The Hanging Garden was the biggest test site, an entire hilltop shaved level by Jaworski’s crew. It looked like an Aztec pyramid forty yards across at the top, but instead of a bloody altar there was a steel pad blackened by carbon and fire and attended not by priests but by a dozen draft-deferred graduate students in shorts and baseball caps. The overall litter of burned cables and broken glass gave a false impression of disorder. There was a pattern. At the outer edges were the periscopes for the flash and rotating prism cameras that would record every microsecond of a blast. Halfway to the pad were deep trenches for pressure gauges. Nearer, the buried mother cable emerged from the ground to be attached to exposed detonator cables. Almost nudging the pad was an X-ray bunker with the distinctive aluminum nose cone from which the rays would emanate to take their ghostly pictures. On the pad itself was a waist-high wooden table stamped “U.S.E.D.,” for United States Engineers Detachment, and in the middle of the table was a quarter-size model of a plutonium bomb, a 20-inch sphere with a steel shell of bright pentagonal plates bolted together at the edges. The team in baseball caps was connecting black cables to the detonator ports in each plate.

Leopold Jaworski wore suit, suspenders, a military brush of gray hair and mustache dyed as dark as arrowheads. He had soldiered against Kaiser Wilhelm of Germany, Tsar Nicholas of Russia and Marshal Pilsudski of Poland. In fact, he was the only scientist on the Hill who knew anything about war.

“You see,” he explained to Joe, “a uranium bomb is child’s play compared to this. Simply put half your uranium at one end of a barrel, half at the other end, shoot them together with gun cotton and you have your critical mass and chain reaction. But plutonium has to be brought together into a critical mass much faster with high explosive, at three thousand yards per second. Explosion is not good enough. The explosive in this device crushes and implodes a plutonium core into a critical mass.”

“That will take a lot of explosive,” Joe said, trying to sound intelligent.

“Joe, the energy released by the nuclear fission of one kilogram of plutonium is equal to seventeen thousand tons of TNT.”

Joe nodded to the model on the pad. “You don’t have a plutonium core in this, do you?”

“No.” Harvey arrived, puffing; he’d gone back down to the jeep for his clarinet, which he carried around like a riding crop. “Leo wants to blow up the table, not the mesa.”

“I used a squash ball for this test,” Jaworski said. “I assume the core in the full model will be the size of a croquet ball.”

“About,” Harvey said.