To make matters worse, different parts of the sustaining surface of the machine may receive different impulses. One wing, for example, may graze a violent uprush of air not encountered by the other, giving the aeroplane a tilt to one side, or the tail of the machine may be driven in one direction and the nose in the other. Again, the whole machine may suddenly enter an air current of quite different speed and direction from the one in which it has been flying. To take an extreme case, it may run into a stream of air flowing just as fast, and in the same direction, as the machine itself, with the result that the _relative_ wind becomes zero, and the machine, deprived of all lift, drops like a stone until it acquires a velocity with respect to its new environment. When such conditions prevail, the pilot is kept busy with his “controls”; now moving his elevator to adjust his fore-and-aft balance, and now his ailerons to set him on an even keel laterally, and occasionally turning his rudder to offset the effects of horizontal gusts. The elevator and the ailerons are worked with a single lever, colloquially called the “joy-stick,” and the rudder with a bar which the pilot operates with his feet. Ordinary adjustments of this kind are performed automatically by the trained aviator, but violent disturbances call for the exercise of skill and judgment. Generally speaking, no amount of atmospheric turbulence causes any serious trouble to the trained pilot, except when he is flying close to the ground, as in starting and landing. Before we leave the subject of wind it will be well to emphasize once more the fact, which the average layman has difficulty in grasping, that the only movements of the air that affect the safety and comfort of flight are the movements relative to the machine, and not those relative to the ground. To the aviator, when he is once clear of the ground, a steady wind of any speed is merely a mass of calm air. Hence an aviator will sometimes have perfectly smooth flying when the wind, as measured on the earth, is blowing 40 or 50 miles an hour; and again he will describe the air as rough and bumpy when flags are hanging limp from their staffs and dwellers on _terra firma_ declare that not a breath of air is stirring. In the early days of flying aviators themselves were afraid of a strong wind. Thus Wilbur Wright, during his pioneer exhibition flights in France, would never go up unless the smoke from his cigarette rose in a straight line, and until about the end of 1909 no aviator attempted to fly in a wind of 20 miles an hour.