After ordering the men to empty their pockets, they marched them downstairs, and through the door connecting the back part of the house with the officers’ billets. Captain Adams, like the others, had gone to bed. He came to the door of his room in his pyjamas. "We’ve caught Obernai and three others signalling with a lamp, sir," said Kenneth. "You don’t say so! What have you done with them?" "They are below, sir." "Take them off to the provost-marshal: I don’t want to see them." "Stoneway is in it, sir, I am sorry to say." "Arrest him, as quickly as you can. Then come back and tell me all about it." The spies were marched off to prison. Then Ginger with a corporal’s guard went to the cottage where Stoneway was billeted. Stoneway was not there. Enquiry and search were alike fruitless. It was not until an hour later that Ginger hit on a possible explanation of his absence. "By jinks!" he exclaimed, with a gesture of vexation. "I forgot the old woman." He hastened back to Obernai’s house. The old woman had disappeared. On returning to the house some time before, Kenneth and Harry found the officers, all in their night attire, examining the signalling apparatus in the upper room. "They are all safely locked up, sir," Kenneth reported. "That’s well. How did you catch them?" Kenneth gave an account of the night’s work. "You did very well, Amory," said the captain. "The battalion is lucky in having the Three Musketeers. And the whole brigade is indebted to you. This is a fiendishly ingenious arrangement." He explained the working of the apparatus. The acetylene lamp faced one end of a long tube, which pierced the outer wall of the house. By means of a delicate mechanism the position of the tube could be altered by millimetres. The length of the tube prevented the rays from converging like the rays of a searchlight, so that the light, directed eastward, was not likely to be seen except by a person at an equal height. "I have no doubt at all," said the captain, "that some miles away in the German lines there is an operator with a similar lamp, at the same height and in the same straight line with this. We have kept a look-out but seen nothing; no doubt the cessation of the flashing gave them warning. To them the light would appear like a star on the horizon, and the alternate exposure and dousing of it by means of the disc made the signals. No wonder we’ve got it unexpectedly hot sometimes." Here Ginger came in. "Stoneway’s got away, sir," he reported. "I guess the old woman gave him the tip." "Poor wretch! He can’t get far. I’ll circulate the news at once and he’ll be hunted down. Now get to your billets, men; I shall want your evidence in the morning." As they were returning through the silent streets, talking over the exciting incidents of the night, Kenneth suddenly exclaimed: "By George! I remember now. That fellow was the man I saw talking French to Stoneway at St. Pancras station."