After he has flown several times as a passenger, and watched all that his instructor does while in the air, the pupil reaches the second stage of his tuition; and in this he is seated alone in a machine, and allowed to drive it here and there across the aerodrome. The biplane is adjusted, as a rule, so that it will not fly; and this process of the training--which is called “rolling” at the schools--is to enable a novice to handle his craft neatly while on the ground, to learn to steer to right or left, to keep it when necessary upon a straight course, and to accustom himself to the starting and stopping and control of his engine. This practice is important. With his motor giving just sufficient power to make the controls of the machine effective, but not enough to lift it into the air, the pupil “rolls” from one side of the aerodrome to the other; then he turns by a movement of his rudder, which swings the tail of the machine, and returns again to his starting-point, striving always to keep his craft from swinging sideways. He finds, while making these long, straight runs, that the machine shows a tendency, if the controls are untouched, to swerve a little to the left; and this is caused by the influence of the propeller, which is revolving to the left. But the inclination may be checked quite easily by a movement of the rudder to the right. With a school biplane, adapted for its work, a novice may soon learn to steer while on the ground; but with some other types of machine--notably a fast monoplane--“rolling” is by no means easy. A racing craft, for instance, when it is moving across the aerodrome, needs most expert handling: even the smallest error of judgment in the use of the rudder may send it whirling round upon itself in a violent turn--rather like an angry bird; while if the pilot attempts to swerve too abruptly, in avoiding any obstacle before him, the machine may reel sideways and overturn--and, seeing that it is travelling at a speed greater than that of an express, the resulting smash may be a serious one.