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thrilled.
    “I’m so glad we saw that,” he said. “Do you know, I’ve been designing them, and writing letters about them, and giving advice about them for a long time and this is the first time I’ve actually seen one!”
    Good God, Derek said to himself.
    “How stable are these boats in rough weather?” the Wavemaker asked.
    “Pretty good. The fin keeps them more or less dry, not like the older boats with low towers. The stability has to be pretty carefully worked out, of course. We do a trim dive in the dockyard basin after every refit. Occasionally they make a mistake. One boat I went to sea in very nearly capsized. We heeled over to about fifty degrees and stayed there. I thought we’d all had it.”
    “Of course,” said the Wavemaker, “in a case like that we’ve got to differentiate between actual danger , and mere discomfort .”
    Derek ground his teeth and repressed an almost overwhelming urge to howl out loud.
    “Now, is there anything else, gentlemen?”
    The Senior Scientist looked sheepish.
    “I wonder. . . .”
    “Yes, sir?”
    “I wonder. ... It seems silly but ... I wonder if you could explain something I’ve always been puzzled about. . . .”
    “Yes, sir?”
    “How exactly does a submarine dive?’’
    “Well sir, all along the outside of the boat we’ve got a row of very large tanks, called main ballast tanks. They’re open to the sea at the bottom and closed at the top by very large valves, called main vents. When we open the main vents, the sea rushes in at the bottom and the air rushes out at the top, the submarine in effect shrinks in volume, displaces less water and therefore becomes heavier and therefore sinks. When we want to come up again we shut the main vents and blow the water out with compressed air. That makes the boat sort of swell again, displaces more water, become in effect lighter, and up she comes again. All done by Archimedes’ principle, sir.”
    “Archimedes?”
    “You remember the chap, sir,” said the Wavemaker. “He lived in a barrel.”
    “Ah yes,” said the Senior Scientist.
    In the control room, the Schools Liaison Officer was explaining technical matters to a crowd of schoolboys. Keep it simple, The Bodger had said. Dagwood began his address on first principles.
    “These levers raise and lower the periscopes, and these open and shut the main vents. The main vents are. . . .”
    “Solenoid-operated, I suppose?” said a treble voice, casually.
    “Huh?” Dagwood was thrown out of his stride. “As a matter of fact, they are. This is the starter for the L.P. Blower. . .”
    “It puts the final bit of air into the ballast tanks after surfacing,” said another treble voice confidently. “Naturally you wouldn’t use air from the bottles for all of it. You would use only enough to get you to the surface. H.P. Air is too precious in a submarine.”
    Dagwood felt the hair on the back of his neck prickle with the first cold feeling of foreboding.
    “Quite right,” he said. “Now this. . .”
    “The Germans used to use the exhaust gases from the engine starting instead.”
    “ Did they?” said Dagwood.
    “Yes.”
    A very small boy whose face was almost entirely extinguished by hair and by an enormous blue school cap said: “What would you do if the submarine began to drop towards the bottom, sir?”
    'Dagwood thought rapidly.
    “I would go hard a port, or hard a starboard, and full astern. That would tend to bring the bows up.”
    “And if that didn’t work, sir?”
    “Blow the forrard main ballast tank.”
    “And if that didn’t work?”
    “Blow all main ballast tanks.”
    “And if that didn’t work?”
    Dagwood had by now the attention of everyone in the control room; there was a hush as they waited for his answer.
    “That would work all right,” he said finally. But he did not feel that he had convinced anybody.
    Far aft in the after torpedo space, Leading Seaman Miles, the torpedo rating in charge of the compartment, was being