A message from Burke was waiting for Kennedy at the wharf.  He read it quickly, then handed it to Captain Shirley and myself.

Have just received a telegram from Washington.  Great excitement at the embassy.  Cipher telegram has been despatched to the Titan Iron Works.  One of my men in Washington reports a queer experience.  He had been following one of the members of the embassy staff, who saw he was being shadowed, turned suddenly on the man, and exclaimed, “Why are you hounding us still?” What do you make of it?  No trace yet of Nordheim

Burke.

The lines in Craig’s face deepened in thought as he folded the message and remarked abstractedly, “She works all right when you are aboard.”  Then he recalled himself.  “Let us try her again without a crew.”

Five minutes later we had ascended to the aerial conning-tower, and all was in readiness to repeat the trial of the night before.  Vicious and sly the Z99 looked in the daytime as she slipped off, under the unseen guidance of the wireless, with death hidden under her nose.  Just as during the first trial we had witnessed, she began by fulfilling the highest expectations.  Straight as an arrow she shot out of the harbour’s mouth, half submerged, with her periscope sticking up and bearing the flag proudly flapping, leaving behind a wake of white foam.

She turned and re-entered the harbour, obeying Captain Shirley’s every whim, twisting in and out of the shipping much to the amazement of the old salts, who had never become used to the weird sight.  She cut a figure eight, stopped, started again.

Suddenly I could see by the look on Captain Shirley’s face that something was wrong.  Before either of us could speak, there was a spurt of water out in the harbour, a cloud of spray, and the Z99 sank in a mass of bubbles.  She had heeled over and was resting on the mud and ooze of the harbour bottom.  The water had closed over her, and she was gone.

Instantly all the terrible details of the sinking of the Lutin and other submarines flashed over me.  I fancied I could see on the Z99 the overturned accumulators.  I imagined the stifling fumes, the struggle for breath in the suddenly darkened hull.  Almost as if it had happened half an hour ago, I saw it.

“Thank God for telautomatics,” I murmured, as the thought swept over me of what we had escaped.  “No one was aboard her, at least.”

Chlorine was escaping rapidly from the overturned storage batteries, for a grave danger lurks in the presence of sea water, in a submarine, in combination with any of the sulphuric acid.  Salt water and sulphuric acid produce chlorine gas, and a pint of it inside a good-sized submarine would be sufficient to render unconscious the crew of a boat.  I began to realise the risks we had run, which my confidence in Captain Shirley had minimised.  I wondered whether hydrogen in dangerous quantities might not be given off, and with the short-circuiting of the batteries perhaps explode.  Nothing more happened, however.  All kinds of theories suggested themselves.  Perhaps in some way the gasoline motor had been started while the boat was depressed, the “gas” had escaped, combined with air, and a spark had caused an explosion.  There were so many possibilities that it staggered me.  Captain Shirley sat stunned.