Apart from the mining operations, the construction of dug-outs, and the creation of entanglements, little new work was carried out in this sector. The French had already begun a system of defence in depth. This was worked upon and improved. The only important new work undertaken was the construction of the corps reserve lines, known as the Bouzincourt Switch, which ran from Martinsart to Bouzincourt and across the river Ancre to Albert. In this reserve line there were constructed what are believed to be the first concrete "pill-boxes" made either by the Allies or the Germans on the Western Front. These were formed of walls of reinforced concrete five feet thick, with loopholes through which machine-guns could fire. After the British lines had been driven back by the great German offensive in March 1918, these pill-boxes were situated in or about No Man's Land, and were on several occasions struck by direct hits. They were, however, undamaged, and, it is hoped, played the part with success for which they were intended. During the period in which the Division was in this sector, great progress towards efficiency was made by the Divisional artillery. By the time they had arrived in France mobile warfare was for the time being in abeyance. This was a distinct advantage to Territorial gunners, as the conditions of service in peace time afforded them few opportunities for training for warfare of movement. They had, however, devoted a considerable amount of study to technical gunnery. Consequently, they had less ground to make up than would have been the case if they had been engaged in a war of movement. Their training at home had, however, been carried out under difficult conditions, and when they arrived they still required considerable training as units. This was mostly carried out in the line, as from the time of its arrival in France the artillery was practically continuously in action until early in 1916. In spite of this they made great progress. An artillery brigade commander describes them as follows: "By the end of their first year they were good; by the end of their second year they were better; and by the end of their third year they were first-class field artillery,--very mobile, very quick, and full of initiative." On the 31st August the 15-pounders were replaced by 18-pounders, to the great relief of all concerned. The 15-pounders had always suffered from a very restricted supply of ammunition. What ammunition they had was so erratic as to be a positive danger to our own troops. The gunners also had not had the opportunities at home of becoming complete masters of their weapons. The result was that the artillery, working in these adverse circumstances, did not at first enjoy the confidence of the infantry. However, their work in this sector went far towards inspiring in the infantry full confidence in their gunners.