The trench was reconnoitred throughout its length, and it was found to have fallen in to a depth of about four feet, and to have been completely filled in with entanglements of barbed wire. There was, further, a fairly strong wire entanglement in front of it. The saps connecting this trench to the present front-line trench had all been completely blocked when they joined the existing front line, and had also been wired up throughout their length. A large amount of work was therefore required to clear this trench so that the troops could occupy it for assembly. It was, however, decided to do no work in it beyond removing the barbed wire from it so as not to attract the enemy's attention. The whole area was found to be very deficient of dug-outs suitable for headquarters. Three new brigade battle headquarters had therefore to be tunnelled as well as some additional battalion battle headquarters and regimental aid posts. Machine-gun positions had also to be constructed for guns firing in the machine-gun barrage during the attack, and accommodation had to be provided for their crews. The Divisional area also contained an insufficiency of artillery positions, particularly of forward positions required by the guns firing in the barrage. Considerable preparations had therefore to be made by the artillery. The system of buried telephone cables was also incomplete. In fact, it was only completed in time for the engagement by concentrating every available man on this work. As regards the preparations of the enemy's position for the attack, the chief consideration was his wire entanglements, which had been erected in profusion. Those which were to be encountered in the first phase of the attack, being on the western slopes of the Vimy Ridge, were visible, and could be cut by the artillery with direct observation. The later stages of the attack were, however, planned to pass over the ridge and reach the plains beyond it. The wire to be encountered in this phase of the attack was, therefore, on the reverse slope of the Vimy Ridge, and could only be seen from the air. Unfortunately, at this period the German air service established a marked superiority over our reconnaissance machines. In a single period of twenty-four hours, three R.E. 8's (reconnaissance aeroplanes) were crashed within full view of a single brigade headquarters, one of them being hunted right down to the ground, where it overturned, with both its occupants wounded.