goes; when the need for troops became more and more pressing and when our transport service had demonstrated great skill in the work, a few slower vessels were used; but the great majority of our troop transports were those which made twelve knots or more. Now one of the greatest protections which a ship possesses against submarine attack is unquestionably high speed. A submarine makes only eight knots when submerged and it must submerge immediately if its attack is to be successful. It must be within at least a mile of its quarry when it discharges its torpedo; and most successful attacks were made within three hundred yards. Now take a pencil and a piece of paper and figure out what must be the location of a submarine, having a speed of eight knots, when it sights a convoy, which makes twelve knots and more, if it hopes to approach near enough to launch a torpedo. A little diagramming will prove that the U-boat must be almost directly in line of its hoped-for victim if it is to score a hit. But even though the god of Chance should favour the enemy in this way, the likelihood of sinking its prey would still be very remote. Like all convoys, the troopships began zigzagging as soon as they entered the danger zone; and this in itself made it almost impossible for a submarine to get its bearings and take good aim. I believe that these circumstances in themselves—the comparative scarcity of troop transports, the width of the "lane" in which they travelled, the high speed which they maintained, and their constant zigzagging, would have defeated most of the attempts which the Germans could have made to torpedo them. Though I think that most of them would have reached their destinations unharmed without any other protection, still this risk, small as it was, could not be taken; and we therefore gave them one other protection greater than any of those which I have yet mentioned—the destroyer escort. A convoy of four or five large troopships would be surrounded by as many as ten or a dozen destroyers. Very properly, since they were carrying human cargoes, we gave them an escort at least three times as large per vessel as that given to large mercantile convoys of twenty or more vessels; and this fact made them very uninviting baits for the most venturesome U-boat commanders.
When the engineers build a Brooklyn bridge they introduce an element which they call the factor of safety. It is
