There came into the Union lines a Negro from a farm on the other side of the river, known by the name of Dabney, who was found to possess a remarkably clear knowledge of the topography of the whole region; and he was employed as a cook and body servant at headquarters. When he first saw our system of army telegraphs, the idea interested him intensely, and he begged the operators to explain the signs to him. They did so, and found that he could (readily) understand and remember the meaning of the various movements. …
Not long after, his wife, who had come with him, expressed a great anxiety to be allowed to go over to the other side as a servant to a "secesh woman," whom General Hooker was about sending over to her friends. The request was granted. Dabney’s wife went across the Rappahannock, and in a few days was duly installed as laundress at the headquarters of a prominent General. Dabney, her husband, on the north bank, was soon found to be wonderfully well informed as to the rebel plans. Within an hour of the time that a movement of any kind was projected, or even discussed, among the rebel generals, Hooker (the Union general) knew all about it. He knew which corps was moving, or about to move, in what direction, how long they had been on the march, and in what force; and all this knowledge came through Dabney, and his reports always turned out to be true.
Yet Dabney was never absent, and never talked with the scouts, and seemed to be always taken up with his duties as cook and groom about headquarters.
How he got his information remained for some time a puzzle to the Union Officers. At length, upon much solicitation, he unfolded his marvelous secret to one of the officers.
Taking him to a point where a clear view could be obtained of Fredericksburg, he pointed out a little cabin in the suburbs near the river bank; and asked him if he saw that clothes-line with clothes hanging on it to dry. "Well" said he, "that clothes line tells me in half an hour just what goes on at Lee’s headquarters. You see my wife over there; she washes for the officers, and cooks, and waits around, and as soon as she hears about any movement or anything going on she comes down and moves the clothes on that line so I can understand it in a minute. That there gray shirt is (Confederate general) Longstreet; and when she takes it off, it means he’s gone down about Richmond. That white shirt means (Confederate general) Hill; and when she moves it up to the west end of the line, Hill’s corps has moved upstream. That red one is (Confederate general) Stonewall. He’s down on the right now, and if he moves, she will move that red shirt."
One morning Dabney came in and reported a movement over there. "But" says he, "it don’t amount to anything. There’re just making believe."
An officer went out to look at the clothes-line telegraph through his field-glass. There had been quite a shifting over there among the army flannels. "But how do you know that there is something in it?"
"Do you see those two blankets pinned together at the bottom?" said Dabney. "Yes, but what of it?" said the officer. "Why, that her way of making a fish-trap’; and when she pins the clothes together that way, it means that Lee is only trying to draw us into his fish trap."
As long as the two armies lay watching each other on opposite banks of the stream, Dabney, with his clothes-line telegraph, continued to be one of the most prompt and reliable of General Hooker’s scouts.