The concentration of the troops in the assembly trenches was a matter which required elaborate arrangements. On the night of the 28-29th the two brigades took over their battle fronts each with four companies, the remainder of the troops moving up to the trenches on the night before the battle. "To ensure secrecy, it was impossible, owing to the enemy's activity in the air, to move from the camps in which the brigades were assembled before 8.30 P.M. There was thus barely time for the attacking force to arrive in its position of deployment before zero hour. It was therefore necessary to eliminate by careful arrangement all likely causes of delay. To avoid loss of direction, overland routes to the entrances of the communication trenches were marked out by stakes and tapes. Men who had been made familiar with these routes were employed as guides. Police were also posted at all junctions of tracks and of trenches, to ensure that in the event of gas being encountered the men would adjust their respirators as quickly as possible and continue their march. The troops were also instructed that in no circumstances was the march to the position of deployment to be delayed by enemy action. Previous to the attack this march had been rehearsed in the dark by bodies of troops both with and without respirators, so that it could be ensured, in selecting the zero hour, sufficient time was allowed for the operation. A hot meal was issued at the place of assembly, and soup and tea were issued from hot-food containers at the positions of deployment."[7] These arrangements proved adequate. The hostile artillery was exceptionally quiet, and the assembly was completed without incident. At 3.50 A.M., 31st July, the third battle of Ypres opened. At that hour the sky was covered with clouds, and in consequence there was little natural light. However, the burning oil thrown on the enemy lines by the trench-mortars at zero hour illuminated the whole front. The troops had little difficulty in forming up under the barrage, except that the numerous water-filled shell-holes made the maintenance of direction and of proper extensions no easy matter. The going was also very heavy. The enemy's artillery did not open in reply to our barrage for some ten minutes, and fell on and in rear of the old British front line. The leading battalions were, from right to left, the 5th Seaforth Highlanders, the 8th Argyll and Sutherland Highlanders, the 7th Gordon Highlanders, the 7th Black Watch. These four battalions advanced to the Blue line without a check. The trenches were found to have been almost obliterated by the artillery. Some of the farms had been so demolished by shell-fire that they could not be located until the sun was above the horizon. The gunners had, indeed, carried out their tasks.