Like the subchaser and the submarine, the seaplane operated most successfully in coastal waters. I have already indicated that one advantage of the convoy system was that it forced the U-boats to seek their victims closer to the shore. In our several forms of aircraft we had still another method of interfering with their operation in such quarters. In order to use these agencies effectively we constructed aircraft stations in large numbers along the coast of France and the British Isles, assigned a certain stretch of coastline to each one of these stations, and kept the indicated area constantly patrolled. The advantages which were possessed by a fleet of aircraft operating at a considerable height above the water are at once apparent. The great speed of seaplanes in itself transformed them into formidable foes. The submarine on the surface could make a maximum of only 16 knots an hour, whereas an airplane made anywhere from 60 to 100; it therefore had little difficulty, once it had sighted the under-water boat, in catching up with it and starting hostilities. Its great speed also made it possible for an airplane or dirigible to patrol a much greater area of water than a surface or a subsurface vessel. An observer located several hundred feet in the heavens could see the submarine much more easily than could his comrades on other craft. If the water were clear he could at once detect it, even though it were submerged; in any event, merely lifting a man in the air greatly extended his horizon, and made it possible for him to pick up hostile vessels at a much greater distance. Moreover, the airplane had that same advantage upon which I have laid such emphasis in describing the anti-submarine powers of the submarine itself: that is, it was almost invisible to its under-water foe. If the U-boat were lying on the surface, a seaplane or a dirigible was readily seen; but if it were submerged entirely, or even sailing at periscope depth, the most conspicuous enemy in the heavens was invisible. After our submarines and our aircraft had settled down to their business of extermination, existence for those Germans who were operating in coastal waters became extremely hazardous and nerve-racking; their chief anxiety was no longer the depth bomb of a destroyer; they lived every moment in the face of hidden terrors; they never knew when a torpedo would explode into their vitals, or when an unseen bomb, dropped from the heavens, would fall upon their fragile decks.