One important duty of the Harwich Force was to patrol the mine-net barrage which extended along the Belgian coast, parallel to and at about ten miles distance from the shore, from Dunkirk to Holland. There was nearly always one division of the Harwich Force, consisting of four destroyers, with one or two monitors, patrolling just outside the barrage by day, within effective range of the German guns on the shore (their range was 30,000 yards). By night the division used to patrol and protect the Downs. This patrol, based on Dover, used to carry on this work for three weeks at a stretch, always at sea, or ready to get off at a moment's notice. Its function outside the mine-net barrage was to prevent enemy submarines from passing through the barrage, and to stop the enemy destroyers from leaving their base. This channel patrolled by our destroyers was bordered on its south side by the mine-net barrage and on its north side by our minefields. On the further side of the minefields our light cruisers and destroyers patrolled in support. Our destroyers had frequent scraps with the enemy across the narrow mine-net barrage. It was while engaged in this work that the Harwich Patrol co-operated with the Dover Patrol in the bombardment of the coast. On one occasion, at daylight, the Harwich Force sighted four German destroyers making for Zeebrugge. The _Centaur_, at that time Admiral Tyrwhitt's flagship, with other cruisers and destroyers of the Harwich Force, sank one of the enemy destroyers, the S20, and badly damaged other destroyers.