The Fountains of Paradise

He had seen that two-century-old film at least fifty times, and there were sections that he had examined frame by frame, until he knew every detail by heart. It was, after all, the most expensive movie footage ever shot, at least in peacetime. It had cost the State of Washington several million dollars a minute.

There stood the slim (too slim!) and graceful bridge, spanning the canyon. It bore no traffic, but a single car had been abandoned midway by its driver. And no wonder, for the bridge was behaving as none before in the whole history of engineering.

It seemed impossible that thousands of tons of metal could perform such an aerial ballet; one could more easily believe that the bridge was made of rubber than of steel. Vast, slow undulations, metres in amplitude, were sweeping along the entire width of the span, so that the roadway suspended between the piers twisted back and forth like an angry snake. The wind blowing down the canyon was sounding a note far too low for any human ears to detect, as it hit the natural frequency of the beautiful, doomed structure. For hours, the torsional vibrations had been building up, but no-one knew when the end would come. Already, the protracted death-throes were a testimonial that the unlucky designers could well have foregone.

Suddenly, the supporting cables snapped, flailing upwards like murderous steel whips. Twisting and turning, the roadway pitched into the river, fragments of the structure flying in all directions. Even when projected at normal speed, the final cataclysm looked as if shot in slow motion; the scale of the disaster was so large that the human mind had no basis of comparison. In reality, it lasted perhaps five seconds; at the end of that time, the Tacoma Narrows Bridge had earned an inexpungable place in the history of engineering. Two hundred years later there was a photograph of its last moments on the wall of Morgan's office, bearing the caption "One of our less successful products".

To Morgan that was no joke, but a permanent reminder that the unexpected could always strike from ambush. When the Gibraltar Bridge was being designed, he had gone carefully through von Kбrmбn's classic analysis of the Tacoma Narrows disaster, learning all he could from one of the most expensive mistakes of the past. There had been no serious vibrational problems even in the worst gales that had come roaring in from the Atlantic, though the roadway had moved a hundred metres from the centre line precisely as calculated.

But the space elevator was such a leap forward into the unknown that some unpleasant surprises were a virtual certainty. Wind forces on the atmospheric section were easy to estimate, but it was also necessary to take into account the vibrations induced by the stopping and starting of the payloads – and even, on so enormous a structure, by the tidal effects of the sun and moon. And not only individually, but acting all together; with, perhaps, an occasional earthquake to complicate the picture, in the so-called "worst case" analysis.

"All the simulations, in this tons-of-payload-per-hour regime, give the same result. The vibrations build up until there's a fracture at around five hundred kilometres. We'll have to increase the damping – drastically."

"I was afraid of that. How much do we need?"

"Another ten megatons."

Morgan could take some gloomy satisfaction from the figure. That was very close to the guess he had made, using his engineer's intuition and the mysterious resources of his subconscious. Now the computer had confirmed it; they would have to increase the "anchor" mass in orbit by ten million tons.

Even by terrestrial earth-moving standards, such a mass was hardly trivial; it was equivalent to a sphere of rock about two hundred metres across. Morgan had a sudden image of Yakkagala, as he had last seen it, looming against the Taprobanean sky. Imagine lifting that forty thousand kilometres into space! Fortunately, it might not be necessary; there were at least two alternatives.

Morgan always let his subordinates do their thinking for themselves; it was the only way to establish responsibility, it took much of the load off him – and, on many occasions, his staff had arrived at solutions he might have overlooked.

"What do you suggest, Warren?" he asked quietly.

"We could use one of the lunar freight launchers, and shoot up ten megatons of moon-rock. It would be a long and expensive job, and we'd still need a large space-based operation to catch the material and steer it into final orbit. There would also be a psychological problem –"

"Yes, I can appreciate that; we don't want another San Luiz Domingo –"