The three days went by. Four great cases of machinery had been duly shipped on board, and Engstrom, Thornton, and Tegelmund spent many hours daily at their work in the interior of the submarine. Of course I could not appear--I should have been recognised at once--but among the crew of the submarine were a couple of the smartest men of the Surete, who kept the bogus engineer and his associates under the closest scrutiny. They reported to me that Engstrom appeared to be the only one of the three with any great amount of mechanical knowledge, and that, while Tegelmund worked assiduously at his machine, the others spent most of their time carefully examining the details of the British vessel, in which they showed the greatest interest. I began to get at last an inkling of the plot! The fourth day dawned--the day of the _denouement_. Early in the morning I slipped on board the submarine, and when the two conspirators arrived we made our coup. Engstrom, when he came on board with Tegelmund, found himself suddenly confronted by the Commander, with a stalwart bluejacket standing on either side of him. He was curtly informed that he could not go below. "But you promised!" he shrieked, livid with vexation. "True!" said the Commander. "But you call yourself Oscar Engstrom, of Malmo, and I happen to have the real Mr Engstrom here." The engineer went white to the very lips as Mr Engstrom, who had come post-haste from Stockholm in response to my urgent cable, emerged from behind the conning-tower, closely followed by myself. The false Engstrom began a vehement protest, but ceased suddenly, for, glancing round, he saw Tegelmund also under guard. The game was up! A few minutes later, with Engstrom and Tegelmund safely in irons, the Admiralty experts who had come over from London began a minute examination of the wonderful "invention." They soon discovered that the cases contained a jumble of wires and odds and ends of mechanical scraps simply thrown together to look complicated, and of no value whatever for the renewal of vitiated air. The real object was only revealed when we had got to the very heart of the amazing collection of rubbish. There, cunningly hidden among much that was superfluous, was a highly efficient electric motor, wonderfully made and controlling a powerful bomb by machinery, set to detonate the explosive after six hours' running. The machinery was to have been operated by the electric batteries of the submarine, and had the E77 gone to sea and begun the "tests" of the bogus apparatus, not a vestige of the vessel or the crew would have been seen again, and the secret of her loss would have been locked for ever in the depths of the Atlantic.