"We've got a new thing on hand in the submarine line," he told me as soon as the waiter had left the room. "You know we have been trying some experiments in German waters lately, and the Hun destroyers have been so confoundedly active that our fellows have had to pass a lot of their time sitting on the bottom. As a consequence, some of the crew have suffered terribly for want of fresh air. We have a very good system of purifying the atmosphere, but it is not sufficient owing to the long periods the boats have to stay under water, and a number of men have collapsed and died from suffocation. Indeed, one boat only escaped with more than half her crew totally incapacitated." I was keenly on the alert. Was I, I wondered, coming to grips at last with our problem? "Well," Captain A--went on, "we have been offered a new apparatus, which, if half of what the inventor tells us is true, will enable us to give the Hun a very bad time. We are assured that by its help a boat can stay under water for five days without the slightest risk." "Five days!" I repeated incredulously. "Why, it's impossible!" "So I thought," he rejoined, "but when Engstrom and Linner vouch for anything, you've got to listen." "Engstrom and Linner!" I gasped. Things were getting "warm" indeed. "Yes," he replied. "Mr Engstrom is in Paris now with his invention, and we are going to test it off Havre." Then I sprang my mine. "Would you be surprised to learn," I asked, as coolly as I could, "that your Mr Engstrom is not Mr Engstrom at all, but a German agent passing under his name?" I have seen a good many badly surprised men in my life, but I never witnessed before or since such a spectacle of hopeless astonishment as Captain A--presented when he grasped the full significance of this announcement. He sat staring at me, his mouth wide open, and with dismay written legibly on every line of his countenance. "But, Sant," he gasped. "Are you sure? Mr Engstrom came to us in London and told us all about it. He explained that the inventor was a Spaniard who would not trust the `neutrality' of the Spanish Government in the matter, and that he had brought his invention to Engstrom's with the idea of getting the best terms from one of the Allies." "I have no doubt that the man posing as Engstrom came to you," I replied. "But, none the less, he is not Engstrom at all." "Then what is his game?" countered A--. "He has offered us the fullest test before we adopt his machine, and has not asked for a cent." "That remains to be seen," I answered, "but it bodes no good to the Allies. What does he propose?" "He has offered to instal the apparatus on one of our newest types," replied A--, "and she is on her way to Havre for the purpose. We are to make any test we like, and, in fact, I am here to see the test carried out. The only condition he makes is that his machinery shall be sealed, and not opened until after the test has proved it to be satisfactory."