In appearance the lifting craft is like a huge, flat barge with a covered deck. Its hull contains a series of great tanks, or watertight compartments, which can quickly be flooded or emptied, just as the salvage expert desires. As the tanks are flooded, so the craft sinks lower and lower in the water, and as they are pumped out so she rises again. When the tanks are full, the lifting craft sits 4½ feet lower in the water, and if she is then attached to a wreck and her tanks be emptied she is capable of lifting a weight of 1200 tons from the seabed. [Illustration: IN RAISING THE U-44 AND CARRYING HER TO PORT, COMMANDER DAVIS, R.N.R., THE NEAREST FIGURE ON THE LIFTING VESSEL, ACCOMPLISHED A BRILLIANT FEAT. THE PHOTOGRAPH SHOWS THE U-BOAT JUST AFTER SHE WAS BROUGHT TO PORT AND ALSO GIVES AN EXCELLENT IDEA OF WHAT A LIFTING VESSEL LOOKS LIKE] Say that the difference between low tide and high tide is 16 feet. If the lifting craft be placed in position over a wreck at low tide and pumped out, the cables between the lifting craft and the wreck being made taut, as the tide rises, so the lifting craft swings the wreck off the seabed, and at high tide the wreck lies slung under the lifting craft over 20 feet from the bottom. She can then be towed inshore until she grounds again. [Illustration: HOISTING OUT THE DEADLY CARGO OF MINES FROM THE U-44] In other words, a vessel floating on the surface is nearest to a submerged wreck at low water. If the tide happen to rise and fall 20 feet, the vessel will be 20 feet nearer the wreck at low tide than at high tide. By filling their lifting craft with water the salvors can bring it another 4½ feet nearer the wreck, and if they then pump out the water tanks they can raise the wreck 24½ feet from the bottom at the top of the tide, provided they have craft capable of lifting a weight as great as that of the wreck. Towing into shallower water follows, as before described. Commander Davis placed his lifting vessel in position exactly over the wrecked submarine, and the cables running under the wreck were brought up on each side of the surface craft and securely fastened. The tanks of the lifting craft were blown out with compressed air and, as the tide began to rise, the lifting craft rose with it and dragged the U-boat from her bed 90 feet below the surface. Just before the tide was at the full the salvors began to tow the lifting craft with her burden inshore and succeeded in covering a distance of three-quarters of a mile before the submarine grounded again. Next day, at the top of the tide, the performance was repeated, and the wreck was carried inshore for another three-quarters of a mile. In two days the salvors thus gained a mile and a half, and the wreck now rested on the bottom, about three miles from the beach.