Work could not be commenced before the 25th January, owing to the whole Lys Valley, which is very low lying, becoming water-logged by heavy rain. At first the labour available was very limited, owing to the urgent demands for defence works in the battle zone, and for the construction and wiring of a new emergency line of five trenches on the north bank of the river. Early in February the labour position improved, and frequently over one thousand five hundred men were employed simultaneously. The sappers had a strenuous time, and, but for the assistance of a party from the Corps Cyclists, who by long association with Signals had become relatively skilled, could not have kept up the pace. By unremitting effort the Corps Section of the trenches was completed by the beginning of March, and a portion of the work originally assigned to divisions taken over. While the other sections were so engaged, B.F. was employed on another section of the scheme in and around Armentières itself. In this town a considerable underground sewer system existed, and though not comparable with the underways of Arras, yet these sewers proved extremely valuable as affording ready-made covered ways for cable. B.F. accordingly spent two months in laying securely many miles of cable. The new 91 Air Line Section meanwhile pushed on the necessary additions to the open-wire routes forward from Corps Headquarters. Hostile artillery activity began to increase considerably in March, and about the middle of the month became so marked that from this and other symptoms an immediate attack was expected. This cut off the supply of labour, but the sappers were busier than ever in bringing the cables already laid into use by temporary joints, without waiting for the construction of the test dug-outs. This alarm proved for the time false, the offensive commencing instead on the Third and Fifth Army fronts on the 21st March. The immediate effect was the withdrawal of all the divisions in the Corps, and their replacement with exhausted divisions from the south—the 34th and 40th—who naturally could provide no labour. The greatest anxiety was now felt lest the communication should not be finished in time, and work was rushed day and night. With the assistance of the Corps Cyclists temporary sandbagged test points were erected where the concrete work was still lacking, and when the storm finally broke on the 9th April practically all cable laid had been joined up and was working, and three-quarters of the original scheme had been completed.