Without going into a technical discussion of all the forces that enter into the flight of an aeroplane we must, however, realize that if the pressure of the atmosphere is uniform in all directions, in order to make the air forced under a wing or plane lift more than the air above forces down, the wing of the plane must be curved in such a way that the forward motion of the edge of the wing causes the air underneath to force any particle of the surface upward, while the upper surface is relieved of the pressure. This is done by curving the surface of the planes so that the under surface is concave while the upper part is almost convex, like the outspread wing of a bird. When this wing is forced horizontally through the air it creates a vacuum immediately behind the upper or convex part, the under pressure is still constant and the surface is lifted upward. That is why a plane covered with a curved surface will fly and a plane with a flat surface will not. In short, a curved surface when moving through atmosphere causes eddies in the air, and if the curvature of the wings is properly calculated, it leaves a vacuum near the rear edge of the surface of the plane and it climbs upward. The smaller the angle the smaller the lift or climbing power of the plane. Thus a 15-degree angle will lift one pound; if reduced to 10 degrees it will only lift two-thirds of a pound, but because a wing is curved a plane could fly at several degrees less than 0 degree, but its “stalling” or critical angle beyond which it is not safe to go is 15 degrees. It must be borne in mind that the larger the wing surface the larger load the aeroplane can carry, for the lift of a heavier-than-air machine depends entirely on the number of square feet of surface in the plane or wings. The larger the planes the more power is required to force them through the air and the less easy they are to manœuvre and land. The Nieuports, Spads, Sopwiths, and Fokkers, with their small wing spread of less than 30 feet, made them much easier to fly, even though they land faster than the “big busses.” Therefore every pound of weight added to an aeroplane decreases its speed proportionately and requires an equivalent increase in horse-power to force it through the air. Of course, an increase of speed gives an increase in lift, so by doubling the speed of a plane you increase the lift just four times.