But this, we found, was only a part of the plot--perhaps even the least important part. Tegelmund, finding himself trapped, turned craven and revealed the whole story. The real object of the spies was to get the fullest possible details of the internal arrangements of a British submarine of the latest type, and how well they had succeeded was shown when we cast our net a little wider. Directly Engstrom and Tegelmund were in custody, an innocent-looking signal flag flew from the masthead of the submarine, and the officials of the Surete ashore made their pounce. Thornton and Halbmayr were seized at once at their hotel, and in their possession we found a wonderful series of drawings in which many of the secrets of the submarine were fully explained. A telegram to Paris brought about also the arrest of Madame Bohman, and a few days later the German agents were safely immured in the convict prison at Tours, where they were sent by the sentence of a court-martial summoned immediately to deal with their case. Their guilt in this particular case was too clear for any possibility of denial, but I am glad to say that their arrest opened up a way to us to deal the Hun Secret Service a blow from which it has never fully recovered. Enormous piles of documents were seized and carefully examined, with the result that numerous associates of Thornton in England found themselves in durance vile "for the duration," and so many fingers of the Hidden Hand were lopped off that the hand itself was badly crippled for many months to come. How the fingers grew and were again cut off I hope to tell at some later date. CHAPTER EIGHTEEN. THE MYSTERIOUS CYLINDERS. After over two years of strenuous work without a holiday I found myself at length free, and I found myself one morning busy in my rooms in Curzon Street making final arrangements for a trip to Worcestershire to spend a fortnight with Doris and her mother in their lovely country home. I was jaded and fagged, for I do not mind confessing that my work recently had considerably affected me, and I was looking forward with eager anticipation to the delights of a stay in the country. I had not seen Doris for some months, though of course we were in constant communication, and I was naturally longing for a sight of her. But I was destined to another disappointment. Just as I was finishing my packing the telephone rang. I found the call was from Morgan, one of the ablest of the Government Experts on Explosives, and he had a curious story to tell me. When I had listened to what he had to say I realised with a heavy heart that my long-promised holiday must be again postponed. I rang up Doris on the telephone and, having broken the news to her, hurried off to Morgan's office. I found the expert in a state of utter bewilderment. He was an acknowledged authority on explosives, but a problem had been set before him which had baffled him completely.