The efficacy of gunboats for the defense of harbors and of other smooth and inclosed waters may be estimated in part from that of galleys formerly much used but less powerful, more costly in their construction and maintenance, and requiring more men. But the gunboat itself is believed to be in use with every modern maritime nation for the purposes of defense. In the Mediterranean, on which are several small powers whose system, like ours, is peace and defense, few harbors are without this article of protection. Our own experience there of the effect of gunboats for harbor service is recent. Algiers is particularly known to have owed to a great provision of these vessels the safety of its city since the epoch of their construction, Before that it had been repeatedly insulted and injured. The effect of gunboats at present in the neighborhood of Gibraltar is well known, and how much they were used both in the attack and defense of that place during a former war. The extensive resort to them by the two greatest naval powers in the world on an enterprise of invasion not long since in prospect shews their confidence in their efficacy for the purposes for which they are suited. By the northern powers of Europe, whose seas are particularly adapted to them, they are still more used. The remarkable action between the Russian flotilla of gunboats and galleys and a Turkish fleet of ships of the line and frigates in the Liman Sea in 1788 will be readily recollected. The latter, commanded by their most celebrated admiral, were completely defeated, and several of their ships of the line destroyed. From the opinions given as to the number of gunboats necessary for some of the principal seaports, and from a view of all the towns and ports from Orleans to Maine, inclusive, entitled to protection in proportion to their situation and circumstances, it is concluded that to give them a due measure of protection in times of war about 200 gunboats will be requisite. According to first ideas the following would be their general distribution, liable to be varied on more mature examination and as circumstances shall vary; that is to say: To the Mississippi and its neighboring waters, 40 gunboats. To Savannah and Charleston, and the harbors on each side from St. Marys to Currituck, 25. To the Chesapeake and its waters, 20. To Delaware Bay and River, 15. To New York, the Sound, and waters as far as Cape Cod, 50. To Boston and the harbors north of Cape Cod, 50. The flotillas assigned to these several stations might each be under the care of a particular commandant, and the vessels composing them would in ordinary be distributed among the harbors within the station in proportion to their importance.