Later, a vertical rudder was also added for horizontal steering. The combined movements of these devices maintained equilibrium. The importance of the system of torsion of the main carrying surfaces cannot be overestimated. We have only to look to nature for its _raison d’être_, and observe a flight of seagulls over the sea: how varied are the flexings of nature’s aëroplanes in their wondrous manœuvrings to maintain and recover equilibrium! Since the appearance of the Wright motor-driven aëroplane, the principle of moving either the main surface or attachments to the main surface has been very generally adopted in other types of flying machines. A feature of these early experiments was the placing of the operator prone upon the gliding machine, instead of in an upright position, to secure greater safety in alighting, and to diminish the resistance. This, however, was only a temporary expedient while the Wrights were feeling their way. In the motor-driven aëroplanes the navigator and his companion were comfortably seated. After the experiments of 1901, the Wrights carried on laboratory researches to determine the amount and direction of the pressures produced by the wind upon planes and arched surfaces exposed at various angles of incidence. They discovered that the tables of the air pressures which had been in use were incorrect. Upon the results of these experiments they produced, in 1902, a new and larger machine. This had 28.44 square metres of sustaining surfaces--about twice the area that previous experimenters had dared to handle. The machine was first flown as a kite, so that it might be ascertained whether it would soar in a wind having an upward trend of a trifle over seven degrees; and this trend was found on the slope of a hill over which the current was flowing. Experiment showed that the machine soared under these circumstances whenever the wind was of sufficient force to keep the angle of incidence between four and eight degrees. Hundreds of successful glides were made along the full length of this slope, the longest being 22½ feet, and the time 26 seconds. A motor and screw propellers were then applied in place of gravity, in 1903, and four flights made, the first lasting 12 seconds, and the last 59 seconds, when 260 metres were covered at a height of two metres.