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face. Monlamville received 330 hits by 42-cm. shell. The more exposed forts, such as Douaumont and Souville, deluged with medium and field as well as heavy shell, received over 30,000 hits apiece. Thus, cupolas of the thickness and quality of those of Verdun are capable of all necessary resistance.

FIG. 10. Austrian Traditore.

Even more marked is the discrepancy between the behaviour of concrete at Liege and Antwerp and its behaviour at Verdun; as in the case of armoured gun mountings, the Belgian sieges brought concrete into discredit. Here, however, it is a question of quality of material and of conscientious painstaking work. The best concrete has a resistance such that a test bar will break across the stones rather than the cement; the worst is feebler than virgin earth, especially rooty earth. The Verdun concrete had not only been well made, but had also been given an adequate afety margin of thickness; and it was found that, in spite of the volcanic bombardment, 3 metres of special concrete or 2 metres of einforced concrete, or 25 metres of special concrete (or 1-75 tietres reinforced) disposed as an apron overlying an old mason- vault, with a sand cushion interposed, would steadily resist alibres up to 42-cm. inclusive. Although the effects of these aonster mine-shell were felt in the ground to a depth of 14 netres (nearly 47 feet), the fissuring which had ruined the Bel- gian forts does not seem to have occurred at Verdun.

In the Metz Feste the characteristic disposition is a mass of ordinary concrete with a thick outer casing of ferro-concrete. An arrangement which is favoured by some engineers is to dis- pose reinforced concrete in layers separated by air-spaces; this had not in 1921 been subjected to the test of siege. The lesson of Verdun, however, is held to be not so much that it proved 3 me- tres to be a sufficient thickness, but that it demonstrated the importance of mass as a material element of resistance and depth as a moral element. The smaller concrete masses suffered under bombardment much more severely in proportion than the larger, and it is considered therefore that organs small in themselves such as caponnieres should form part of a larger body of concrete, as in fig. 2, or, or in fig. ro. As regards depth, it was found that in tunnelled habitations well below the level of the forts the foundations of which at Verdun are in rock moral remained unimpaired by the heaviest bombardment. At Antwerp, from the nature of its site, no such burrowing was practicable, and the moral of the defenders was subjected to a fresh strain at the explosion of every monster shell.

In sum, armour and concrete in masses calculated according to the lower standards of a previous epoch, but given a safety

margin in application, proved capable of meeting the heaviest strain of bombardment which history records. The problem of the engineer to-day is to determine what safety margin is re- quired for new work or reconstruction calculated according to the standards of Verdun.

A fixed resistance, in situ, of several months, then, is practicable with the materials and means of permanent fortification. That it is not so with those of field fortification is shown by the various trench-warfare offensives of 1916-7. At Verdun the German attack progressed rapidly till it confronted a close-defence system which had permanent works as its backbone. At the Somme, though the Allies' progress was slow, it was sure, and the defense finally evolved a system of " elastic " or " coilspring " resistance, which by its very nature implied the giving-up of ground until an organized counter-attack could be mounted for its recapture. The success of this type of defence, so long as the moral of the defending troops, and their skill in group, platoon and company tactics, remain high, is a matter of history. But it is equally a matter of history that there are positions which, strategically or morally, must be defended to the end without yielding ground, on which the defender must say, with Petain at Verdun, " On ne passera pas." And it is for these cases, so far as they can be foreseen in peace, that permanent fortification exists.

The very words of Petain's defiance, however, carry the im- plication of a linear system. Fournier at Maubeuge, Kusmanek of Przemysl governors of ring-fortresses could not say that the enemy " shall not pass," for the enemy could go round. It was not Verdun the ring-fortress, but Verdun the end-redoubt of the fortified line Verdun-Toul, that compelled the Germans to traverse Belgium in August T9i4. In igi6 Falkenhayn hesitated between Belfort and Verdun as the objectives for his attack. Both had been ring-fortresses, but both were now key-points in a long line of battle, and derived their whole strategic importance from-that fact. At Liege, we find the ring-fortrees splitting itself into two halves which acted to the best of their ability as suc- cessive barrier positions. At Antwerp the attacks on the defence were purely frontal; and indeed, save Przemysl, not one ring- fortress attacked in the World War was vigorously invested and attacked by convergent radial efforts. But it is perhaps Metz which affords the clearest illustration of the modern tendency to convert the stronghold into the barrier. From 1899 onward the old ring becomes of less and less importance, and the Feste were not only disposed on a wider perimeter, but collectively took the form of a sort pf a parabola instead of that of a circle or ellipse. Facing westward, the system began with a group of Feste, form- ing an end-redoubt; and at the same time the pivot of the field armies that were to swing through Belgium, at Diedenhofen (Thionville), then ran southward along the Moselle to the neighbourhood of Metz, where it curved westward, passed round the S. of the old ring, and E. of the Moselle, took a new direction north-eastward, till finally it was completed by the " Nied posi- tion " (created on mobilization) which connected it to the stable line of battle that would be formed by the middle Saar. The southern bend of the system is about r4 m., the westward-facing limb 36 m. and the south-eastward-facing limb about 38 m. in frontage, the whole differing in scale rather than in principle from the familiar " Western Front " of the trench-warfare years which constituted between the Oise and the Moselle a great salient with the apex at Verdun. Outside this blunted redan. Dieden- hofen-Metz-^Rehlingen, the Germans were prepared to yield ground, but on it the defence was meant to be absolutely stable. The growth of the linear tendency is discernible even in the crea- tion of the successive parts of the system. Beginning with the endeavours to increase the perimeter of Metz itself, the Germans presently bore away at a tangent on each side of the southern sector. The individual Feste became longer in proportion to depth, and finally, in the case of the Amanvillers-Horimont works, of 1912, attained a frontage of nearly 2 m. and consisted sub- stantially of a great ditch with its flanking-organs.

Correspondingly, a change follows in ideas as to interval organ- ization. It is evident that, in a ring-fortress of the 1873-1903 type, the individual " forts " occupy only a small proportion