MARINE FORTIFICATION differs from land fortification in that the approaches of the enemy which are to be resisted take place on the level of the sea, so that he can come near without having to overcome the dangerous slope of the glacis. The combat is shnply one between two powerful batteries, and the question to be decided is whether the ship or the fort wilI first be placed hors de combat; the ship having ordinarily the largest number of guns, while the fort has more solid battlements, and its fewer guns of great caliber can be fired with a steadiness unattainable on so shifting a base as the ocean. Under these circumstances, the less relief a sea-fortress has the better, as by so much the less is it likely to be hit from the shipping. Its walls are usually built per pendicular, or nearly so. The magazines and quarters for the men are bomb-proof, as also are the casemates, from which the guns are usually fired, although sometimes, as in the martello-tower, the gun is worked on the top of the structure.
Sea-fortifications may be of various importance, the simplest being the battery con sisting of a mere parapet formed in a cliff or on a hill, and mounted with guns to com mand the sea; these are generally built in such concealed situations that it is hoped the hostile ships will not perceive them until they actually open fire. They are numerous all around the British coast. Next greater in importance is the martello-tower (q.v.). 3Iore powerful still are the beach-forts, such as those which on either shore defend the entrance to Portsmouth harbor these are constructed of the most solid masonry, faced with massive iron plates, and armed with guns of the heaviest caliber, sweeping the very surface of the sea, so as to strike an approaching ship between wind and water.
The guns are usually in bomb-proof casemates, and the fort is often defended on the land side if the coast be level; if, however, higher ground be behind, this would be useless, and then the sea-front alone is defensible. Most terrible of an sea-forts, how _ ever, are the completely isolated forts, with perpendicular faces and two or three tiers of heavy guns. uch are the tremendous batteries which render Cronstadt almost inap proachable, and by which Spithead and Plymouth sound are now fortified. These forts, are generally large, with all tbe requisites for a garrison to maintain itself; against them wooden ships stand no chance, and in the American civil war fort Sumter, at Charles ton, showed itself no mean antagonist for ironsides. sueli forts iron is employed as the facing, in plates of such vast thickness and weight that it is supposed no ship can ever possess any comparable resisting power; and, as they are armed with guns the smallest of which will probably be 300-pounders, it is expected that they will be able to destroy any fleet that could be sent against them.
At the present day the value of sea-fortifications is disputed, as iron-plated vessels may pass them with impunity unless the artillery in the fort be so heavy as to destroy the armor of the ships. In the long run, however, it is apparent that the fort can com mand the greater power, for its armor may be of any thickness, while that of the ship must be limited by her floating powers, and, on the other hand, the limit to the size of artillery must be sooner reached in a ship than in a solid and stationary fortress.