The British forces have been preparing to attack Messines Ridge. Since 21 March the Corps aircraft have been carrying out spotting for the preliminary artillery bombardment.
At conference on the 30 May, captured German documents revealed that the enemy would rely, for defence, mainly on prearranged schemes of artillery fire. This raised the importance of counter-battery work.
To induce the Germans to disclose the positions of their barrage batteries, it was arranged that a full-dress rehearsal of the artillery bombardment, as it would be at zero hour, with a smoke demonstration along the front of attack, should take place today. The hour for this rehearsal was fixed on the advice of the Royal Flying Corps because it was essential to choose conditions favourable for the placing of the maximum strength in the air to discover the enemy guns.
The full-dress rehearsal of the artillery barrage on the Messines ridge was made this afternoon when thirty-one Corps aeroplanes kept watch to note the positions of the German batteries. They were ill-rewarded. The enemy retaliation was feeble, and not many new emplacements were discovered. However air photographs revealed much about the accuracy of the barrage. Artillery staff officers were also flown over the front while the bombardment was in progress enabled many minor errors of timing to be adjusted.