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.

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.

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.

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.

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.