A brief reference must be made to the work of the 179th Tunnelling Company, R.E., in the right sector. Most of their heavy work, such as the construction of tunnelled dug-outs and emplacements, and the maintenance of infantry sub-ways, proceeded unseen by the men of other arms, for the entrances to the deep-mining system were closed to the curious. The strata in which mining operations were possible had become so honeycombed with galleries and listening-saps that no new offensive mining could be undertaken without instant detection. The methods of keeping touch with enemy activities were full of interest. In a little chamber off the main gallery a man sat in front of a board on which was a plan of the mine system. A telephone receiver was fixed over his ears. By inserting a plug into the board at any of the sapheads he could listen by a microphone to the enemy’s subterranean activities in the vicinity of this particular sap. A pump at work in an adjacent trench, an enemy miner patrolling the gallery or tiptoeing to an adjacent saphead—even such sounds as these could be detected with uncanny distinctness. The system of holding the line by tactical localities and immediate counter-attack, which afterwards proved so successful as compared with continuous line, was made the subject of strenuous training. The reorganization was planned with the double object of reducing the number of men required to garrison the trenches, and at the same time of strengthening the defence by a series of self-contained, mutually supporting localities, disposed in depth and connected, so far as possible, by underground, shell-proof passages, with wire-entanglements so sited as to bring an approaching enemy under the fire of these localities. The term “locality” is used to denote an area of ground of tactical importance which is organized for defence in depth to form a centre of resistance. The guiding principle is that its front and flanks should be held by infantry posts and machine-guns while reserves are kept in hand ready for counter-attack within the locality. A large number of concrete machine-gun emplacements, command posts, etc., were constructed, and as there was no adequate system of light railways or tramways serving the front, a heavy strain was thrown upon the transport in bringing up R.E. material alone. The value of this work was seen later during the German offensive on the Lys in April, 1918, the sector being then held by the 55th (West Lancashire Territorial) Division. Major-General Jeudwine, commanding the 55th Division, in a letter to Major-General Solly-Flood, stated that the work of the East Lancashires had greatly assisted the West Lancashires to defeat the onslaughts of the enemy—and thus hold inviolate that most important tactical feature, Givenchy—and that he had pleasure in acknowledging the debt they owed to their predecessors. [Sidenote: _Raids_]