Nor is the problem as simple as the illustration of the kite suggests, for the air is compressible, and is moreover set in motion in the form of a current by a body passing through it at anything like the ordinary speed of an aeroplane. This has caused the curving of the planes (from front to rear) of the flying machine, in contrast with the flat plane of the kite. The reasoning is along this line: Suppose the main plane of an aeroplane six feet in depth (from front to rear) to be passing rapidly through the air, inclined upward at a slight angle. By the time two feet of this depth has passed a certain point, the air at that point will have received a downward impulse or compression which will tend to make it flow in the direction of the angle of the plane. The second and third divisions in the depth, each of two feet, will therefore be moving with a partial vacuum beneath, the air having been drawn away by the first segment. At the same time, the pressure of the air from above remains the same, and the result is that only the front edge of the plane is supported, while two-thirds of its depth is pushed down. This condition not only reduces the supporting surface to that of a plane two feet in depth, but, what is much worse, releases a tipping force which tends to throw the plane over backward. In order that the second section of the plane may bear upon the air beneath it with a pressure equal to that of the first, it must be inclined downward at double the angle (with the horizon) of the first section; this will in turn give to the air beneath it a new direction. The third section of the plane must then be set at a still deeper angle to give it support. Connecting these several directions with a smoothly flowing line without angles, we get the curved line of section to which the main planes of aeroplanes are bent.