One more patrol story must be added--a story in some ways unique, with mysterious details which haunt the imagination, but can never be finally explained. The vessels of the patrol on this occasion were not yachts, or P-boats in the strict sense of the classification. One was the _Sarba_, an armed trawler like those we have already met, and commanded by Lieutenant George G. Astbury, R.N.R.; the other was a small boat, with no name but T.B. 055, commanded by Gunner T. H. Britton. On the morning of October 31, 1917, T.B. 055 was accompanying the trawlers who were engaged in sweeping an important channel outside a British harbour. At 3.0 P.M. when the sweep was practically over, Mr. Britton noticed an oil track on the surface of the channel. This was in itself an astonishing sight, and not to be accounted for in a moment. How could a submarine have ventured into a channel only thirteen fathoms deep, and daily swept by a highly efficient force of trawlers? And for what possible reason could she be lying there on the bottom at 3 o’clock in the afternoon, in a position where she could use none of her weapons, and was certain to be found and attacked? Mr. Britton went into the oil track to investigate; stopped his boat and listened on the hydrophone. His astonishment was redoubled--the submarine was there, and not only there, but busy and audible. The case was so extraordinary that he and his trained hydrophone listener took counsel together and classified the sounds they heard. First there were the usual ‘water noises’; these were continuous and perfectly familiar. Secondly, there was an almost continuous high-pitched sound, somewhat similar to that of a turbine engine running. Thirdly, at intervals of a few seconds, came a noise as of knocking or hammering upon metal; the speed of the tapping varied from slow to fairly rapid blows. Lastly--and this was the most unexpected and mysterious of all--on two occasions there was audible, over all the other noises, a sound as of wireless letters on a high musical note. For three minutes these sounds were heard, noted, and compared. T.B. 055 was then taken forward about 200 yards, to the end of the oil track, and the hydrophone was used again. Precisely the same sounds were heard, except that this time the musical note, as of a wireless message, was not repeated. Mr. Britton had no desire to lose time; but he was not troubled with nerves, and he was determined to make sure of his evidence. He took precautions to stop all ship’s noises. The fact only became clearer that the sounds below came from a live submarine. What her crew were doing no one could know; but she was there for an evil purpose, and she must pay the penalty.