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it is only necessary here to consider examples of the newer group- fortification. Three forms may be taken, one of which, the Metz form, has been applied on a large scale, while the others, though academic examples, are fully representative of principle.

Common to all, it will be seen, are: (a) a wire obstacle round the whole group, and behind it an infantry trench-position; (b) very large area, equalling that of town and fortifications together in some of the old Vauban fortresses, and six to eight times that of the typical 1873-1903 fort; (c) batteries, closed and under armour, for the guns of the main armament (or safety armament) irregu- larly disposed within the wired area.

So far, all are in agreement. But beyond, there are some impor- tant differences. Thus, the Metz group, and those proposed by de Mondesir, both possess powerful infantry works with ditches, whereas the Austrian type lacks this element. Again, de Mondesir and the Austrian text-books agree in attaching the greatest impor- tance to the traditore element, remarkably neglected in the Metz works at least as originally built. Lastly, the Austrian and Ger- man engineers tend to place the centre of gravity of the artillery, and even that of the infantry, defence well forward, while the French author puts them as far back as possible, with only observa- tories and frontal trenches in the forepart of the area.

The Austrian design (fig. i) as the simplest, is taken first. On the height 130 is an armoured battery P B, containing four 6-in. howitz- ers in cupolas, with an observation post in a small cupola in the centre. 1 Between the cupolas are magazines for the storage of 800 rounds per gun. A passage runs along the backs of the cupolas and ammunition rooms, and two barrack rooms are provided at the ends, with other small rooms as offices, etc., in the centre. In the actual design the thickness of the concrete is, in places, less than 2 metres, which is considerably below present-day standards.

On the forward slopes at S, S, S, are small works, combining in each 2 cupolas for quick-firing guns (intended for frontal close- defence, not main-armament work), with an armoured observation post between them and a shelter for infantry and machine-guns in waiting, to man the trench-line against assault. These are built with a roof of about the same thickness as that of the main-arma- ment battery. On the rear slopes are two powerful traditore batter-

FIG i. Austrian Type

ies, T T with quick-firing guns (4 to 6 in each, in order to have suffi- cient for a distributed fire over the interval, in case fog or darkness makes accurate aim impossible). The inner parts of the concrete masses are organized as barracks (U) and magazines. The traditore is in two tiers, the upper commanding the country outside and the lower sweeping the (wired) bottom of the ditch. Armour is used for the faces of the gun casemates and nearly 3 metres of concrete form the roof. Those parts of the wired ditch not swept by the traditores are flanked by counterscarp casemates (F F) containing machine- guns or pompoms and protected by 2-8 metres of concrete. Tunnels connect the various elements of the group. In this design, which is simple and, owing to the absence of refinements that would not stand bombardment, strong, there are nevertheless some points of weakness, which may be discussed here, not by way of criticism but because they afford convenient illustrations of certain practical points which the engineer cannot ignore.

The whole of the front wire depends for its intimate flanking upon the counterscarp casemates, F F. In such cases it is necessary to protect the backs of the casemates and their communicating galleries from mine attack, by providing the roots of a counter- mine system at the outset. This was a form of attack which played a considerable r&le at Port Arthur. It is perhaps the only way of

1 This is a miniature of the gun cupola, with a telescope placed in the port. The development of the periscope now makes the pro- vision of protected command ports much easier than it was at the time of this design.

dealing with counterscarp casemates, but it is an effective one. At Fort Vaux (Verdun) the Germans made their way through a counterscarp casemate into the tunnel system of the fort, and the terrible gallery fighting of Port Arthur repeated itself. But, unlike the Japanese, the Germans had no difficulty in gaining access to the galleries in the first instance, as the French had themselves blown away the backs of the casemates in order to get convenient access to some external trenches. It is noteworthy that in the final stages of refortifying Metz on the group principle, the Germans were careful to provide the foundations of counter-mine systems. In the Austrian design here considered, nothing prevents the devel- opment of mine attack on these casemates except the fire of the central battery P B and the batteries, S, S, S, all of which are exposed and liable to neutralization or destruction by counter-battery. The depth of the ditch containing the main wire varies, and no walls exist to make it an important obstacle in itself. The integrity of the obstacle therefore depends purely on the fire of the counter- scarp casemates, and quite apart from the question of mining attack on these-^later war experience has shown that there is great risk of the flanking fire being impeded or intercepted by the debris produced by intensive bombardment. This weakness is common to all ditches, and the problem of keeping the field of fire open had not yet been solved in 1921. But it is evident that the longer the ditch, the more chance there is of a heap of debris collecting at some point in it. In any case, it would seem that to attempt ditch protection for the whole perimeter of a group work involves the expenditure of money that might more profitably be devoted to other elements of defence. Another defect seems to be the small number of the infantry shelters, having regard to the time required for the defend- ing infantry to come out and man the parapet. This is the more important, as this design altogether lacks the strong self-contained infantry work which is the kernel of those now to be described. The evolution of Metz as a ring-fortress is dealt with at 10-696. Allu- sion is there made to new works in progress outside the existing perimeter. These were the famous Feste. They were built in suc- cession from 1899 to the outbreak of the World War, and were con- tinued and practically completed in 1915. Their characteristics were only approximately known at that time, but when Metz was retroceded to France by the -Treaty of Versailles, not only their present condition, but their history and cost accounts became available. (See the French official Revue du Genie of Jan.-Feb. 1921.)

The Feste in fig. 2 (from the Revue du Genie) shows an actual example. It should be understood that the Germans designed the earlier works of this class with a minimum of defensive precautions, notably in respect of external interval flanking, but that, in the later works constructed in the two or three years prior to the World War, there was a marked tendency to develop the hitherto inade- quate external flanking, even at the expense of the main armament, which on this line of evolution would, in due course, have become a " safety " armament only. The group-work illustrated is rather of the earlier than of the later kind, as it is lacking in the traditore element. But it is one of the greatest advantages of the group-work over the cramped fort that additions and alterations can be made as required, and in fact many such works at Metz were provided later with 57-mm. and 77-mm. traditore batteries.

The Feste forms an irregular quadrilateral, measuring, from outer edge to outer edge of wire, 1,200 yd. from front to rear and the same from flank to flank, with an area within the outermost wire of about 1 20 acres. At the front and rear angles there are strong and minutely organized infantry works, which form the basic units of the system : their role is to flank the wired perimeter and to look after their own close-defence as well. At the right and left angles, the perimeter trench takes the form of redoubts, which contain, in their forward sides, infantry observation posts, and, in their rear sides, both observa- tion posts and organs for flanking the rearward wire. In the interior of the Feste, four armoured batteries for main armament are dis- posed irregularly and each has a war barracks attached, communi- cating with it by underground passages. The perimeter trench is provided at intervals with armoured sentry posts. The artillery observatories are aligned on the front slope, and have tunnel con- nexions with their batteries. The fifth battery is a dummy a device freely used in these Metz works, in which there is plenty of room. The perimeter wire is sunk to a depth of 2 metres, and the ground in which it is bedded is sloped up to the infantry line, which has the lowest command compatible with its functions. This perimeter wire is carried round the main works (01, 02) also (though partly unflanked), but the strength of these lies in their inner system. Behind the perimeter wire and the advanced parapet or covered way lies a deep ditch (20 ft.), wired at the bottom and provided with a concrete counterscarp. The floor of this ditch is flanked (in the case of the forward work 01) by a double counterscarp casemate at the apex and a small caponniere in the gorge.

About the same time as these Metz works were being evolved in Germany, Lt.-Col. (afterwards General) Piarron de Mondesir, in France, advocated another type of group-fortification, which, though generally of the same class as the Feste, shows some characteristic differences.

De Mondesir's group is in general outline oval, or rather lens- shaped, with the curved front towards the enemy and the flattened front towards the defended region. Like the Feste it bestrides the