The destruction of a bridge impedes the transport of troops and supplies; a sudden freshet, occurring in the midst of an extensive movement, may imperil an army by sundering its forces; while of the utility of such natural trenches to the purposes of shelter and of defence, of awaiting attack, or resisting an advance, both the Tugela and Paardeberg have given recent striking illustration.
As a general rule such conditions favour the defence relatively to the offence; the former, remaining comparatively motionless, is shielded by obstacles, to surmount which the assailant must expose himself in the open.

Thus they compensate for inferior numbers, which is usually the condition of the defence; and they conduce to delay, ever a leading object in defensive warfare.

Consequently, in the {p.021} present hostilities they have helped the Boers.

It may be added that their influence is most felt when the armies are face to face, or at least in touch.

Hence their existence near the scene of probable conflict, as in Natal, is a matter of more concern to the invader than when, as upon the Cape extreme of the scene of war, they are found beyond the range to which the defendant can safely extend his operations.
These successive watercourses indicate natural lines of defence, stronger or weaker according to their individual distinctive features.
As the railroad, in its progress north, draws near the mountains in the neck of Natal, the streams show smaller volume and less developed channels.