The Major said briefly: “Tell me.”
Joe said: “All the tricks but one, that were used on the plane I came on, were the same kind of trick. They were all arrangements for getting regular destructive items—bombs or rockets or whatever—where they could explode and smash things. The saboteurs were adding destructive items to various states of things. But there was one trick that was different.”
“Yes?” said the Major, on the telephone.
“Putting explosive gas in the CO2 bottles,” said Joe painstakingly, “wasn’t adding a new gadget to a situation. It was changing something that was already there. The saboteurs took something that belonged in a plane and changed it. They did not put something new into a plane—or a situation—that didn’t belong there. It was a special kind of thinking. You see, sir?”
The Major, to do him justice, had the gift of listening. He waited.
“The pushpots,” said Joe, very carefully, “naturally have their fuel stored in different tanks in different places, as airplanes do. The pilots switch on one tank or another just like plane pilots. In the underground storage and fueling pits, where all the fuel for the pushpots is kept in bulk, there are different tanks too. Naturally! At the fuel pump, the attendant can draw on any of those underground tanks he chooses.”
The Major said curtly: “Obviously! What of it?”
“The pushpot motors explode,” said Joe. “And they shouldn’t. No bomb could be gotten into them without going off the instant they started, and they don’t blow that way. I make a guess, sir, that one of the underground storage tanks—just one—contains doctored fuel. I’m guessing that as separate tanks in a pushpot are filled up, one by one, one is filled from a particular underground storage tank that contains doctored fuel. The rest will have normal fuel. And the pushpot is going to crash when that tank, and only that tank, is used!”
Major Holt was very silent.
“You see, sir?” said Joe uneasily. “The pushpots could be fueled a hundred times over with perfectly good fuel, and then one tank in one of them would explode when drawn on. There’d be no pattern in the explosions....”