The most conspicuous feature of the convoy room was a huge chart, entirely covering the wall on one side of the office; access to this chart was obtained by ladders not unlike those which are used in shoe stores. It gave a comprehensive view of the North and South American coast, the Atlantic Ocean, the British Isles, and a considerable part of Europe and Africa. The ports which it especially emphasized were Sydney (Cape Breton), Halifax, New York, Hampton Roads, Gibraltar, and Sierra Leone and Dakar, ports on the west coast of Africa. Thin threads were stretched from each one of these seven points to certain positions in the ocean just outside the British Isles, and on these threads were little paper boats, each one of which represented a convoy. When a particular convoy started from New York, one of these paper boats was placed at that point; as it made its way across the ocean, the boat was moved from day to day in accordance with the convoy's progress. At any moment, therefore, a mere glance at this chart, with its multitude of paper boats, gave the spectator the precise location of all the commerce which was then _en route_ to the scene of war. But there were other exhibits on the chart which were even more conspicuous than these minute representations of convoys. Little circles were marked off in the waters surrounding the British Isles, each one of which was intended to show the location of a German submarine. From day to day each one of these circles was moved in accordance with the ascertained positions of the submarine which it represented, a straight line indicating its course on the chart. Perhaps the most remarkable fact about the Allied convoy service was the minute information which it possessed about the movements of German submarines. A kind of separate intelligence bureau devoted its entire attention to this subject. Readers of detective stories are familiar with the phenomenon known as "shadowing." It is a common practice in the detective's fascinating profession to assign a man, known as a "shadow," to the duty of keeping a particular person under constant observation. With admirable patience and skill an experienced "shadow" keeps in view this object of his attention for twenty-four hours; he dogs him through crowded streets, tracks him up and down high office buildings, accompanies him to restaurants, trolley cars, theatres, and hotels, and unobtrusively chases him through dense thoroughfares in cabs and automobiles. "We get him up in the morning and we put him to bed at night" is the way the "shadow" describes the assiduous care which he bestows upon his unsuspecting victim. In much the same fashion did the Allied secret service "shadow" German submarines; it got each submarine "up in the morning and put it to bed at night." That is to say, the intelligence department took charge of Fritz and his crew as they emerged from their base, and kept an unwearied eye upon them until they sailed back home.