When the box had filled itself to the level of the rest of the sphere, the acrobat drew it slowly out of the sphere and brought it to rest again, so that its base was placed perhaps five microns above the surface. Now the blue Air slid over the sides and, in a thin stream which poured from the base of the vessel, returned eagerly to the central sphere.

The acrobat troupe maintained this display at all hours of the day, at quite remarkable expense. Adda watched the cycle through a couple of times, his good eye empty of expression.

Muub watched him surreptitiously, then shook his head. "Don't you have any interest in this? Even your eye-leech is showing more awareness, man!" He felt driven, absurdly, to justify the display. "The Fount is demonstrating superfluidity. When the vessel is lowered into the pool, a thin layer of the fluid is adsorbed onto the vessel's surface. And the Air uses that fine layer — just a few neutrons thick — to gain access to the interior of the vessel. When the vessel's withdrawn the Air uses the same channel to return to the main bulk, the sphere. Quite remarkable.

"The hoops maintain a slight magnetic gradient from the geometric center of the cylinder. That gradient restricts the residual Air to that sphere at the center... and it is the resulting difference in electromagnetic potential energy which drives the cycle of the fountain. And..."

"Riveting," Adda said drily.

Muub bit back a sharp comment. "Well, I know you people have different priorities in life. Let's view the rest of the Garden... perhaps some of it will remind you of the world you have left behind. I'm curious as to how you lived, actually."

"We upfluxers?" Adda asked acidly.

Muub replied smoothly, "You Human Beings. For example, superfluidity... Have you retained much knowledge of such matters?"

Adda said, "Much of the lore absorbed by our children is practical and everyday... how to repair a net; how to keep yourself clean; how to turn the battered corpse of an Air-pig into a meal, a garment, a source of weapons, a length of rope."

Muub felt himself shudder delicately.

"But knowledge is our common heritage, City man," Adda murmured. "We would scarcely allow you to rob us of that, as you robbed us of our place here ten generations ago."

Turning, Muub led Adda slowly away from the Fount. Beside the youthful grace of the acrobats, Adda's ungainly stiffness was laughable — and yet heartbreaking, Muub thought. They passed through one of Hork's experimental ceiling-farm areas. Here a new strain of wheat — tall and fat-stemmed — thrust from a simulated section of Crust-forest root-ceiling.

"Tell me, Adda. What are your plans now?"

"Why should you care?"

"I'm curious."

Adda was silent for a while; then, grudgingly, he replied: "I'm going to go back. Back to the upflux. What else?"

"And how do you propose to achieve that?"