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 from the ground by pulling at your boot straps!" laughed Pedro. "Listen, Luis. Man is subject to certain natural laws. The horse, it is true, carries more than his own weight, but by a device of Nature's own—his legs. Had you the artificial force you dream of you would have to apply it naturally. I have it! It would have to be applied to poles to push your waggon from behind!" "I hold to applying the force to the wheels," insisted Luis. "By the nature of things you would lose power," said Pedro. "A wheel is harder to force on from a point inside its circumference than when the motive power is applied to that circumference directly, as by pushing or pulling the waggon." "To relieve friction I would run my power waggon on smooth iron rails, then the loss in power would be gained in speed." "Smooth iron rails!" laughed Pedro. "Why, the wheels would slip on them. You would have to put notches all round their circumference and corresponding notches in the rails. And what would there be to prevent the power waggon slipping off the rails even then?" The boys had been walking briskly. Now a