A naval base capable of doing this for a large fleet must be a very great establishment. In such a naval base, one must be able to build, dock, and repair vessels of all kinds, and the mechanisms needed in those vessels; anchor a large fleet in safety behind adequate military and naval protection; supply enough fuel, ammunition, and supplies for all purposes, and accommodate large reserves of material and personnel. Inasmuch as a naval base is purely a means for expending energy for military purposes, and has no other cause for its existence, it is clear that it cannot be self-supporting. For this reason it is highly desirable that a naval base shall be near a great city, especially if that city be a large commercial and manufacturing centre. It is true that many large naval bases, such as Malta and Gibraltar are not near great cities; and it is true that most large naval bases have no facilities for building ships. But it is also true that few large naval bases fulfil all the requirements of a perfect naval base; in fact it is true that none do. The most obvious requirement of a naval base is a large sheet of sheltered water, in which colliers and oil-carriers may lie and give coal and oil to fighting craft, and in which those fighting craft may lie tranquilly at anchor, and carry on the simple and yet necessary repairs and adjustments to machinery that every cruising vessel needs at intervals. Without the ability to fuel and repair, no fleet could continue long at work, any more than a man could do so, without food and the repairs which nature carries on in sleep. The coming of oil fuel and the consequent ease of fuelling, the practicability even of fuelling in moderate weather when actually at sea, subtract partially one of the reasons for naval bases; but they leave the other reasons still existent, especially the reasons connected with machinery repairs. The principal repair, and the one most difficult to furnish, is that given by docking in suitable docks. The size and expense of docks capable of carrying dreadnaughts and battle cruisers are so great, and their vulnerability to fire from ships and from aircraft is so extreme, that the matter of dry-docks is perhaps the most troublesome single matter connected with a naval base. The necessity of anchorage areas for submarines is a requirement of naval bases that has only recently been felt; and the present war shows a still newer requirement in suitable grounds for aircraft. The speed of aircraft, however, is so great that little delay or embarrassment would result if the camp for aircraft were not at the base itself. Instead of the camp being on Culebra, for instance, it might well be on Porto Rico. The extreme delicacy of aircraft, however, and the necessity for quick attention in case of injuries, especially injuries to the engine, demand a suitable base even more imperiously than do ships and other rugged things.