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The first idea about unsteady motion in the atmosphere seems to have been the vague notion of “ Air Pockets”—areas or gaps, wherein the air just does not generate lift, and an aeroplane falls in consequence. It is astonishing how widely this view is still held.Those engaged in aviation soon learned that unsteady flight was due to turbulence and two sorts, friction and orographical, were quickly recognised. Friction turbulence, which affects only the lower layers of the atmosphere, can best be observed by watching a soaring sea-bird in a breeze. The friction of the water surface creates a velocity gradient or variation of windspeed with altitude, and the bird will probably be diving down-wind, and climbing into wind, with no effort— from the bird's point of view a fair approach to perpetual motion. However, the air is really a mass of tiny eddies where friction is at work and close observation of the bird's wings and feathers will show them to be constantly in motion as they yield to each puff.