Von Loringhoven confessed that he did not. In spite of a careful perusal of all works dealing with numbers and nomenclature of British shipping--and Berlin was kept fairly up-to-date in such matters--the mystic symbol "X 5" was to him an unknown quantity. Incidentally it recalled days when he was studying mathematics at the Kiel Naval College. The ober-leutnant steadied the periscope and touched a switch. Immediately, by the introduction of a special lens, the "field" covered by the eye-piece of the periscope was reduced, but the object actually seen was considerably magnified. It was like looking through a telescope. "They are men of the English Air Force," he observed. "I believe--here, Eitel, look--the man walking for'ard. What do you make of him?" "_Donnerwetter!_" ejaculated von Loringhoven. "Surely it is our friend von Preussen?" "Yes," replied the ober-leutnant. "Von Preussen playing the part of a Jonah to an English whale. I wonder what he does there?" "It would be well to clear out and leave him alone, Herr Kapitan," suggested von Loringhoven. "It could only be that von Preussen is engaged in highly important confidential work that brings him afloat again. _Himmel!_ He is a clever fellow." The ober-leutnant tugged at his moustache thoughtfully. Eager to have a finger in any pie without the risk of burning himself, he was loth to take his subordinate's advice. Here, apparently, was an unarmed craft, crewless, with the exception of a few officers. To him it suggested that highly confidential experiments were being carried on--so important that no one beneath the rank of officer was permitted to be present. Perhaps they were staff officers of high rank? Eagerly von Preugfeld kept each man under observation. The trench-coats gave no indication of their wearers' rank, but --disappointing fact--none of the officers wore gilt leaves round the peaks of their caps. The sailing-boat alongside was also a puzzle. Why should the experimenters make use of an insignificant sailing-boat when there were steam pinnaces and motor launches available? "Stand by!" he ordered. "Guns' crews prepare to take your stations. Blow main and auxiliary tanks." Bells clanged, valves hissed and pumps grated, men hurried to and fro in execution of loud-voiced orders. Von Preugfeld turned to his unter-leutnant. "Bring her up," he ordered. "I am going to take those fellows prisoners." CHAPTER XII PRISONERS OF WAR "WHAT in the name of goodness is that?" exclaimed Captain Cumberleigh. He knew perfectly well. The sight of a slender pole inclined slightly from the perpendicular and throwing out a double feather of spray as it cleft the water told him that it was the periscope of a submarine.