For mountainous frontiers it is deemed necessary to secure all the important passes with small redoubts or military works, and to defend with strong forts the grand interior strategic points on which these communications are directed.  For a frontier of moderate extent there may be some six or eight gorges in the mountains by which an army might penetrate; but it will always be found that these roads concentrate on two or three points in the great valleys below.  Take, for example, the frontier of France towards Switzerland and Italy.  The passes of the mountains are secured by the little works of Fort L’Ecluse, Fort Pierre-chatel, Fort Barraux, Briancon, Mont Dauphin, Colmars, Entrevaux, and Antibes; while Besancon, Grenoble, and Toulon, form a second line; and Lyons a grand central depot.

Where a great river or chain of lakes forms the boundary of a state, the system of defence will be much the same as that of an open land frontier, the works of the first line being made to secure the great bridges or ferries by which the enemy might effect a passage; those of the second line, to cover the passes of the highlands that generally approach more or less near the great watercourse; and those of the third line, far enough in rear to protect the great internal communications of the country.  Let us take, for example, the side of France bordering on the Rhine.  Wissembourg and Lauterbourg, Fort Louis, Haguenau, Strasbourg, Schelstadt, Neuf-Brisach, and Huneguen, cover the several passages of the river; while Bitche, Phalsbourg, and Befort form a second line; Thionville, Metz, and Toul, a third line; and Verdun a grand central depot.

The following are the principal objects proposed to be accomplished by fortifications on a sea-coast.

1st.  To close all important harbors to an enemy, and secure them to the navy of the country.

2d.  To prevent the enemy from forming an establishment on our shores, from which, by his naval superiority, he might destroy our commerce and keep the whole frontier in continual alarm.

3d.  To cover our great cities against a maritime attack and bombardment.

4th.  To cover our ship-yards and great naval depots.

5th.  To prevent, as much as possible, the great avenues of interior navigation from being blockaded by naval means at their entrance into the ocean.

6th.  To give to our navy facilities for protecting our coast trade from the enemy’s ships of war, and our internal communications, which lie near the coast, from maritime descents.

Let us notice how France has attempted to accomplish this object.  The Mediterranean frontier has Fort Quarre, Fort St. Marguerite, St. Tropez, Brigancon, the forts of Point Man, of l’Ertissac, and of Langoustier, Toulon, St. Nicholas, Castle of If, Marseilles, Tour de Boue, Aigues-Montes, Fort St. Louis, Fort Brescou, Narbonne, Chateau de Salces, Perpignan, Collioure, Fort St. Elme, and Port Vendre.  Toulon is the