But now there was an answer. From the Bay of Biscay red lights with black dots on them began to wink on as the mammoth tabulating machine in the room below recorded the information from thousands of hidden electronic tubes, totaled it, and presented it on the Map. The President hardly watched them. His screen with its principal power-plant in Philadelphia would stop the rockets, up to a total of some seventy-five octillion macro-ergs. On the off chance that Forgacs would forget to close his screen after his rockets had passed it, Jeffrey fired a salvo from the Bahamas. Forgacs answered with three salvos from Brest, and Jeffrey gave him back ten from Long Island, then Hoshawk frowned as he saw the President rub his stomach. Hoshawk had always opposed that abominable atavistic confection called ice cream. It was a game of incredibly swift calculation and rapier thrusts from strong point to strong point in the effort to break through the screen. Once the screen should be broken, anything might happen. * * * * * Jeffrey could see when his own screen was up, but their science had devised no way to detect the enemy's screen except by firing into it. Jeffrey pressed a pedal with his left foot, and a thin golden line flashed on in a flattened arc from Greenland down through the Atlantic and curved around the Falkland Islands. Jeffrey's screen was up. The Biscay salvos began to wink out against it. Jeffrey's hands began to flash. Red lights winking up along the coast of Europe and from North Africa showed that Forgacs was opening up. Jeffrey cut in the oxygen for a second and flicked it off, then his left foot slashed at the pedal as he cut his screen to let his own rockets through and then threw it on again to stop the enemy. Forgacs was beginning a drive on Philadelphia, the site of the power plant. Jeffrey was watching for an opening to Marseilles, vulnerable for the same reason. Jeffrey kept firing rockets, but his mutant mind would be racing ahead, calculating with infinite precision the times of discharge and times of arrival. It was apparent by now that Forgacs' most powerful defenses were centered around Marseilles, because Forgacs was not using them. This meant he was not taking a chance on opening the Marseilles sector of the screen. Jeffrey calculated the probable interchange of batteries for some sixty moves ahead, Hoshawk knew, then he began to fire the Philadelphia batteries at intervals. The firing rose in intensity, and Jeffrey's faster-than-light fingers played the great keyboard like a master organ. A bell sounded and his right foot threw on the western screen with its automatic cut-out. And all the time Jeffrey fired his big Philadelphia batteries at intervals with a definite rhythm--five, three, and six seconds. He shot to the right and manipulated a bank of keys and was back in the center almost instantaneously.