The two alternatives, however, that of attacking mercantile convoys or troop convoys, hardly existed in fact. Let us suppose for a moment that the Germans had changed their programme, had taken their group of operating submarines from the northern trade routes, and had stationed them to the south, in the track of the troop transports. What would the results have been? "Lane," though a convenient word for descriptive purposes, is hardly an accurate one; for this ocean passage-way was really about 200 miles wide. Imagine eight or ten submarines, stretched across that expanse and hunting for troopships. At this rate the Germans would have had about one submarine for every twenty miles. Instead of finding themselves sailing amid a swarm of surface ships, as they were when they were stationed in the busy trade routes of the Irish Sea or the English Channel, the submarines would have found themselves drifting on a great waste of waters. Our troop convoys averaged not more than three a week even in the busiest period; in all probability the submarines would therefore have hung around for a month without catching a glimpse of one. Even if by some chance the patient vigil should finally have been rewarded, it is extremely unlikely that the submarine would ever have found a favourable opportunity to attack. We must keep in mind that the convoy room at the Admiralty knew, within certain limits, the location of submarines from day to day; any time one was located in the track of a troop convoy, therefore, a wireless to the convoy would have conveyed this information and directed it to reach the coast of France by another route.