Admiral Heyworth nodded comprehendingly.
"By the way," he said, "there's one thing I do not quite understand."
"Put a name to it," said John Dene tersely.
"How do you propose to keep at sea for any length of time without recharging your batteries?"
"We shall be lying doggo most of the time," was the reply.
"Then in all probability the U-boats will pass over you."
"We shan't be lying at the bottom of the sea, either," said John Dene.
"What!" exclaimed Admiral Heyworth, "but if your motor's cut off, you'll sink to the bed of the sea—the law of gravity."
"The Destroyer is fitted with buoyancy chambers, and she can generate a gas that will hold her suspended at any depth," he explained. "This gas can be liquefied in a few seconds. Her microphone will tell her when the U-boats are about; it's my own invention."
Sir Lyster looked from one to the other, unable to grasp such technicalities; but conscious that Admiral Heyworth seemed surprised at what he heard.