"That's true," averred John.  "The closer you are to the earth the more you will feel the 'bumps,' as we call them.  They are a whole lot like the waves of the ocean, only invisible, and there will be one straight over every protuberance or depression of size in the surface of the earth.  Mountains, hills, houses, lakes, valleys, rivers, forests, all cause bumps or holes in the air up above them.  At one thousand feet they are pretty bad.  At ten thousand feet they are scarcely noticeable.  That's why most pilots prefer to fly high whenever they can."