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 sub-surface 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.
I have said that the destructive achievements of aircraft figure only moderately in the statistics of the war; this was because the greater part of their most valuable work was done in co-operation with war vessels. Aircraft in the Navy performed a service not unlike that which it performed in the Army. We are all familiar with the picture of airplanes sailing over the field of battle, obtaining information which was wirelessed back to their own forces, "spotting" artillery positions, and giving ranges. The seaplanes and dirigibles of the Allied navies performed a similar service on the ocean. To a considerable extent they became the "eyes" of the destroyers and other surface craft, just as the airplanes on the land became the "eyes" of the army. As part of their equipment all the dirigibles had wireless telegraph and wireless telephone; as soon as
