[Illustration: Blimps too may use masts aboard surface ships as anchorage point on long cruises, as the U.S.S. Los Angeles successfully demonstrated when moored to the U.S.S. Patoka. (U. S. Navy photo)] [Illustration: The Army’s TC-7 demonstrates the first airplane pick-up at Dayton. Army pilots found that at flying speed the plane weighed nothing, was sustained by dynamic forces. (U. S. Army photo)] Flight was here, though it would be a long time becoming practical. Dr. Charles and many others contributed, even at that early day. Knowing that hydrogen expanded as the air pressure grew less, at higher altitudes, Charles devised a valve at the top of the balloon, so that the surplus gas could be released, not burst the balloon. He devised a net from which the basket could be suspended, distributing its load over the entire bag. The drag rope was evolved, an ingenious device to stabilize the balloon’s flight in unstable air. If the balloon tended to rise it would have to carry the entire weight of the rope. If it grew sluggish and drifted low, it had less weight to carry, as much of the rope now lay on the ground. These ballooning principles, early found, are still in use. But the “dirigible” balloon, or airship must wait for light weight, dependable motors, despite the hundreds of ingenious experiments made by men over a full century. Since this is an airship story, we should first make clear the difference between the airship and the airplane. The French hit on an apt phrase to distinguish them, dividing aircraft into those which are lighter than the air, such as airships, and those which are heavier than the air, like airplanes. Airships are literally lighter than air. So are all free balloons, used for training and racing, and all anchored balloons, such as the observation balloon widely used in the last war and the barrage balloons of the present war. The airship goes up and stays up because the buoyancy given by its lifting gas makes it actually lighter than the air it displaces, and even with the load of motors, fuel, equipment and passengers, must still use ballast to hold it in equilibrium. The airplane, on the other hand, is heavier than the air. Even the lightest plane can stay up only if it is moving fast enough to get a lifting effect from the movement of air along the wings, similar to that which makes a kite stay up. A kite may be flown in calm weather only if the one who holds the cord keeps running. On a windy day, the kite may be anchored on the ground, and the movement of the wind alone will have sufficient lifting effect. So powerful are these air forces that a plane weighing 20 tons may climb to an altitude of 10,000 feet if its speed is great enough, and its area of wing surface broad enough to produce this kiting effect.