The trawlers were engaged in patrol duty, convoy escort service, and minesweeping. The drifters worked drifting nets fitted with mines as an anti-submarine weapon, and also in the case of the Dover area they laid and kept efficient a barrage of mine nets off the Belgian coast. Some were also fitted with hydrophones and formed hunting flotillas, and some were engaged in minesweeping duties, or in patrolling swept channels. At Fleet bases a small number were required to attend on the ships of the Fleet, and to assist in the work of the base. The whalers, being faster vessels than the trawlers, were mostly engaged on escort duty or on patrol. The motor launches were employed for anti-submarine work, fitted with hydrophones, and worked in company with drifters and torpedo-boat destroyers, or in minesweeping in areas in which their light draught rendered it advantageous and safer to employ them instead of heavier draught vessels to locate minefields, and in the Dover area they were largely used to work smoke screens for operations on the Belgian coast. As the convoy system became more general, so the work of the small craft in certain areas altered from patrol and escort work to convoy duty. These areas were those on the East Coast and north-west of Scotland through which the Scandinavian and East Coast trade passed, and those in the Channel frequented by the vessels employed in the French coal trade. The majority of these ships were of comparatively slow speed, and trawlers possessed sufficient speed to accompany them, but a few destroyers of the older type formed a part of the escorting force, both for the purpose of protection and also for offensive action against submarines attacking the convoys, the slow speed of trawlers handicapping them greatly in this respect. The difficulty of dealing with submarines may be gauged by the enormous number of small craft thus employed, but a consideration of the characteristics of a submarine and of the great volume of traffic passing up and down our coasts will assist in a realization of the varied and difficult problems set to the British Navy.