The system of convoy protection had instant effect in curbing the activities of the U-boats. They could no longer work at sea on the lines that had proved so safe for them and disastrous for us. To get at the ships they had now to come within range of the destroyers' armament. Hydrophones and depth-charges reduced their vantage of submersion. The risks of sudden rupture of their plating by the swiftly moving keel of an escorting vessel did not tend to facilitate the working of their torpedo problem. In the coastal areas aircraft patrolled overhead the convoys, to add their hawk-sight to the ready swerve of the destroyers. The chances of successful attack diminished as the hazard of discovery and destruction increased. Still, they were no fainthearts. The German submarine commanders, brutal and hell-nurtured, are no cowards. The temptation of a massed target attracted them, and they sought, in the confusion of the startled ships, a means of escape from the destroyers when their shot into the 'brown' had run true. Convoy has added many new duties to the sum of our activities when at sea. Signals have assumed an importance in the navigation. The flutter of a single flag may set us off on a new course at any minute of the day. Failure to read a hoist correctly may result in instant collision with a sister ship. We have need of all eyes on the bridge to keep apace with the orders of the commodore. In station-keeping we are brought to the practice of a branch of seamanship with which not many of us were familiar. Steaming independently, we had only one order for the engineer when we had dropped the pilot. 'Full speed ahead,' we said, and rang a triple jangle of the telegraph to let the engineer on watch know that there would be no more 'backing and filling'--and that he could now nip into the stokehold to see to the state of the fires. Gone--our easy ways! We have now to keep close watch on the guide-ship and fret the engineer to adjustments of the speed that keep him permanently at the levers. The fires may clag and grey down through unskilful stoking--the steam go 'back' without warning: ever and on, he has to jump to the gaping mouth of the voice-tube: "Whit? Two revolutions? Ach! Ah cannae gi' her ony mair!"--but he does. Slowly perhaps, but surely, as he coaxes steam from the errant stokers, we draw ahead and regain our place in the line. No small measure of the success of convoy is built up in the engine-rooms of our mercantile fleets.