Indicator nets and high explosive mines might give some protection to harbors, might be stretched across steamship lanes and planted around the hiding places of the submarines, if those could be discovered. But troop ships and munition ships and food ships must be dispatched without interruption across the tricky waters of the English Channel to France, and for this purpose convoy escorts were devised, with camouflaged warships zigzagging alongside, while high aloft in lookout stations men with binoculars strained their eyes, searching the waters, ahead, astern, alongside, their search lingering long over every bit of floating wreckage—and there was a lot of it—to make sure it was not a periscope. These lookouts aboard ship quickly had a new ally in the air. As the submarine menace grew, binoculars began to flash too from the fuselages of bobbing blimps overhead. At a few hundred or perhaps a thousand feet elevation they could see deep below the surface, and quickly learned to recognize at considerable distance the tell-tale trail of bubbles or feathered waters or smear of oil which denoted the enemy’s presence, might even pick out the shadowy form of the submerged craft itself. The value of the airship in convoy was that it could fly slowly, could throttle down its motors and march in step with its charges, cruise ahead, alongside, behind. The very speed of its sister craft, the airplane, handicapped its use in this field. This characteristic of the blimp was even more useful in hunting U-boat nests. The blimp could head into the wind, with its motors barely turning over, hover for hours at zero speed over suspect areas. It could fly at low altitudes, follow even slender clues. Seagulls following a periscope sometimes gave highly useful information. An orange crate moving against the tide attracted the attention of one alert pilot, for the crate concealed a periscope, and the blimp dropped bombs—successfully. When a blimp discovered a submarine, it would give chase. With its 50 knots of reserve speed it was faster than any warship, much faster than the poky wartime submarine, which could do only 10 or 12 knots on the surface, much less than half that when submerged. If it was lucky the airship might drop a bomb alongside before the sub got away. And run for cover the submarine always did. It wanted no argument with a ship which could see it under water, could out-run it, and might plunk a bomb alongside before its presence was even suspected. Airships did get their subs during the war. The records, always incomplete in the case of submarines, whose casualties were invisible, show that British blimps sighted 49 U-boats, led to the destruction of 27. But their greater usefulness lay in the fact that their mere presence sent the underseas craft scuttling for submerged safety.