"You probably know," the aviator went on, while Dick, Paul, and Innis, with several other cadets, listened interestedly, "you probably know that one of the great problems of aviation is how to keep a machine from turning turtle, or turning over, when it strikes an unexpected current, or 'air pocket' in the upper regions. Of course a birdman may, by warping his wings, or changing the elevation of his rudder, come out safely, but there is always a chance of danger or death. "If there was some automatic arrangement by which the airship would right itself, and take care of the unexpected tilting, there would be practically no danger. "I realized that as soon as I began making airships, and so I devised what I call a gyroscope equilibrizer or stabilizer. A gyroscope, you know, is a heavy wheel, spinning at enormous speed, on an anti-friction axle. Its great speed tends to keep it in stable equilibrium, and, if displaced by outside forces, it will return to its original position. "You have probably seen toy ones; a heavy lead wheel inside a ring. When the wheel is spinning that, and the ring in which it is contained, may be placed in almost any position, on a very slender support and they will remain stable, or at rest. "So I put a gyroscope on my airship, and I found that it kept the machine in a state of equilibrium no matter what position we were forced to take by reason of adverse currents. Of course it was not an entire success, but I was coming to that. "In the biplane which was wrecked in the river I had my latest gyroscope. It seemed to be perfect, and, with Jack and Harry, I had made a number of beautiful flights. I even flew alone upside down, and had no trouble. "Before that I had made the acquaintance of Lieutenant Larson, who is also an expert aviator. He worked for me before he went in the army. He had his own ideas about equilibrium, and his plan, which he wanted me to adopt, consists of tubes of mercury that can automatically be tilted at different angles. I do not believe they will ever work, and I told him so. I refused to use them, and he and I parted, not the best of friends. He wanted his invention exploited, but I refused to try it, as I thought it dangerous. "When my gyroscope worked fairly well, I presume Lieutenant Larson was professionally jealous. At any rate he, left me, and I am glad of it." "But he was around our workshop just before we made this last flight!" insisted Jack. "He came in pretending he had left some of his important drawings behind when he went away, but I noticed that he hung around the airship a good bit. I saw him looking at, and running the gyroscope, and I'm sure he did something to it that caused it to fail to work, and so wrecked us." "You should not say such things," chided Mr. Vardon. "Well, I believe it's true," insisted Jack. "And you found something wrong with the gyroscope, when you took it from the airship; didn't you?"