"Dear me!" exclaimed Dorothy, rubbing her nose, which had struck something hard, although she could not see what it was; "this isn't as easy as it looks. What has stopped us, Ozma? Is it magic of some sort?"
Ozma was feeling around, her bands outstretched before her.
"Yes, dear, it is magic," she replied. "The Flatheads had to have a way from their mountain top from the plain below, but to prevent enemies from rushing up the stairs to conquer them, they have built, at a small distance before the entrance a wall of solid stone, the stones being held in place by cement, and then they made the wall invisible."
"I wonder why they did that?" mused Dorothy. "A wall would keep folks out anyhow, whether it could be seen or not, so there wasn't any use making it invisible. Seems to me it would have been better to have left it solid, 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."