however difficult, cannot be compared in that respect with those over a large river; it is, therefore, much easier for the enemy to make a turning movement against us. But at the same time, such a movement carries him out of his natural direction (for we suppose, as is plain in itself, that the valley crosses that direction at about right angles), and the disadvantageous effect of a confined line of retreat only disappears gradually, not at once, so that the defender will still always have some advantage over the advancing foe, although the latter is not caught exactly at the crisis of the passage, but by the detour he makes is enabled to get a little more room to move.
As we are not speaking of rivers in connection only with the mass of their waters, but have rather more in view the deep cleft or channel formed by their valleys, we must explain that under the term we do not mean any regular mountain gorge, because then all that has been said about mountains would be applicable. But, as every one knows, there are many level districts where the channels of even the smallest streams have deep and precipitous sides; and, besides these, such as have marshy banks, or whose banks are otherwise difficult of approach, belong to the same class.