The countermeasures which were adopted by the navy, however, were successful in reducing the attack to manageable proportions, though they involved a drain upon the national resources both in man power and material which is often not fully recognized, and which is by no means the least important of the contributions of the British Empire to the war. The number of men engaged either in the navy or in supplying naval needs now exceeds a million. Unfortunately it is not possible to set forth in detail the immense scope of the Admiralty operations. But they include a very great addition to the armed craft in the service of the navy from torpedo boat destroyers to mine-sweepers, airships, and airplanes, and the organization of a vast system of patrols and mine-sweepers. As a result of the self-sacrificing devotion on the part of the men of the navy and the auxiliary services, and the steadfast performance in all weathers and seasons of their monotonous and dangerous duties, the enemy never succeeded in interfering to any vital degree with the sea communications of the Allies. The Shipping Problem The naval preparations, however, were only part of the measures which were necessary to deal with the shipping situation. The second step was to create the Ministry of Shipping. At the end of 1916 the tonnage requisitioned by the State was less than one-half of the whole, and this was mainly used on purely military and naval services for the British Government or the Allies. During 1917 practically the whole of the remainder of the British ocean-going mercantile marine was brought under requisition at Blue Book rates and organized as a national war service. The Dominion Government also liberated much overseas shipping for war purposes, and neutral shipping was brought as far as possible into allied service. A close scrutiny was then made of the countries from which the necessary imports could be derived, and shipping was concentrated on the shortest routes, thereby multiplying the number of voyages the ships could make in the year. Leading regulations were revised, which increased the carrying capacity from the 1913 figure of 106 to 150 tons per 100 tons net of shipping entering our ports, and arrangements were made for shortening the time occupied in the turn round of ships at the ports. In the latter part of the year the convoy system was introduced, which reduced the shipping losses, though it involved certain delays to individual ships.