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.
"Exactly. That is one force I refer to," said the Demon. "The force
of repulsion, which is little known, but just as powerful, is another
that mankind may direct. Then there are the Polar electric forces,
attracting objects toward the north or south poles.
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