The surface shape of the plane is an important consideration. It has been found that a slight upward arch from beneath, making the under surface concave, gives the best results. The concavity should reach its maximum about one-third of the distance from the front or entering edge to the rear edge of the plane and should be the same whether one or more planes are used. In flight the forward or entering edges of the planes are tipped slightly upward to give the machine lifting power for the same reason that the top of a kite is given an angle of elevation so that the air will lift it as it is drawn forward by the string. Balancing the Machine The balancing of a machine in mid-air is one of the most difficult problems in aviation. In the balloon this is easily accomplished because the principal weight, the basket with the passenger, is below the gas-filled sphere or compartment, and the balloon tends to right itself after any disturbance by the wind, much like a plummet when swayed out of its position. Professor Langley, Lilienthal and others had sought to take advantage of this tendency in the construction of their machines by placing or arching the wings above the pilot or heavier portion of the mechanism. After a slight disturbance in mid-air the machine would then tend to right or balance itself and assume its former position. The practical difficulty of this arrangement, however, arose from the fact that when once set to swaying the gliders thus constructed continued to sway like the pendulum of a clock. The Wright brothers set themselves the task of finding some other method of preventing the biplane from dipping downward or upward at either side with the shifting of air currents. The first device to give steadiness of motion was a small movable horizontal plane, supported parallel with and in front of the two main planes, and by means of a lever, under control of the pilot. At Kitty Hawk Having after much study completed their glider, the Wright brothers sought a suitable place for their first tests. By correspondence with the United States Weather Bureau they learned that at Kitty Hawk, North Carolina, the winds are stronger and more constant than at any other point in the United States. This treeless waste of sand dunes along the solitary shore near the village afforded the privacy where they might carry on their work unmolested. Here in October, 1900, they spent their vacation testing their biplane glider. They sought to fly it in the face of the wind like a kite. This they succeeded in doing but it would not support the weight of a man. They then experimented with it, using light ropes from below to work the levers and guide it through the air. It was sufficiently responsive to encourage them and they went back home to make at their leisure a number of improvements.