The frequent bombing of German supply centres in Belgium and North France came into being with the development of aerial photography. Owing to the difficulty of correct aim, before the advent of modern bomb-sights, all the early raids were carried out from a low altitude, sometimes from only a few hundred feet. For every purpose, moreover, low altitudes were the rule in the earlier months of the war, as most of the machines would not climb above 4000-7000 feet. Much of the observation was performed at something between 1000 and 2000 feet, so that aircraft often returned with a hundred or so bullet-holes in them. Meanwhile the important work of artillery spotting was being developed. New systems of co-operation between artillery and aeroplanes were devised, tested, and improved. At first lamps or Very's lights were used to signal code-corrections, but these were soon replaced by wireless transmission from the observation machine. Targets which could not be ranged on through ground observation posts became targets no longer, after one shoot ranged from the air. As the number of available aircraft increased, so did the amount of observation for the guns, until finally the entire front opposite the British was registered for bombardment and divided into sections covered by specified artillery machines. Aerial fighting, now so essential and scientific a branch of modern war, was rudimentary in 1914. Pilots and observers of the original Flying Corps carried revolvers, and many observers also equipped themselves with rifles, but the aeroplanes were not fitted with machine-guns. Such scraps as there were consisted of one machine manoeuvring round an opponent at close quarters for the chance of a well-aimed shot. Under these circumstances to "bring down" or "drive down out of control" an enemy was extremely difficult, though a very gallant officer, since killed in action, once killed two German pilots within five minutes with his revolver. Soon the possibilities of aerial machine-guns were quickly recognised. The R.F.C. adopted the Lewis, which from the points of view of lightness and handiness was well suited for aircraft, and the German airmen countered with a modified Hotchkiss and other types. But the stable observation machines, while excellent for reconnaissance and artillery spotting, allowed their crews only a small arc of fire, and not until the German single-seater scouts and our Bristol scout, then a comparatively fast machine, appeared on the western front in the spring of 1915 did the destruction of aeroplanes become an everyday occurrence. With the introduction of scouts for escort and protective duties came formation flying and concerted attack.