A far-fetched but, as it proved, entirely correct deduction was made by the operator of a listening-post whose curiosity was aroused by the sudden change in the nature of the conversation taking place over the enemy’s lines, familiarity interspersed with profanity abruptly giving way to studied politeness. From this he reasoned that a new division had moved in during the night. Prisoners captured the next day verified his deduction. Just before the St. Mihiel offensive one of our operators noted that the telephone conversations between the enemy units opposite his station had almost ceased, presumably because a troop movement was in progress which they did not dare to discuss for fear of being overheard, the truth being that the Germans were quietly withdrawing. Though he had practically no conversation to guide him, this by no means discouraged the American listener, who, by comparing the intensity of the T. P. S. (_telegraphie par sol_) signals he overheard, deduced with amazing accuracy the movements of the retiring troops. In comparison with such feats of deduction, Sherlock Holmes’s ability to deduce a stranger’s occupation from the condition of his finger-nails or the soles of his boots seems absurdly commonplace, doesn’t it? A youth in search of excitement beyond that usually provided by battle could always find it by joining the Listening-In Service. In March, 1918, the American troops holding a certain sector were suddenly ordered to retire to a second line of resistance, but through an oversight the orders for withdrawal were not passed on to the Signal Corps men who were operating the listening-stations out in front. Serenely unconscious, therefore, of the fact that their comrades had fallen back and that German raiding-parties were prowling all about them in the darkness, they remained at their post throughout the night. It was not until the American infantry reoccupied their original position in the morning that the men in the listening-station learned that for eight hours they had been the only occupants of the sector. While crawling over No Man’s Land to repair a break in a line connecting his station with a ground-plate, a Signal Corps man discovered a wire leading straight toward the enemy’s position. Being of an inquiring turn of mind, he followed it up on hands and knees until he actually penetrated the German trenches, where he made the interesting discovery that the enemy’s listening-station had tapped the same ground which we were using. Needless to say, he lost no time in crawling back and changing his ground-plates. This feat was paralleled by a soldier who followed an American raid into the German trenches, and, unobserved during the excitement, succeeded in attaching a wire to one of their ground-plates which was well within their lines, and, therefore, presumably in no danger of being tampered with. By this means he listened-in on the enemy’s conversations for several days before his wire was discovered and cut.