But that was not all the Wright brothers did to make man-flight over a sustained and steered course in a heavier-than-air machine possible. Directional control or power to steer the glider in a straight line or to vary it had not yet been acquired, so the Wrights installed a vertical rudder which they also operated by lever, just as the rudder on a power-boat is controlled, and the effect on directional steering was the same. Indeed, passage through the medium of the air is in many ways similar to passage through water. Thus the moment the glider swerved from right to left the rudder was pulled in the opposite direction and the planes came back to the steered course. But this was not invented at once nor installed until after the Wrights discovered that whenever the glider was in flight the effect of warping the wings to control the rolling had a serious unexpected secondary effect, namely, a tendency for the high wing, which they desired to bring down, to advance faster through the air than the low wing, and solely by its higher velocity to develop a higher lifting capacity and thus to neutralize the benefit of the warp. After much experimenting they hit upon the rudder idea and that corrected the difficulty. Thus the Wrights gained complete mastery of the glider; they could steer it up and down, turn it from right to left, and bring it back safely to the earth. This is the basis of the Wright patents to-day. The next thing to be done was to install upon an aeroplane a power plant sufficient to drive it through the air fast enough to make the air lift it off the ground and sustain it in the “liquid blue” until the pilot saw fit to glide to the earth again. This was by no means a simple matter, for from 1900, when the Wrights began their glider experiments, to 1903, when they made their first flight, the gasoline motor was in its impotent infancy. They set about building a small light motor, however, to install in their planes. In the meantime they experimented further with wing surfaces. Langley and Chanute had proved flat wings inefficient and curved wings necessary for lifting capacity. Of course, those early experimenters did not know how much those curvatures affected the climbing angle of a glider, so the Wrights set out to find out by using the wind-tunnel method and testing scale models in the same, with a blast of air generated by an engine-driven fan. This tunnel was cylindrical in form, sixteen inches in diameter. The smaller models of wings were hung in the centre, the air-blast turned on, and the balance arm, which projected into the tunnel and on which the wings were mounted, measured the air forces and the efficiency of the varied wing shapes from the standpoint of rounded wing tips and curvature.