same--the wing warping appeared to have no effect at all. The Wrights now made the discovery that in free flight, when the wing on one side of the machine was presented to the wind at a greater angle than that on the other side, the wing with the greater angle, instead of rising as it was expected to do, sometimes descended. The explanation was that the greater angle of the wing at one side gave more resistance to forward motion and thus reduced the speed on that side. This decrease in speed more than counter-balanced the effect of the larger angle of the wing in producing lift. (The Wrights had not discovered this when flying the glider as a kite, because, when held by ropes, the wings always maintained equal air speeds, even when their resistances were unbalanced.) It was evident to the brothers that their present method of controlling equilibrium was not yet complete. Something was needed to maintain equal speeds at the two wing tips. The idea occurred to them that the addition of a vertical fin attached to the machine at some distance in the rear of the wings might be the solution of the problem. But the test of such a fin had to be left until another season. The behavior of the glider in these various flights forced the Wrights to give thought to another scientific problem, that regarding the center of air pressure on curved surfaces. Contrary to the teachings of scientific books on the subject, it was becoming more and more evident that the travel of the center of pressure on a cambered surface is not always in the same direction as the travel on a plane surface. When the angle of attack on a plane surface is decreased, the center of pressure moves toward the front edge; but on a cambered surface this is true only when larger angles are being decreased. When the angle of attack on a cambered surface is decreased from, say, thirty degrees to twenty-five degrees, the center of pressure moves forward, as it does on a plane surface; but when a certain angle (between twelve and fifteen degrees) is reached, then the movement of the center of pressure is reversed. From there on, the center of pressure moves toward the rear so long as any further decrease is made in the angle of attack. The Wrights proved this by a series of experiments with a single surface from their plane. Knowledge of the phenomenon of this reversal of center of pressure was of great importance to them in their later work of designing aeroplanes. Scientific problems were not the only ones to perplex the Wrights. A sore trial were the mosquitoes and sandfleas, particularly numerous and aggressive in that summer of 1901. As Orville Wright recalled in later years, there were times when he thought, while fighting mosquitoes through the night, that if he could just survive until morning he would pack up and return home. Those mosquitoes might have caused a long postponement of the conquest of the air.