The question was still open whether more shielding would be brought up to make the overlap certain, or whether it would be best to maintain a personnel rotation policy indefinitely. Some factions on Earth seemed determined that rotation must remain not only a procedural but an actual requirement--their voices spoke plainly through the directives and edicts of U.N. Budget Control--but from what source behind this bureaucratic smokescreen it would have been difficult to say. As a heat sink, the water provided stability of temperature that would have been difficult to achieve without it. Bathed in the tenuous solar atmosphere that extends well beyond the orbit of Earth, and with a temperature over 100,000 C, maintenance of a livable temperature on board the big wheel was not the straight-forward balancing of radiation intercepted/radiation outgoing that had been originally anticipated by early writers on the subject. True, the percentage of energy received by convection was small compared to that received by radiation; but it was also wildly variable. As a biological cultural medium, the hydraulic system provided a basis for both air restoration and food supplies. When the proper balance of plankton and algae was achieved, the air jets that gave the ship its spin would also purify the ship's air, giving it back in a natural manner the oxygen it was now fed from tanks. As a method of controlling and changing the rate of rotation of the wheel, the rivers of water had already proven themselves; and as a method of static balancing to compensate for off-center weights, masses of it could be stopped and held in counterbalance tanks around the rim, thus assuring that the observatory, in its stationary position on the hub, would not suddenly take up an oscillatory pattern of motion as the balance within the wheel was shifted either by moving equipment or personnel. * * * * * In effect, the entire ship operated against a zero-M-I calculation which could be handled effectively only by the computer. The moment of inertia of the ship must be constantly calculated against the moment of inertia of the hydraulic mass flowing in the rim. And the individual counterbalance tanks must constantly shift their load according to the motions of the crew and their masses of equipment that were constantly being shifted during installation. For already the observatory was hard at work, and its time must not be stolen by inappropriate wobbles of the hub. A continuously operating feedback monitor system was capable of maintaining accuracy to better than .01% both in the mass inertial field of centrifugal force affecting the rim; and in overall balance that might otherwise cause wobbles in the hub.