limits the speed of advance of an army by the rate at which the railhead on which it is based can be moved forward. Throughout the whole of the present advance and the greater one which was to follow, the movement of our railhead proved to be the decisive factor. So thoroughly had the German Engineers done their work, that the position of railhead was never certain for two or three days together. Our own Railway Engineers would work night and day repairing the permanent way, the rails of which had been blown up with small gun-cotton charges at intervals of ten or twenty yards, and would successfully get the line completed as far as Bohain or Vaux Andigny. A delay-action mine would then go up between Fresnoy and Bohain or between Bohain and Vaux Andigny, and back would go the railhead again for some days, while the gap was being filled by gangs of Chinese coolies, or German prisoners. Once more the line would be put through and trains would arrive with rations and supplies for a few days, when again a mine would throw the railhead back several miles. Thus, the question of supplies was a very difficult one indeed, and one which definitely limited the progress made by the Division.
In its broader aspect, therefore, from the view of the pursuing troops, the chief disadvantage of the delay-action mine was undoubtedly its effect on the transport of the Army. The Divisional troops, however, pressing on in the van after the retreating Germans, were more intimately concerned with the mines placed at the cross-roads, or at irregular intervals along main roads, and timed to explode within a few hours of the German retreat. A party of our men would be scouting carefully along the road when, without warning, several of these mines would explode with a roar, throwing a column of débris and
