"Well," said Paul, "you know, sir, that it is far more difficult to drive an airplane than to guide an automobile, not merely because you have two steering-gears or rudders to take care of, one for sidewise and the other for up-and-down travel, but also because there are movable planes in the wings of the machine, which have to be worked to tip or 'bank' it when making a turn or to keep it on an even keel when a gust of wind strikes it.  The 'rudder' is the vertical plane at the tail of the machine, and is used for steering sideways, while the 'elevators' are the two horizontal movable planes just below the rudder, which are used for steering up and down.  Similar planes to the latter, one situated in the back edge of each upper wing, are called 'ailerons,' and one or the other is raised or depressed according to whether the aviator wishes to bank to the right or left.