A fact to be considered, also, when the problem of aeroplanes and gun-fire is under review, is the distance at which aircraft are visible from the ground. In ordinary weather, and under normal conditions, it is generally estimated that a reconnoitring aeroplane should be sighted when it is about three miles away. But, even in clear atmospheric conditions, the aircraft is an elusive object to locate. Even when one is expected to appear, from a certain direction, and all eyes are fixed upon the sky, awaiting its advent, it is frequently almost at its destination before anyone locates it. More difficult, as can be imagined, is the task of sighting an aeroplane when it is not known from what point of the compass it is likely to appear. And yet this, of course, will be the position of the gunner in war-time. A hostile aircraft may loom up from anywhere—even from over his own troops. It will be possible for a reconnoitring machine to ascend to a great height, and conceal itself in low-lying clouds. From these it will be able to descend swiftly, effect a rapid reconnaissance, and then "climb" again until lost to sight. In such circumstances, the artilleryman will need to be phenomenally handy with his gun if he is to note the approach of so cunning a scout, and "wing" him before he has slipped out of range. A point which has been referred to before—but which artillery experts are prone to ignore—is the skill a military pilot will be able to exercise, in avoiding fire from below. In many cases, during a reconnaissance, the observer should be able to obtain all the information he seeks without once coming within range of the enemy’s guns. Naturally, the aeroplanist will never fly intentionally over artillery, or court infantry-fire. Long-distance observations will often be possible, giving the gunner no chance of using a shell; or, if it is necessary to come fairly close to troops, for a detailed piece of reconnaissance, the airman will swoop down, and as speedily get clear again. When he knows he is likely to be within range of any of the enemy’s guns, he will pursue an erratic course. Therefore, the gunner, when he does obtain a chance of firing at a machine, will find his target darting about in disconcerting fashion. A point arises as to establishing the identity of an aircraft, when it is sighted during time of war. Machines will fly flags, indicating their nationality, but these flags are not likely to be seen at any great distance. Therefore, if an artilleryman detects an aeroplane, approaching at an appreciable altitude, it will frequently be impossible for him to determine whether it is friend or foe.