It is to be noted, however, that while the Germans have adhered to the three-line principle in the sectors where they believe themselves but slightly threatened, on the fronts where they are heavily pressed by the Allied Armies, they have organized, as their advanced lines weakened or were forced, a series of very strong positions, one behind the other. It is impossible to be precise as to the number of these lines. According to the reports of the aviators, several complete systems of defences exist between the positions they are now defending and the Meuse.
It is interesting to remark that the multiplication of large-calibre artillery has caused changes to be made by both belligerents in the construction of shelters and intrenchments.
During the winter of 1914–1915, no serious bombardments took place before April. Both sides organized themselves on their positions, excavating shallow shelters which were braced with wooden beams and roofed with two or three layers of logs, over which earth was more or less thickly packed. Such shelters resisted well enough 150 mm. shells.
But, in 1917, during the operations in Artois and Champagne, the adoption of larger calibres, and the use of torpedoes fired by trench machines, compelled the belligerents to bury themselves more deeply in the ground, wherever the soil permitted, and to build more solid shelters. When water interfered with deep excavations, the shelters were covered in with very strong T-shaped iron railway ties, or with several layers of steel rails; but these proved insufficient, and the Germans were the first to construct those bombproofs of reinforced concrete which the British for the first time encountered on the Somme. The concrete blocks are very large and the steel reinforcing bars extremely strong. Such works certainly impede and delay the operations of the enemy, especially when they are extensively employed, but events have proved that, given time, they can always be destroyed by gun-fire. The French have constructed similar works only at points of capital importance. They prefer the old wooden shelters, well reinforced with earth.
Ailles and its western approach
February 10, 1917—2.30 P.M.
At all the inhabited places in their lines, and at points of natural strength, the Germans have organized independent centres of resistance. They have transformed whole villages into fortresses. Mention need only be made of the labyrinth of Carency-Thiepval, Beaumont-Hamel, the tunnels of Cornillet, of Hill 304, of Mort-Homme, etc.
In every one of those places a surprising number of concrete constructions and superimposed subterranean galleries were discovered. In them the enemy had collected reserve troops, food, and munitions. Such shelters, no doubt, afforded the Germans great protection, and in order to destroy them, it was necessary to have recourse to more and more powerful methods.
An officer of the 81st Regiment of Infantry which captured Mort-Homme writes as follows: “ ... and on the hill where the 81st Regiment is encamped, what an accumulation of defensive agencies! Wire, tunnels, trenches, observatories, shelters of every description, machine-gun posts, light cannon, nothing is lacking. To these ordinary means of defence, other extraordinary ones had been added, consisting of three immense and very deep subterranean systems (82 steps led down to one and the length of another exceeded one kilometre) provided with ventilators, Decauville narrow-gauge railways, electricity, posts of command and relief, rooms for the men, and stores for food, arms, munitions, and material. All these extraordinary fortifications could not resist the impetuous assault of our troops, which had been preceded by a six-day bombardment so intense that the entire first line was enveloped in a thick cloud of smoke about two hundred metres high, and the ground shook all the time.”