A croquet-sized ball of plutonium coated in glittering nickel was the Dragon, the core of the bomb. It nestled snugly in a twenty-inch paraffin bowl on top of a hydraulic piston. Over it, suspended facedown by a chain, was a second, hollow bowl of paraffin. The idea was to check whether an outer sphere of high explosive lenses would, simply by being in place around a nearly critical core, reflect enough neutrons for the plutonium to go critical and explode prematurely and relatively ineffectively. The paraffin was mixed with sooty-gray carbon flour so that it had basically the same atomic makeup as high explosive, without the risk, in case of mishap, of wiping out the eastern end of the canyon.

Only Harvey, Joe and Oppy were in Omega. Harvey’s usual Critical Assembly team was scouting Trinity, and Fermi’s team refused to be near the hangar when the Dragon’s Tail was being tickled. Harvey had protested that at least two physicists had to be present for the experiment, and Oppy had answered that while General Groves had done his best to convert him into an administrator, he was still a physicist. Oppy especially insisted that Joe stay and push the red and green hydraulic buttons on the wall that raised the lower bowl holding the Dragon up to the hanging hollow bowl.

Drawn up to the Dragon on steel tables were a Geiger counter, a neutron scaler that measured radiation with a bank of six red lights, and another radiation graph that drew a red line on rolling paper. If the Dragon got too hot and Joe didn’t react in time, the three counters were wired to drop the hydraulic piston, lower bowl and core to the floor.

Wearing a long white lab coat, Harvey plotted the curve of criticality with a slide rule and clipboard. “Raise it ten inches,” he said.

Joe pushed the green button as Oppy continued the argument that had gone on all morning.