Again he listened, and he thought the droning sound settled down into the throb of engines. He was anxious, however, not to call out the guard on a false alarm, for he had once been severely reprimanded for so doing. "They cannot be hostile 'planes attempting an early morning raid; it is far too thick. It would be like a nigger trying to find a black cat in a dark cellar," he muttered. A quarter of a minute later, however, he thought he had discovered the real cause, for the throbbing of aerial engines could now be distinctly heard. "It's a Zeppelin!" he exclaimed. "They're going to find the aerodrome with their search-light, and bomb the place, then make off before our machines can get up," and he instantly yelled out at the top of his voice, "Turn out, guard!" The alarm was caught up, passed on to the next sentry, who repeated it, and the next moment, after turning out the main guard, the sergeant came running up, and asked: "What's the matter, Pearson?" "Zeppelin approaching from the eastward, sergeant!" replied the air-mechanic. "Zeppelin, man! What the deuce do you mean? Where is it?" "Up there, sergeant. I can hear it quite plainly now." "By Jove, so can I!" The next moment the sergeant was back in the guard-room. From thence he dashed into the orderly-room, and knocked at the inner door, where the orderly officer for the night was on duty. "Come in," cried the officer in answer to the knocking. Then, as the sergeant, all puffed with his exertion, entered and saluted, he said: "What's the matter, sergeant?" "Zeppelin approaching from over the German lines, sir. Hadn't we better 'phone to the anti-aircraft guns, and the searchlights to pick up the raider before he bombs the place?" for to the sergeant's mind, visions of falling bombs and terrific explosions were present. "Zeppelin?" laughed the orderly officer. "Yes, sir. I can hear the engines as plainly as possible outside." "No, you're mistaken. It's the 'Gertie' returning. She's been out on secret service work behind the German lines. I've been expecting her for a couple of hours. Not a word of this to the men, now. I am expecting a secret service man back before dawn, and the 'Gertie's' been to fetch him. Picked him up at some secret place in the dark, far behind the enemy's lines." Now, the "Gertie" was a baby-airship detailed for special service, and not the least important part of her work was the secret journeying to and fro, across the German lines, to quiet rural places, where, in the dark, she dropped messages, carrier pigeons, etc., and occasionally brought back some daring member of the British Secret Service, who had been collecting information behind the enemy's lines. By this time the orderly officer was out on the aerodrome, and the squads of air-mechanics were being roused by the orderly sergeant. Suddenly there came a cry from one of the guard. "Airship signalling to the aerodrome, sir!" "What signal was that?" demanded the officer.