Wilville and Orbur had already begun to mark out the outlines of the boat with stakes and twine. It looked like a large flat-bottomed barge.

“No, no!” screamed Purple, when they explained to him.

“It should be narrower, and it should have a keel, like so!”

Put away the blue-drawings,” I insisted. “We don’t need them.”

After he calmed down, we began again — this time at the beginning. Wilville and Orbur moved the stakes in to form a narrower outline. They shook their heads. “What will keep it from capsizing?” they asked.

“Outriggers, we will have outriggers,” Purple explained that the boat should have narrow pontoons, held out like so from the sides.

Then what will keep the thing level when it is suspended in the air?”

“A keel, of course — a heavier beam of wood at the bottom of the hull.”

“But if it is heavier, won’t it weigh down the boat too much?”

He considered that. “You may be right. If it does, we may have to add another gasbag.”

To tell the truth, I didn’t understand much of the discussion. It began to get too technical for me — but once Wilville and Orbur started to understand what Purple meant they began discussing the project in excited terms. The three of them argued happily back and forth, Wilville and Orbur nodding and gesticulating with every new idea.

Indeed, at one point they began scratching diagrams in the dirt in order to help them understand. When they did this, Purple tried to bring out his blue-drawings again, but they rejected them as having little or no relevance at all to the project. It was the dirt-drawings which were necessary to the construction of the device.

Obviously my sons understood what needed to be built and how to do it. The why of it sometimes eluded them, but Purple was willing to explain. Several times the boys suggested alternative and better ways — especially when the discussion turned to how they would rig the gasbags to the boat frame.

“Why not sew up just one very big bag as large as all the others?” Orbur asked.

Purple held up the hem of his robe of office in two hands and gave it a yank. It did not rip, but the weave parted easily. It looked like a piece of strainer cloth. “If all I have is one bag and this happens,” said Purple, “then I am marooned at sea, or even high in the air! But if I have many bags and this happens, I can only lose one at a time.”

Orbur nodded excitedly. “Yes, yes, I see. I see.” They turned back to the problem of rigging the boat with a variable number of gasbags.

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