WHILE sirens shriek, searchlights comb the night sky with bright fingers of light and anti-aircraft guns bark lustily at tiny moving objects high above, a trimotored seaplane flies low, and slowly, off the English coast. Unnoticed in the din of the air-raid alarm, it drops one fluttering shape after another which floats to the water’s surface and sinks - and thus is recorded the successful planting of mines from the air, a new terror in the war at sea. This, British authorities believe, is the way Germany sows deadly mines in sea lanes. Fully as important as the ability of the seaplane to launch parachute – supported mines is the mine itself, which some experts say is a magnetic type designed so that a steel-hulled ship entering its, electromagnetic field causes a relay to unleash the explosive. with terrible effects to the unsuspecting vessel and all aboard it. The mine is believed to require only two or three midget tubes, of the type used in your radio, with their filaments set so low that batteries much smaller than those in portable receivers produce radiations for long periods. Electromagnetic poles arranged transversely to each other would insure a sufficient area of distribution of energy to induce reactions with ships coming from any direction. For protection of the crew planting the mine, military experts say, a small bar of hard salt interposed between two small plates prevents the “trigger” of the mine from becoming set until the water has dissolved the salt. A number of mines of 200 to 300 pounds weight can be carried easily in large seaplanes. How the mines could be released from a plane is purely speculative, but some military authorities believe that it might be accomplished by an adaptation of the aft gunner’s hatch, which ordinarily is susceptible to the release of parachutes bearing supplies for relief purposes. Refinements of technique conceivably could include cylinders in which the 'chutes maybe packed and swung over to a point above the open hatch before the heavy mine is manipulated into position by movement on rollers along a miniature railway to the hatch. Once the mine is released, it is pointed out, the weight would drag the parachute from its cylinder and the opening would follow as the plane continued. Experts say that separation of the ’chute from the mine, after contact with the wa- ter's surface, might be accomplished by attaching the shroudsto a shackle through which is passed a bar of hard salt and from this hangs the mine, so that the dissolving of the salt frees the parachute, permitting it to drift away. During the World War, experiments with parachute mines were conducted, but with so little success that there is no record of adoption of the idea. In one test, the 'chute was carried away by the wind and the mine was lost. More familiar to military expert and ci- vilian is the so-called “floating” mine, which is a metal case containing 200 to 400 pounds of high explosive, a detonating device and sufficient air space to make it buoyant. Usually round or pear-shaped, it has a number of spikes or long horns on its upper surface. Each horn is a soft metal casing containing a glass bottle of acid, which, when broken by contact with a ship’s hull, permits the acid to run into the body of the mine and explode a fulminate of mercury detonator by completing an electric circuit. The detonator flashes a primer of black powder or dry gun cottonwhich explodes the main charge - the complete operation being almost instantaneous. Laid mostly by surface ships, these mines are mounted on small trucks or “sinkers” that run on rails fixed to the deck. They are tipped over the stern at regular intervals as the ship steams along, and float for a short time until the heavy truck fills with water. At the same time a large lead plummet sinks and trips a catch on the truck when it touches bottom. This tripping of the catch results in an automatic measurement of the depth of the water, less the distance the mine is designed to float below the surface. The cable by which the mine is fastened to the truck is automatically shortened to this length. By this time the heavy truck is sinking but the mine is trying to float, and thus causes the ‘cable to unwind from its reel until it reaches the point set by the trip catch, locking the cable and dragging the mine to its predetermined depth, anchored to the sea bed by the truck or “sinker.” Submarines also lay mines, employing an interior launching structure. While a surface mine-layer can carry 200 to 400 mines, the submarine has a capacity of about forty, all of which can be planted without the craft coming to the surface. To the conventional mine may be added an antenna maintained vertically by a float of special design. Thirty-five feet belowthe surface the mine, at the end of the antenna, can hardly be seen from an airplane and becomes less visible when painted a dull green or if it becomes covered with sea weed. The specific value of the antenna is its sensitivity to an approaching ship’s metal hull, thus setting off the explosive. ‘When a mine field laid by the enemy is discovered, usually by the sinking of a ship, a flotilla of sweepers is sent out to clear it. Two main types of mine sweepers are used by the British navy. The first is an 800-ton craft which has relatively high speed and heavy gun armament and can be used to sweep ahead of warships. The second is a converted fishing boat - trawler - which can be used for sweeping inshore. For work in even more sheltered places there is the motor mine sweeper, similar to small motor fishing vessels. Of the two types of sweep, the commoner has a float and a “kite” which strains a wire away at a wide angle from the side of the sweeper, enabling the craft to work up and down the edge of a mine field like a reaping machine. The mine cables are cut by the sweep wire which has a rough saw edge. As the mines bob to the surface they are destroyed by rifle fire. The second sweep is a wire slung between two ships. For combating the magnetic mine, the British are said to have considered dragging large sheets of magnetic metal with awooden vessel through suspected areas, thus causing the mines to explode with no damage except to the sheets. Another proposal calls for use of a wooden-hulled “mother” ship, ahead of which on either side would be crewless, lightweight, motor-equipped rafts or boats deriving their power and steering abilities from cables running from the ship. Between these rafts would be spread another cable to which would be affixed electromagnets and sound makers, in case the mines were found sensitive to noise waves, and a number of spark gaps with antennas. Subjecting the ship’s generator, which would be connected to the magnets and the spark gaps, to flashlike overloadings would create an intense electrical barrage that would explode the mines on sea bottom some thirty-five to fifty feet below.