Meanwhile, strategic reconnaissance was perfected as the line of the front became firmly established, and more and more importance was accorded to the search for objectives.  Remarkable results were attained by air photography from December, 1914; and after January, 1915, the regulation of artillery fire by wireless telegraphy was in general practice.  It was necessary to protect the airplanes attached to army corps, and to clean up the air for their free circulation.  This role devolved upon the most rapid airplanes, which were then the Morane-Saunier-Parasols, and in the spring of 1915 these formed the first escadrilles de chasse, one for each army.  Garros, already popular before the war for having been the first air-pilot to cross the Mediterranean, from Saint-Raphael to Bizerto, forced down a large Aviatik above Dixmude in April, 1915.  A few days later a motor breakdown compelled him to land at Ingelminster, north of Courtrai, and he was made prisoner.[18] The aviators, like the knights of ancient times, sent one another challenges.  Sergeant David—­who was killed shortly after—­having been obliged to refuse to fight an enemy airplane because his machine-gun jammed, dropped a challenge to the latter on the German aerodrome, and waited at the place, on the day and hour fixed, at Vauquois (noon, in June, 1915, above the German lines), but his adversary never came to the rendezvous.

[Footnote 18:  The romantic circumstances under which he escaped in February, 1918, are well known.]

The Maurice Farman and Caudron airplanes were used for observation.  The Voisin machines, strong but slower, were more especially utilized for bombardments, which began to be carried out by organized expeditions.  The famous raids on the Ludwigshafen factories and the Karlsruhe railway station occurred in June, 1915.  It was at the battle of Artois (May and June, 1915) that aviation for the first time constituted a branch of the army; and the work was chiefly done by the escadrilles belonging to the army corps, which rendered very considerable services as scouts and in aerial photography and destructive fire.  But as an enemy chaser, the airplane was still regarded with much distrust and incredulity.  Some said it was useless; was it not sufficient that the airplanes of the army corps and those for bombardments could defend themselves?  Others of less extreme opinions thought it should be limited to the part of protector.  This opposition was overcome by the sudden development of the German enemy-chasing airplanes after July, 1915, subsequent to our raids on Ludwigshafen and Karlsruhe, which aroused furious anger in Germany.