The expansion of the navy occasioned by the war was notable in other directions. Since January 1, 1917, the naval force increased from 4,500 officers and 68,000 men to 15,000 officers and 254,000 men; the number of stations operated by the navy from 130 to 363; the number of civil employees from 35,000 to 60,000; the strength of the Naval Reserve from a few hundreds to 49,246 men; the average monthly expenditures from $8,000,000 to $60,000,000; the Hospital Corps from 1,600 to 7,000; the National Naval Volunteers from zero to 16,000 men; the Marine Corps from 344 officers and 9,921 men to 1,197 officers and 30,000 men. The navy placed great reliance on destroyers to fulfill the part allotted to it in the sea warfare against German submarines. A formidable fleet of these vessels was planned at a cost of $350,000,000, and contracts for the construction were placed with five shipbuilding concerns in October, 1917. Their actual number was guarded as a military secret. It was the largest project the navy department had undertaken, and would probably give the United States a destroyer fleet exceeding those of all other countries. The expenditure embraced the expansion of existing shipbuilding plants and the building of additional engine and boiler factories, as the destroyer program taxed the full capacity of the shipbuilding industry. The destroyer had proved to be the deadliest weapon utilized against the submarine, and was superior to the submarine chaser, even for harbor and in-shore patrol work, besides having better seagoing qualities. Submarine chasers were regarded as a necessity, but the navy evinced little enthusiasm for them as a weapon of permanent effectiveness, and rather pinned its faith to an overwhelming destroyer armada to combat the U-boats. In aviation the Government made no less impressive strides. The building of 20,000 aeroplanes, for which Congress had voted $640,000,000, was undertaken for the creation of a great American aerial force to operate against Germany. Their types covered the whole range of training machines, light, high-speed fighting airships, and powerful battle and bombing planes of heavy design. The training of aviators, the building of motors, and the assembling and framing of the wings proceeded uniformly so that men and equipment would be ready for service simultaneously. Numbers of American aviators were already abroad undergoing intensive training behind the battle fronts. The thousands in training at home were coached by a corps of Allied air experts of various nationalities, forming virtually an international aviation general staff for organizing the American aerial force.