"She may not make all the speed I hope she will, and I may not be able to push her high into the air quicker than I used to before I made the changes," admitted Tom, "but I'm sure of one thing. She'll fly, and she won't come down until I'm ready to let her. So you needn't worry about getting hurt." "All right--if you say so. Now what do you want me to do, Tom?" "Go over the wire guys and stays for the first thing. There's going to be lots of vibration, with the re-built motor, and I want everything tight." "Aye, aye, sir!" answered Ned with a laugh. Then he set at his task, tightening the small nuts, and screwing up the turn-buckles, while Tom busied himself over the motor. There was some small trouble with the carburetor that needed eliminating before it would feed properly. "How about the tires?" asked Ned, when he had finished the wires. "You might pump them up. There, the motor is all right. I'm going to try it now, while you attend to the tires." Ned had pumped up one of the rubber circlets of the small bicycle wheels on which the aeroplane rested, and was beginning on the second, when a noise like a battery of machine guns going off next to his ear startled him so that he jumped, tripped over a stone and went down, the air pump thumping him in the back. "What in the world happened, Tom?" he yelled, for he had to use all his lung power to be heard above that racket. "Did it explode?" "Explode nothing!" shouted Tom. "That's the re-built motor in action." "In action! I should say it was in action. Is it always going to roar like that?" Indeed the motor was roaring away, spitting fire and burnt gases from the exhaust pipe, and enveloping the aeroplane in a whitish haze of choking smoke. No, I have the muffler cut out, and that's why she barks so. But she runs easier that way, and I want to get her smoothed out a bit. "Whew! That smoke!" gasped his chum. "Why don't you--whew--this is more than I can stand," and holding his hands to his smarting eyes, Ned, gasping and choking, staggered away to where the air was better. "It is sort of thick," admitted Tom. "But that's only because she's getting too much oil. She'll clear in a few minutes. Stick around and we'll go up." Despite the choking vapor, the young inventor stuck to his task of regulating the motor, and in a short while the smoke became less, while the big propeller blades whirled about more evenly. Then Tom adjusted the muffler, and most of the noise stopped. "Come on back, and finish pumping up the tires," he shouted to Ned. "I'm going to stop her now, and then I'll give her the pressure test, and we'll take a trip." Having cleared his eyes of smoke, Ned came back to his task, and this having been finished, Tom attached a heavy spring balance, or scales, to the rope that held the airship back from moving when her propellers were whirling about. "How much pressure do you want?" asked Ned.