"That is what John and I call an 'automatic pilot,'" answered Paul. "It is a new form of stabilizer, and made so as to overcome the defects of others which are on the market. A stabilizer should automatically keep an airplane on a fairly level keel no matter how air conditions are, even so steady that it will travel along on its course for a considerable distance with the pilot paying no attention to his controls, perhaps eating his lunch or reading his orders." "A mighty useful contrivance," commented Mr. Giddings. "I should think that would also prevent lots of accidents in bad winds." "It will--if it turns out as we expect," Paul remarked. "Give me the full details of this," was the request. "Remember, I am not much of an airplane man." "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. "The driver of an automobile has nothing to do but watch his steering-wheel, and be ready to touch a pedal when he wishes to slow up or go faster or stop. If he makes a curve he does not have to bank his machine owing to his comparatively slow speed; but the aviator, traveling much faster through the air, must do this, bringing his airplane to a steep angle if he makes a very short turn. If he does not calculate just right, he is likely to turn upside down and meet this death in a nasty fall. "While the careful automobilist can always see the road in front of him and avoid rough spots or obstacles before he reaches them, the aviator cannot do this. It is true that he can see another airplane if it gets in his way, or a church steeple when he is flying low; but his greatest dangers are in the clear air itself, where they cannot be detected. He may suddenly drop into a 'hole,' which is really a downward current of air, or he may get a terrific bump when he strikes a rising current. A freakish whim of the winds may unexpectedly take away the air support from under one of the wings, and he will lurch and dip sharply to one side." "And I suppose sometimes lose all control?" said Mr. Giddings.