In 1901 and 1902 the Wrights continued their gliding experiments at Kitty Hawk. Their new machines were much larger, and they added a vertical tail in order to secure better lateral balance. Sometimes the wind was strong enough to lift the aviator above the point from which he had started and hold him motionless in the air for half a minute. They made new tables of calculation for aerial flight, and found that a wind of eighteen miles an hour would keep their plane and its operator in the air. Their next step was to place a gas-engine on their aeroplane and attempt actual mechanical flight. After many experiments they succeeded, and on December 17, 1903, the first airship made four flights at Kitty Hawk. In the longest flight it stayed in the air fifty-nine seconds, and flew against a twenty-mile wind. It weighed, with the aviator, about 745 pounds, and was propelled by a gas-engine weighing 240 pounds, and having twelve or thirteen horse-power. That test assured them that mechanical flight was possible. The Wrights had now solved the real problem of aviation, equilibrium. They were ready to try mechanical flights in places where the wind-conditions were less favorable than at Kitty Hawk. They secured a swampy meadow eight miles east of Dayton, and, using that secrecy which they have always believed was necessary to the protection of their interests, began to fly there. Their airship flew well in a straight course, but there was difficulty in turning corners. Sometimes it could be done, but occasionally the plane would lose its balance as it turned, and have to be brought to the ground. In time they remedied this, and on September 20, 1904, they were able to make a complete circle. Later in that same year they made two flights of three miles each around a circular course. The Wrights’ system of balance, the great original feature of their invention, is attained by what is called the warping of the wings. When they are flying, and some cause, such as a change in their position, or a sudden gust of wind, makes the airship tip, a lever is moved, and the two planes warp down on the end that is canting toward the earth. Simultaneously the two opposite ends of the planes warp up. The lower ends at once gain greater lifting power, the upper ends less. Therefore the airship stops tilting and comes back to an even flight. The lever is instantly moved to keep the machine from tipping to the other side. WILBUR WRIGHT ORVILLE WRIGHT CABLE ADDRESS: WRIGHTS, DAYTON WRIGHT BROTHERS 1127 W. THIRD STREET DAYTON, OHIO July 22, 1911. George W. Jacobs & Co., Philadelphia. Gentlemen:-- Replying to yours of June 26th we are herewith enclosing a photograph of our first flight made at Kitty Hawk, North Carolina, on December 17, 1903. Yours truly, [Signature: Wright Brothers.] [Illustration: THE WRIGHT BROTHERS’ AIRSHIP]