Maubeuge had fallen on the 7th of September and, in addition to the Seventh Reserve Corps and other troops, the siege artillery which had been used to reduce Maubeuge was brought down to the Aisne, and the British guns were outranged and outnumbered. The spotting of hostile batteries became an operation of the first importance, and the Flying Corps quickly rose to its opportunities. When trench warfare began, the aeroplanes attached to corps commands took up artillery officers daily from each division over the German batteries. The positions of these batteries were noted on maps, and the maps were sent in every day to the divisional artillery commander, who allotted the targets to his batteries. When any part of the British lines was shelled, information was obtained from the air and orders were given to those of our batteries which could best reply, to concentrate on the enemy's guns. The wireless machines of No. 4 Squadron had been attached to the army corps direct during the battle of the Marne, but their opportunities had been few. On the Aisne they were first used to observe for the artillery. Two pioneers of wireless telegraphy are associated in work and in memory with these early attempts at wireless co-operation with the artillery--Lieutenants Lewis and James. Donald Swain Lewis had joined the Royal Engineers in 1904, and, after qualifying as a pilot in May 1912, had transferred to the Royal Flying Corps in December 1913. By example and precept he had done all that he could before the war to adapt wireless telegraphy to the uses of the Flying Corps and to convince others of its necessity. Before the battle of the Aisne ended he had won his victory. He was in the habit of going out alone in a B.E. machine, piloting the machine and operating the wireless at the same time. A brother-officer noted of him in a diary: 'Lewis, R.E., came in from spotting with his machine shot full of holes; I believe he likes it!' Later on in the war, at home and in the field, he continued his work. In April 1915 he was appointed to command No. 3 Squadron, in succession to Major J. M. Salmond, and did much to maintain and advance the great reputation of that pioneer among squadrons. After a spell at home during the winter of 1915-16, he returned to France in February 1916, to command the Second Wing, co-operating with the Second Army in the Ypres salient. By this time he held the rank of lieutenant-colonel, but he continued to fly over the enemy lines. On the 10th of April 1916, flying a Morane parasol, east of Wytschaete, with Captain A. W. Gale, an officer of the Trench Mortars, as passenger, he was brought down by a direct hit from the enemy's anti-aircraft guns. He had been showing Captain Gale some of the objectives on which the trench mortar fire had been directed during the week, and was killed in action while he was carrying out the duties of that artillery observation which he had done so much to perfect.