Let us stand in the rear of an aeroplane, the main wing of which is represented by _ab_. Let the small fan-shaped wings _c_ and _d_ be attached near the ends, and let the control wires, _e_, _f_, passing to the operator at _g_, be employed to close and unclasp the fans. If these fans are given a forward inclination at the top, as indicated in the end view, they will when spread out exert an extra lifting force. A fan will be placed at each end. They will be ordinarily folded up: but when rounding a curve the aviator will open the fan on the inner or more slowly moving limb of the main plane. This represents one of the first forms of the _aileron_ or wing-tip for lateral control. The more common present form of aileron is that shown in the lower sketch, at _s_ and _t_. The method of control is the same. [Illustration: WING TIPPING] The cellular Voisin biplanes illustrate an attempt at self-sufficing control, without the interposition of the aviator. Between the upper and lower sails of the machine there were fore and aft vertical partitions. The idea was that when the machine started to revolve, the velocity of rotation would produce a pressure against these partitions which would obstruct the tipping. But rotation may take place slowly, so as to produce an insufficient pressure for control, and yet be amply sufficient to wreck the apparatus. The use of extra vertical rudder planes, hinged on a horizontal longitudinal axis, is open to the same objection. Wing Warping In some monoplanes with the inverted _V_ wing arrangement, a dipping of one wing answers, so to speak, to increase its concavity and thus to augment the lifting force on that side. The sketch shows the normal and distorted arrangement of wings: the inner limb being the one bent down in rounding a curve. An equivalent plan was to change the angle of inclination of one-half the sail by swinging it about a horizontal pivot at the center or at the rear edge: some machines have been built with sails divided in the center. The obvious objection to both of these plans is that too much mechanism is necessary in order to distort what amounts to nearly half the whole machine. They remind one of Charles Lamb's story of the discovery of roast pig. [Illustration: WING WARPING] The distinctive feature of the Wright machines lies in the warping or distorting of the _ends only_ of the main planes. This is made possible, not by hinging the wings in halves, but by the flexibility of the framework, which is sufficiently pliable to permit of a considerable bending without danger. The operator, by pulling on a stout wire linkage, may tip up (or down) the corners _cc´_ of the sails at one limb, thus decreasing or increasing the effective surface acted on by the wind, as the case may require. The only objection is that the scheme provides one more thing for the aviator to think about and manipulate. Automatic Control