A year elapsed after the first successful flight at Kitty Hawk before the aviator became able to describe a circle in the air. A later date, 1907, is recorded for the first European half-circular flight: and the first complete circuit, on the other side of the water, was made a year after that; by both biplane and monoplane. It was in the same year that Louis Blériot made the pioneer cross-country trip of twenty-one miles, stopping at will _en route_ and returning to his starting point. What Happens When Making a Turn [Illustration: CIRCULAR FLIGHT] We are looking downward on an aeroplane _ab_ which has been moving along the straight path _cd_. At _d_ it begins to describe the circle _de_, the radius of which is _od_, around the center _o_. The outer portion of the plane, at the edge _b_, must then move faster than the inner edge _a_. We have seen that the direct air pressure on the plane is proportional to the square of the velocity. The direct pressure _P_ (see sketch on page 22) will then be greater at the outer than at the inner limb; the lifting force _L_ will also be greater and the outer limb will tend to rise, so that the plane (viewed from the rear) will take the inclined position shown in the lower view: and this inclination will increase as long as the outer limb travels faster than the inner limb; that is, as long as the orbit continues to be curved. Very soon, then, the plane will be completely tipped over. Necessarily, the two velocities have the ratio _om_:_om´_; the respective lifting forces must then be proportional to the squares of these distances. The difference of lifting forces, and the tendency to overturn, will be more important as the distances most greatly differ: which is the case when the distance _om_ is small as compared with _mm´_. The shorter the radius of curvature, the more dangerous, for a given machine, is a circling flight: and in rounding a curve of given radius the most danger is attached to the machine of greatest spread of wing. Lateral Stability This particular difficulty has considerably delayed the development of the aeroplane. It may, however, be overcome by very simple methods--simple, at least as far as their mechanical features are concerned. If the outer limb of the plane is tilted upward, it is because the wind pressure is greater there. The wind pressure is greater because the velocity is greater. We have only to increase the wind pressure at the inner limb, in order to restore equilibrium. This cannot be done by adjusting the velocity, because the velocity is fixed by the curvature of path required: but the total wind pressure depends upon the _sail area_ as well as the velocity; so that by increasing the surface at the inner limb we may equalize the value of _L_, the lifting force, at the two ends of the plane. This increase of surface must be a temporary affair, to be discontinued when moving along a straight course. [Illustration: THE AILERON]