We have already mentioned the "head resistance" of the airplane wing. If the wing could strike the air in such a way as to sharply divide it into currents flowing above and below, there would be no head resistance. But the very arch of the wing in front gives it a certain amount of thickness where it strikes the air, so that instead of flowing above or below, a portion of the air is pushed along in front, retarding the progress of the airplane. This resistance is called by aviators the "drift." The best wing is the one which has the maximum lifting power with the minimum head resistance, or, to use technical language, the greatest "lift" in proportion to its "drift." Of course, not only the wing but all parts of the airplane offer resistance to the air. In order to reduce this total head resistance to the minimum, every effort is made to give the body or "fuselage" of the airplane a "streamline" form,--that is, a shape, such as that of a fish or a bird, which allows the air to separate and flow past it with little disturbance. For this purpose the fuselage of the airplane is usually somewhat rounded and tapering toward the ends, often "egg shaped" at the nose. The method of "wing warping" invented by the Wright brothers is still used on all modern airplanes to preserve lateral stability. The part of the wing which can be warped is called the _aileron_. There are two ailerons on every wing, one on each side at the rear, and they may be raised or drawn down by the action of a lever operated by the pilot. If the pilot feels that the left side of his machine is falling, he draws down the aileron on that side and raises the right hand aileron. The aileron which is lowered catches the air currents flowing beneath the wing on that side. At the same time the raised aileron on the right lessens the pressure under the wing on that side and so gives it a tendency to fall. In this way, in a fraction of a minute the wings are brought level again and lateral stability is restored. Whereas the old Wright biplane had an elevating plane in front of the main planes, most machines to-day have the elevating surfaces at the rear. By raising the "elevators" an upward motion is obtained, or by lowering them, a downward motion. [Illustration: WRIGHT STARTING WITH PASSENGER] Steering to right and left is accomplished by a rudder at the rear of the airplane body or "fuselage." This rudder may be turned to right or to left, working on a hinge. [Illustration: AN EARLY FARMAN MACHINE PRIOR TO START] CHAPTER III THE PIONEERS