To learn how to fly men studied the wings of huge birds living millions of years ago, made careful mathematical reckonings about them, and then made themselves wings of feathers or skin. But with these wings they could only glide to earth from high towers or cliffs. One useful thing they learned from this study. They found that the wing of a bird is bent as you bend a long piece of paper if you hold it by opposite corners and start to twist it. This is called the principle of the screw, and is now used in making the propeller blades of airplanes. [Illustration: WILBUR WRIGHT] =208. The First Airplanes.= Early airplanes, airplane models and "gliders" were made in the queerest, most outlandish shapes imaginable. They had from one to five or more planes, arranged at almost every possible angle. Some looked like a row of box kites, some like dragons, and some like a collection of old fashioned windmill wheels all fastened together. It was only a little while ago that men were working with these strange models, for it was only about ten years before the World War that a successful airplane flight was first made. [Illustration: ORVILLE WRIGHT] The invention of the balloon came late in the history of flying. Two sons of a French paper manufacturer probably made the first balloon. They filled a large bag with hot air from a bonfire, and found that it rose and sailed away. Early balloons were carried through the air by wind currents, and could not be guided. Their passengers were often blown out to sea and drowned. [Sidenote: =Zeppelins=] A German, Count Zeppelin, invented a balloon called a dirigible, because it could be directed through the air. The Germans named these large cigar-shaped balloons "zeppelins," after their inventor. Dirigibles are now built more than two blocks long, about the length of the largest battleships. They can lift heavy loads, but are very expensive and very easily broken, and require huge sheds or houses to shelter them. [Sidenote: =First successful flight=] An airship properly means a dirigible, while an airplane is a heavier-than-air machine. The first successful flight of any length in an airplane that could be directed was made by Wilbur Wright in 1903, at Kitty Hawk, North Carolina. It was also the first time an airplane had been driven by a gasoline engine. [Sidenote: =Did bicycle repairing=]