In order to maintain itself in equilibrium the center of gravity must coincide with the center of pressure, otherwise there will be a turning action about the center of gravity and the machine will upset. The location of the center of pressure depends upon the angle of the plane. The greater the angle the farther it is from the forward edge of the plane and the pilot can maintain fore-and-aft equilibrium by tilting the horizontal planes of his rudder so as to change the angle of the machine, and hence of the main or forward planes. Unfortunately the air is a turbulent ocean filled with invisible air currents and the aeroplane must be capable of adjustment to meet the variations of pressure due to sudden gusts of wind. This is particularly true of lateral balance. A gust coming from the side will put a greater pressure on one side of the aeroplane than on the other. To meet this the angle of the plane at one side must be less than that at the other. This was discovered by the Wright brothers and their method of overcoming the lateral variations of pressure was the key to their early success. They warped their wings or twisted them so that the angle of the wing was reduced on the side from which the gust came and was increased on the other, and thus the center of pressure over the whole wing was kept on the line of the center of gravity. At the same time there was a variation in head resistance which had to be corrected by moving the rudder, and the Wright brothers used a single lever to warp the wings and at the same time to operate the rudder so as to keep the aeroplane on its course. Before the Wright brothers made public their invention other aeronauts had great difficulty in making turns. When turning, one side naturally has to move through the air faster than the other. This produces an increase of pressure on one side over the other which may be counterbalanced by warping the wings. The same effect is produced by the use of small wing planes, called “ailerons,” at each side of the main planes. An automatic means of stabilizing an aeroplane has also been devised, a description of which will be found in Chapter XXI. CHAPTER XV HELPING THE FARMER