Citizens wishing to make contact with those of another species used ultralong-range aircraft or intra-plane-tary spaceships. Such contacts were encouraged, but only if they did not involve the risk of individual or species invasion of privacy. So touchy were some of the cultures that the stratosphere of the entire Federation World had been rendered opaque so that Citizens would not be able to watch each other through telescopes by looking upward and across their hollow superplanet.

The vast structure was a superthin eggshell of metal which was ultra-hard and fantastically dense, and the soils and atmospheres covering its inner surface were synthetic and produced when population additions required them. Where the atmospheres of adjoining species were mutually toxic, one-hundred-mile-high walls, transparent in the higher altitudes so as not to interfere with the sunlight, separated the two areas. These were similar to the walls which encircled the five-hundred-mile entry ports, positioned at the points of the conical polar extensions, to keep the atmosphere from being lost into extraplanetary space.

In the gravity-free polar areas were the great, thou-sand-mile-square tracts of bare metal on which were built the atmosphere and soil synthesizers, the matter transmitter units, searchship building and maintenance docks, and the power sources for the long-range projectors which could turn aside or destroy any astronomical body large enough to endanger the giant sphere by collision.

All at once the awful immensity of the Federation World, the incredibly high level of knowledge which had created and maintained it, and the inutterable pettiness of his suspicions made Martin want to run away and hide himself. He felt like an aboriginal grasshut dweller suddenly confronted with a block of skyscraper offices and Rush-hour traffic.