When the machine became tilted laterally it began to slide sidewise while advancing, just as a sled slides downhill or a ball rolls down an inclined plane, the speed increasing in an accelerated ratio. If the tilt happened to be a little worse than usual, or if the operator were a little slow in getting the balance corrected, the machine slid sidewise so fast that this movement caused the vertical vanes to strike the wind on the side toward the low wing instead of on the side toward the high wing, as it was expected to do. In this state of affairs the vertical vanes did not counteract the turning of the machine about a vertical axis, caused by the difference of resistance of the warped wings on the right and left sides; on the contrary, the vanes assisted in the turning movement, and the result was worse than if there were no fixed vertical tail. If his explanation was sound, as Orville felt sure it was, then, he said, it would be necessary to make the vertical tail movable, to permit the operator to bring pressure to bear on the side toward the higher wing. (This is the form of the Wright system of control generally used today--the independent control of aileron and rudder.) Wilbur promptly saw that the explanation was probably correct and nodded approvingly. And he immediately made a suggestion. A particular relation existed, he said, in the desired pressures on the tail, no matter whether the trouble was due to difference of resistance of the wing tips or on account of sliding. Whatever the reason, it was desirable to get rid of the pressure on the side toward the low wing, to which a greater angle of incidence must be imparted in restoring lateral balance, and bring pressure on the side of the tail toward the high wing where there must be a reduced angle. So why not have the mechanism that controlled the wing warping and that which moved the tail operated in conjunction? Then the pilot, instead of having to control three things at once, would need to attend only to the front elevator and the wing-warping device. The brothers at once attached the wires controlling the tail to those that warped the wings--and they also changed the tail from two vertical fins to a single vertical rudder. After the changes in the 1902 glider, the Wrights had their machine in about the form pictured and described in the drawings and specifications of their patent, applied for on the 23rd of the next March. With their accurate data for making calculations, and a system of balance effective in winds as well as in calms, the brothers believed that they now could build a successful power-flyer. VI FIRST POWER FLIGHT