoland, and many a U-boat was
destroyed by such mines within its home waters.
[Illustration: (C) Press Illustrating Service
A Dutch Mine-sweeper engaged in clearing the North Sea of German Mines]
PARAVANES
On the other hand, the Allies had a way of sailing right through
fields of enemy mines with little danger. Our ships were equipped
with "paravanes" which are something like the "water kites" used by
mine-sweepers, and they are still used in the waters of the war
zone. Paravanes are steel floats with torpedo-shaped bodies and a
horizontal plane near the forward end. At the tail of the paravane,
there are horizontal and vertical rudders which can be set to make
the device run out from the side of the vessel that is towing it, and
at the desired depth below the surface. Two paravanes are used, one
at each side of the ship, and the towing-cables lead from the bow
of the vessel. Thus there are two taut cables that run out from the
ship in the form of a V and at such a depth that they will foul the
mooring-cable of any mine that might be encountered. The mine cable
slides along the paravane cable and in this way is carried clear
of the ship's hull. When it reaches the paravane it is caught in a
sharp-toothed jaw which cuts the mine cable and lets the mine bob up to
the surface. The mine is then exploded by rifle or machine-gun fire.
[Illustration: Courtesy of "Scientific American"
Hooking Up Enemy Anchored Mines]
In some forms of paravane there is a hinged jaw which is operated from
the ship to shear the cable. The jaw is repeatedly opened and closed by
a line that runs to a winch on the ship. This winch winds up the line
until it is taut and then the line is permitted to slip, letting the
jaw open, only to close again as the winch keeps on turning and winding
up the line.
Guarded by steel sharks on each side, their jaws constantly working,
a ship can plow right through a field of anchored mines with little
danger. To be sure, the bow might chance to hit a mine, when, of
course, there would be an explosion; but the ship could stand damage
here better than anywhere else and unless the bow actually hit the
mine, one or other of the paravanes would take care of it and keep it
from being dragged in against the hull of the vessel.
PENNING IN THE U-BOATS
According to German testimony, mines were responsible for the failure
of the U-boat. However, it was not merely the scattered mine-fields
sown in German waters that brought the U-boat to terms, but an enormous
mine-field stretching across the North Sea from the Orkney Islands
to the coast of Norway. Early in the war, U-boats had been prevented
from entering the English Channel by nets and mines stretched across
the Straits of Dover. As the submarine menace grew, it was urged that
a similar net be stretched across the North Sea to pen the U-boats
in. But it seemed like a stupendous task. The distance across at the
narrowest point is nearly two hundred and fifty miles. It would not
have been necessary to have the net come to the surface. It could just
as well have been anchored so that its upper edge would be covered with
thirty feet of water. Surface vessels could then have sailed over it
without trouble and submarines could not have passed over it without
showing themselves to patrolling destroyers. It would not have been
necessary to carry the net to the bottom of the sea. A belt of netting
a hundred and fifty feet wide would have made an effective bar to the
passage of U-boats. As U-boats might cut their way through the net, it
was proposed to mount bombs or mines on them which would explode on
contact and destroy any submarine that tried to pass. However, laying a
net two hundred feet long even when it is laid in sections, is no small
job, but when the net is loaded with contact mines, the difficulty of
the work may be well imagined.
And yet had it been thought that the net would be a success it would
have been laid anyhow, but it was argued that seaweed would clog the
meshes of the net and ocean currents would tear gaps in it. Even if it
had not been torn away, the tidal currents would have swept it down and
borne it under so far that U-boats could have passed over it in safety
without coming to the surface.
A WALL OF MINES
When America entered the war, we were very insistent that something
must be done to block the North Sea, and we proposed that a barrage of
anchored mines be stretched across the sea and that these mines be set
at different levels so as to make a "wall" that submarines could not
dive under. This would do away with all the drawbacks of a net. Ocean
currents and masses of seaweed could not affect individual mines as
they would a net. Furthermore, an American inventor had devised a new
type of mine which was peculiarly adapted to the proposed mine barrage.
It had a firing-mechanism that was very sensitive and the mine had
twice the reach of any other.
At length the British mine-laying forces were prevailed upon to join
with us in laying this enormous mine. It was one of the biggest and
most successful undertakings of the war. It was to be two hundred
and thirty miles long and twelve miles wide on the average, reaching
from the rocky shores of the Orkney Islands to Norway. There was
plenty of deep water close to the coast of Norway and it was against
international law to lay mines within three miles of the shores of a
neutral nation, so that the U-boats might have had a clear passage
around the end of the barrage. But as it was also against the law for
the U-boats to sail through neutral waters, Norway laid a mine-field
off its coast to enforce neutrality, and this was to join with that
which the British and we were to lay. Most of the mine-laying was to be
done by the United States and we were to furnish the mines.
The order to proceed with the work was given in October, 1917, and
it was a big order. A hundred thousand mines were to be made and to
preserve secrecy, as well as to hurry the work as much as possible,
it was divided among five hundred contractors and subcontractors. The
parts were put together in one plant and then sent to another, where
each mine was filled with three hundred pounds of molten TNT. To carry
them across the ocean small steamers were used, so that if one should
be blown up by a submarine the loss of mines would not be very great.
There were twenty-four of these steamers, each carrying from twelve
hundred to eighteen hundred mines and only one of them was destroyed
by a submarine. The steamers delivered their loads on the west coast
of Scotland and the mines were taken across to the east coast by rail
and motor canal-boats. Here the mines were finally assembled, ready for
planting. Seventy thousand mines were planted, four fifths of them by
American mine-layers and the rest by the British.
MINE RAILROADS ON SHIPS
To handle the mines the ships were specially fitted with miniature
railroads for transporting the mines to the launching-point, so that
they could be dropped at regular intervals without interruption. Each
anchor mine was provided with flanged wheels that ran on rails. The
mines were carried on three decks and each deck was covered with a
network of rails, switches, and turn-tables, while elevators were
provided to carry the mines from one deck to another. The mines, like
miniature railroad cars, were coupled up in trains of thirty or forty
and as each mine weighed fourteen hundred pounds, steam winches had to
be used to haul them. At the launching-point the tracks ran out over
the stern of the boat and here a trap was provided which would hold
only one mine at a time. By the pulling of a lever the jaws of the trap
would open and the mine would slide off the rails and plunge into the
sea.
The mines were dropped every three hundred feet in lines five hundred
feet apart, as it was unsafe for the mine-layers to steam any closer to
one another than that. The mines were of the type shown in Fig. 22 and
automatically adjusted themselves to various depths. The depth of the
water ran down to twelve hundred feet near the Norwegian coast. Never
before had mines been planted at anywhere near that depth.
It was dangerous work, because the enemy knew where the mines were
being planted, as neutral shipping had to be warned months in advance.
The mine-layers were in constant danger of submarine attack, although
they were convoyed by destroyers to take care of the U-boats. There was
even danger of a surface attack and so battle-cruisers were assigned
the job of guarding the mine-layers. The mine-layers steamed in line
abreast, and had one of them been blown up, the shock would probably
have been enough to blow up the others as well. Enemy mines were sown
in the path of the mine-layers, so the latter had to be preceded by
mine-sweepers. Navigation buoys had to be planted at the ends of the
lines of mines and the enemy had a habit of planting mines near the
buoys or of moving the buoys whenever he had a chance. But despite all
risks the work was carried through.
The barrier was not an impassable one. With the mines three hundred
feet apart, a submarine might get through, even though the field was
twenty-five miles broad, but the hazards were serious. Before the
first lines of mines had been extended half-way across, its value
was demonstrated by the destruction of several U-boats, and as the
safety-lane was narrowed down the losses increased. It is said that
altogether twenty-three German submarines met their doom in the great
mine barrage. U-boat commanders balked at running through it, and
U-boat warfare virtually came to a standstill. According to Captain
Bartenbach, commander of submarine bases in Flanders, three U-boats
were sunk by anchored mines for every one that was destroyed by a depth
bomb.