[Illustration: DIAGRAM I. A striking comparison of the length of lines in some past campaigns with the present. The characteristic novelty of the present war is the rapidity with which such lines are established by the great numbers now facing each other, armed as they are by weapons of very long range.] When cannon came and gunpowder for exploding mines underground, the mediæval castle of stone could be quickly reduced. There was, therefore, a phase in which permanent fortification or permanent works were at a discount. The wars of Cromwell in this country, for instance, were fought in the middle of such a phase. The castles went down like nine-pins. But the ingenuity of man discovered a new form of defence valuable even against cannon, in the shape of scientifically constructed _earthworks_. The cannon ball of the day could not destroy these works, and though they could be _sapped_ and _mined_, that is, though tunnels could be dug in beneath them and explosives there fired to their destruction, that was a long business, and the formation of the works was carefully designed to give the garrison a powerful advantage of fire over the besiegers. Works of this kind made the defensive strong again for more than two hundred years. Just as there used to be a stone wall surrounding a town, at intervals from which people could shoot sideways along the “curtain” or sheer wall between the towers, so now there was earthwork, that is, banks of earth backed by brick walls to hold them up, and having a ditch between the outer parapet and the inner. These earthworks were star-shaped, sending out a number of projecting angles, so that an attack launched upon any point would receive converging fire from two points of the star, and the entrances were further protected by outer works called horn works. With the war of 1870, and even for somewhat before it, it was found that the increased range of modern artillery had destroyed the value of these star-shaped earthworks, taking the place of the old walls round a town. One could batter the place to pieces with distant guns. Though the guns within the place were as strong as the guns outside, they were at this disadvantage: that they were confined within a comparatively small space which the besiegers could search by their fire, while the guns of the besiegers could not be equally well located by the gunners of the besieged within the fortress. So the next step was to produce what has been known as the _Ring Fortress_. That is, a series of detached forts lying three or four miles outside the inner place of stores, barracks, etc., which you wanted to defend. Each fort supporting the two others next to it on either side of this ring was thought to be impregnable, for each fort was built within range of the two nearest, and on such a model were built Toul, Verdun, Epinal, Belfort, Metz, Strassburg, Thorn, Cracow, and fifty other great modern strongholds.