[The upper course of the Danube between Ulm and Ratisbon is also employed to illustrate central position, dominating the great European theater of war north of the Alps and east of the Rhine.—EDITOR.] The situation of France relatively to her two opponents of this period—Spain and Austria—illustrates three elements of strategy, of frequent mention, which it is well here to name and to define, as well as to illustrate by the instance before you. 1. There is central position, illustrated by France; her national power and control interposing by land between her enemies. Yet not by land only, provided the coast supports an adequate navy; for, if that be the case, the French fleet also interposes between Spanish and Italian ports. The Danube is similarly an instance of central position. 2. Interior lines. The characteristic of interior lines is that of the central position prolonged in one or more directions, thus favoring sustained interposition between separate bodies of an enemy; with the consequent power to concentrate against either, while holding the other in check with a force possibly distinctly inferior. An interior line may be conceived as the extension of a central position, or as a series of central positions connected with one another, as a geometrical line is a continuous series of geometrical points. The expression “Interior Lines” conveys the meaning that from a central position one can assemble more rapidly on either of two opposite fronts than the enemy can, and therefore can utilize force more effectively. Particular examples of maritime interior lines are found in the route by Suez as compared with that by the Cape of Good Hope, and in Panama contrasted with Magellan. The Kiel Canal similarly affords an interior line between the Baltic and North Sea, as against the natural channels passing round Denmark, or between the Danish Islands,—the Sound and the two Belts.[22] These instances of “Interior” will recall one of your boyhood’s geometrical theorems, demonstrating that, from a point interior to a triangle, lines drawn to two angles are shorter than the corresponding sides of the triangle itself. Briefly, interior lines are lines shorter in time than those the enemy can use. France, for instance, in the case before us, could march twenty thousand men to the Rhine, or to the Pyrenees, or could send necessary supplies to either, sooner than Spain could send the same number to the Rhine, or Austria to the Pyrenees, granting even that the sea were open to their ships.