for then no one would have seen the entrance behind it. Now anybody can see the entrance, as we did. And prob'bly anybody that tries to go up the stairs gets bumped, as we did." 

 Ozma made no reply at once. Her face was grave and thoughtful. 

 "I think I know the reason for making the wall invisible," she said after a while. "The Flatheads use the stairs for coming down and going up. If there was a solid stone wall to keep them from reaching the plain they would themselves be imprisoned by the wall. So they had to leave some place to get around the wall, and, if the wall was visible, all strangers or enemies would find the place to go around it and then the wall would be useless. So the Flatheads cunningly made their wall invisible, believing that everyone who saw the entrance to the mountain would walk straight toward it, as we did, and find it impossible to go any farther. I suppose the wall is really high and thick, and can't be broken through, so those who find it in their way are obliged to go away again." 

 "Well," said Dorothy, "if there's a way around the wall, where is it?" 

 "We must find it," returned Ozma, and began feeling her way along the wall. Dorothy followed and began to get discouraged when Ozma had walked nearly a quarter of a mile away from the entrance. But now the invisible wall curved in toward the side of the mountain and suddenly ended, leaving just space enough between the wall and the mountain for an ordinary person to pass through. 

 The girls went in, single file, and Ozma explained that they were now behind the barrier and could go back to the entrance. They met no further obstructions. 

 "Most people, Ozma, wouldn't have figured this thing out the way you did," remarked Dorothy. "If I'd been alone the invisible wall surely would have stumped me." 

 Reaching the entrance they began to mount the stone stairs. They went up ten stairs and then down five stairs, following a passage cut from the rock. The stairs were just wide enough for the two girls to walk abreast, arm in arm. At the bottom of the five stairs the passage turned to the right, and they ascended ten more stairs, only to find at the top of the flight five stairs leading straight down again. Again the passage turned abruptly, this time to the left, and ten more stairs led upward. 

 The passage was now quite dark, for they were in the heart of the mountain and all daylight had been shut out by the turns of the passage. However, 

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