Machines fitted with cameras photograph every inch of the defences improvised by the enemy, and, as insurance against being caught unprepared by a counter-attack, an immediate warning of whatever movement is in evidence on the lines of communication will be supplied by the reconnaissance observers. Under the direction of artillery squadrons the guns pound the new Boche front line and range on troublesome batteries. The bombing craft are responsible for onslaughts on railways, supply depôts, garrison towns, headquarters, aerodromes, and chance targets. Other guerilla work is done by craft which, from a height of anything under a thousand feet, machine-gun whatever worthwhile objects they spot. A column of troops on the march, transport, ammunition waggons, a train, a stray motor-car--all these are greeted joyfully by the pilots who specialise in ground stunts. And at every hour of daylight the scouts and fighting two-seaters protect the remainder of the R.F.C. by engaging all Huns who take to the air. Doubtless, when sunset has brought the roving birds back to their nest, there will be a few "missing"; but this, part of the day's work, is a small enough sacrifice for the general achievement--the staff supplied with quick and accurate information, a hundred or two Boche batteries silenced, important works destroyed, enemy communications impeded, a dozen or so black-crossed aeroplanes brought down, valuable photographs and reports obtained, and the ground-Hun of every species harried. The German Flying Corps cannot claim to perform anything like the same amount of aerial observation as its British counterpart. It is mainly occupied in fighting air battles and hampering the foreign machines that spy on their army. To say that the German machines are barred altogether from reconnaissance and artillery direction would be exaggeration, but not wild exaggeration. Seldom can an enemy plane call and correct artillery fire for longer than half an hour. From time to time a fast machine makes a reconnaissance tour at a great height, and from time to time others dart across the lines for photography, or to search for gun positions. An appreciable proportion of these do not return. Four-fifths of the Hun bomb raids behind our front take place at night-time, when comparative freedom from attack is balanced by impossibility of accurate aim. Apart from these spasmodic activities, the German pilots concern themselves entirely with attempts to prevent allied observation. They have never yet succeeded, even during the periods of their nearest approach to the so-called "mastery of the air," and probably they never will succeed. The advantages attendant upon a maintenance of thorough observation, while whittling down the enemy's to a minimum, cannot be overestimated.