"Oh, yes," said Bob. "We have tried them out several times, dad, and in connection with the propeller, too. They work tip-top, either connected or disconnected. I tell you, when they're in connection they certainly do make this big propeller hum!" "I can't understand how you can operate the propeller in here," said Mr. Giddings, much puzzled. "All the airplanes I have seen have always dashed forward as soon as their propellers began to revolve under impulse of the motor or motors; there was no restraining them. I should think this machine would run through the front end of the hangar here as soon as you--" "Pardon me, sir," interrupted John, "but we have gone those fellows one better. You forget that in the drawings we showed you there was a set of brakes designed to be worked by a control within reach of the pilot, brakes which will engage these ground wheels a good deal the same as brakes work on automobiles--by a flexible band of steel and grit-filled cotton which is made to compress over a large sort of hub on the inner side of each wheel." "Very good," said Mr. Giddings; "but I understand that has been tried before, with the result that the airplane at once tipped forward and stuck its nose into the ground, or rather tried to, smashing its propeller to smithereens." "They will do that every time unless something has been devised to counteract this tendency to pitch over," explained John. "We have devised the thing to prevent it, Mr. Giddings." "See here, dad," put in Bob at this point. "Stoop down a bit and look under the forward end of the body here." His father did as requested, and Bob pointed out a circular opening about the size of a saucer, from which protruded the end of an aluminum-encased shaft bearing a small rubber-tired wheel of very sturdy proportions. "That is our preventer, dad," smiled his son. "In a few minutes we'll show you how it works," added John Ross. "I see you are wearing a cap, sir, as I suggested. That is all the special dress you will need, as our enclosed cabin makes helmets and close bundling unnecessary. We fellows will wear our regular working togs." Everything being in readiness, the four young men easily pushed the big airplane out of the building and to a place where it would have a smooth runway for a hundred yards ahead. The weather was ideal for the trip. There was little wind, and the few strato-cumulus clouds which were visible showed great stretches of azure-blue sky between them. "Everybody climb in," ordered Tom, with a wave of his hand. "I'll crank her up. You take the joy-stick, John." All hands complied. Then Tom began to turn the big burnished propeller, just as John threw a lever from the inside which caused the auxiliary ground wheel to shoot down and engage the sod. At the same time the movement of another lever by Paul set the airplane's brakes.