Nor are the new practical difficulties of sporadic operations on the great routes the only arguments that minimise the value of convoys. We have also to remember that while the number of vessels trading across the ocean has enormously increased since 1815, it is scarcely possible, even if the abolition of privateering prove abortive, that the number of cruisers available for pelagic attack could exceed, or even equal, the number employed in sailing days. This consideration, then, must also be thrown into the scale against convoys; for it is certain that the amount of serious operative damage which an enemy can do to our trade by pelagic operation is mainly determined by the ratio which his available cruiser strength bears to the volume of that trade. This aspect of the question is, however, part of a much wider one, which concerns the relation which the volume of our trade bears to the difficulty of its defence, and this must be considered later. It remains, first, to deal with the final link in the old system of defence. The statement that the great routes were left undefended will seem to be in opposition to a prevailing impression derived from the fact that frigates are constantly mentioned as being "on a cruise." The assumption is that they in effect patrolled the great routes. But this was not so, nor did they rove the sea at will. They constituted a definite and necessary part of the system. Though that system was founded on a distinction between defended terminals and undefended routes, which was a real strategical distinction, it was impossible to draw an actual line where the one sphere began and the other ended. Outside the regularly defended areas lay a region which, as the routes began to converge, was comparatively fertile. In this region enemies' cruisers and their larger privateers found the mean between risk and profit. Here too convoys, as they entered the zone, were in their greatest danger for fear of their escorts being overpowered by raiding squadrons. Consequently it was the practice, when the approach of convoys was expected, to throw forward from the defended area groups of powerful cruisers, and even battleship divisions, to meet them and reinforce their escorts. Outward-bound convoys had their escorts similarly strengthened till they were clear of the danger zone. The system was in regular use both for home and colonial areas. In no sense did it constitute a patrol of the routes. It was in practice and conception a system of outposts, which at seasons of special risk amounted to an extension of the defended areas combining with a reinforcement of the convoy escorts. Focal points of lesser importance, such as Capes Finisterre and St. Vincent, were similarly held by one or two powerful cruisers, and if necessary by a squadron.