"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."