The young inventor did not know that a key was missing from his ring, nor, as he twirled the dial of the combination-lock, did he realize that a slender lever had been severed from below, thus rendering useless the intricate mechanism. "Who done dis to you', Massa Tom?" asked Rad. "Wish I knew. Anyhow, there's been no damage done except to me! My head's splitting, so I must get to bed. Koku, stay on guard here from now on until I return from Cuba. And get several of the men to relieve you. Another thing: I don't want either of you to mention this affair to anyone. Dad would hear about it and worry." [Illustration: "Koku, Stay on Guard Here."] "If I catchum fella I breakum in little bits!" cried Koku fiercely. He shook the rolling-pin vigorously. "Better him stay 'way fum me!" Tom awoke the next morning little the worse for his experience. Thanks to a rugged constitution, he had been able to throw off the ill effects of the poisonous fumes which had overcome him. [Illustration: "Better Him Stay 'Way Fum Me!"] "I can't make it out, Ned," he said as the boys stood watching the mechanics warm up the engines of the big seaplane. "Nothing is missing. Whoever did the job didn't even rob me, and I had a good deal of cash in my wallet." "Maybe nobody made an attempt on you or your property at all, Tom," Ned remarked slowly. "What d'you mean? I certainly was knocked out!" "Oh, I know that. But couldn't some sort of gas have seeped into your office from your adjoining laboratory? A bottle of acid might have cracked, or--" [Illustration: The Mechanics Warmed up the Engines] "Nothing like that happened. I'm positive, because the same thought struck me. I made a careful inspection this morning. Everything was in perfect order." "It certainly is strange," said Ned. "It looks as if some enemy is camping on your trail, Tom!" "He'll have a hard time picking up that same trail in a few minutes," chuckled the inventor. "Here come Captain Britten and Dad. I guess we can take off soon." [Illustration: "A Bottle of Acid Might Have Cracked."] "So your sea-goin' air-yacht is ready to cast off, is she?" asked the old diver. "Well, when ye haul in the gangplank, so to speak, I'll be aboard!" "Take care of yourself, son," said Barton Swift, shaking Tom's hand. "I hope you will be successful in your attempt." "Good-bye, Dad. And thanks." "Doan git et up by no sharks or allygators!" cautioned Rad. [Illustration: "Take Care of Yourself, Son."] The mechanics had finished their work and were seen climbing down from the fuselage. The passengers took their places in the roomy cabin while Tom seated himself behind the controls. After running a critical eye over the score of instruments he reached for the throttle and clutched the wheel tighter. The intermittent coughing of the powerful motors changed to a deafening roar, and the huge ship lumbered off down the long field, gathering speed every second. [Illustration: Tom Sat Behind the Controls]