"That's fine!" said Rob, as he took the tube.  It was scarcely six inches long, and hollow at one end.
"The busy lives of men," proceeded the Demon, "require them to move about and travel in all directions.  Yet to assist them there are only such crude and awkward machines as electric trolleys, cable cars, steam railways and automobiles.  These crawl slowly over the uneven surface of the earth and frequently get out of order.  It has grieved me that men have not yet discovered what even birds know: that the atmosphere offers them swift and easy means of traveling from one part of the earth's surface to another."
"Some people have tried to build airships," remarked Rob.
"So they have; great, unwieldy machines which offer so much resistance to the air that they are quite useless.  A big machine is not needed to carry one through the air.  There are forces in nature which may be readily used for such purpose.  Tell me, what holds you to the Earth, and makes a stone fall to the ground?"
"Attraction of gravitation," said Rob, promptly.