ELASTICITY OF THE AIR.--Air is the most elastic substance known. The particles constituting it are constantly in motion. When heat or cold penetrate the mass it does so, in a general way, so as to permeate the entire body, but the conductivity of the atmospheric gases is such that the heat does not reach all parts at the same time. AIR HOLES.--The result is that varying strata of heat and cold seem to be superposed, and also distributed along the route taken by a machine, causing air currents which vary in direction and intensity. When, therefore, a rapidly-moving machine passes through an atmosphere so disturbed, the surfaces of the planes strike a mass of air moving, we may say, first toward the plane, and the next instant the current is reversed, and the machine drops, because its support is temporarily gone, and the aviator experiences the sensation of going into a "hole." RESPONSIBILITY FOR ACCIDENTS.--These so-called "holes" are responsible for many accidents. The outstretched wings, many of them over forty feet from tip to tip, offer opportunities for a tilt at one end or the other, which has sent so many machines to destruction. The high center of gravity in all machines makes the weight useless to counterbalance the rising end or to hold up the depressed wing. All aviators agree that these unequal areas of density extend over small spaces, and it is, therefore, obvious that a machine which is of such a structure that it moves through the air broadside on, will be more liable to meet these inequalities than one which is narrow and does not take in such a wide path. Why, therefore, persist in making a form which, by its very nature, invites danger? Because birds fly that way! THE TURNING MOVEMENT.--This structural arrangement accentuates the difficulty when the machine turns. The air pressure against the wing surface is dependent on the speed. The broad outstretched surfaces compel the wing at the outer side of the circle to travel faster than the inner one. As a result, the outer end of the aeroplane is elevated. CENTRIFUGAL ACTION.--At the same time the running gear, and the frame which carries it and supports the machine while at rest, being below the planes, a centrifugal force is exerted, when turning a circle, which tends to swing the wheels and frame outwardly, and thereby still further elevating the outer end of the plane. THE WARPING PLANES.--The only remedy to meet this condition is expressed in the mechanism which wraps or twists the outer ends of the planes, as constructed in the Wright machine, or the ailerons, or small wings at the rear margins of the planes, as illustrated by the Farman machine. The object of this arrangement is to decrease the angle of incidence at the rising end, and increase the angle at the depressed end, and thus, by manually- operated means keep the machine on an even keel. CHAPTER IV FORE AND AFT CONTROL