In kind, preparation is twofold,--defensive and offensive. The former exists chiefly for the sake of the latter, in order that offence, the determining factor in war, may put forth its full power, unhampered by concern for the protection of the national interests or for its own resources. In naval war, coast defence is the defensive factor, the navy the offensive. Coast defence, when adequate, assures the naval commander-in-chief that his base of operations--the dock-yards and coal depots--is secure. It also relieves him and his government, by the protection afforded to the chief commercial centres, from the necessity of considering them, and so leaves the offensive arm perfectly free. Coast defence implies coast attack. To what attacks are coasts liable? Two, principally,--blockade and bombardment. The latter, being the more difficult, includes the former, as the greater does the lesser. A fleet that can bombard can still more easily blockade. Against bombardment the necessary precaution is gun-fire, of such power and range that a fleet cannot lie within bombarding distance. This condition is obtained, where surroundings permit, by advancing the line of guns so far from the city involved that bombarding distance can be reached only by coming under their fire. But it has been demonstrated, and is accepted, that, owing to their rapidity of movement,--like a flock of birds on the wing,--a fleet of ships can, without disabling loss, pass by guns before which they could not lie. Hence arises the necessity of arresting or delaying their progress by blocking channels, which in modern practice is done by lines of torpedoes. The mere moral effect of the latter is a deterrent to a dash past,--by which, if successful, a fleet reaches the rear of the defences, and appears immediately before the city, which then lies at its mercy. Coast defence, then, implies gun-power and torpedo lines placed as described. Be it said in passing that only places of decisive importance, commercially or militarily, need such defences. Modern fleets cannot afford to waste ammunition in bombarding unimportant towns,--at least when so far from their own base as they would be on our coast. It is not so much a question of money as of frittering their fighting strength. It would not pay.