In May, the 4th Australian Division, 11th (Northern) Division and the 24th Division were transferred from Arras aqs reserve divisions for the Second Army corps in the attack on Messines Ridge. Seventy-two of the new Mark IV tanks also arrived in May and were hidden south-west of Ypres. British aircraft began to move north from the Arras front, the total rising to about aircraft in the II Brigade RFC (Second Army) area. The mass of artillery to be used in the attack was supported by many artillery-observation and photographic reconnaissance aircraft, in the corps squadrons which had been increased from twelve to eighteen aircraft each. Strict enforcement of wireless procedure allowed a reduction of the minimum distance between observation aircraft from at Arras in April to at Messines, without mutual wireless interference. Wire-cutting bombardments began on 21 May and two days were added to the bombardment for more counter-battery fire. The main bombardment began on 31 May, with only one day of poor weather before the attack. Two flights of each observation squadron concentrated on counter-battery observation and one became a bombardment flight, working with particular artillery bombardment groups for wire cutting and trench-destruction; these flights were to become contact-patrols to observe the positions of British troops once the assault began. The attack barrage was rehearsed on 3 June to allow British air observers to plot masked German batteries, which mainly remained hidden but many minor flaws in the British barrage were reported. A repeat performance on 5 June, induced a larger number of hidden German batteries to reveal themselves.