Then a new aeroplane was developed, the single-seater tractor, with a Vickers gun, synchronized to shoot through the rapidly revolving propeller so as to avoid the blades. These machines were used to patrol the lines and keep enemy machines from crossing, or to accompany a reconnaissance machine as protector; for they were very much faster, easier to manoeuvre, and altogether very much more efficient fighters. At first they operated singly, but it was soon discovered that two of these scout machines operating together invariably obtained better success than when operating alone. This led to formation flying, and up to the cessation of hostilities these formations grew in size and varied in shape. The reconnaissance work was soon divided into classes: long and short reconnaissance and photographic reconnaissance. The long reconnaissance dealt with enemy movements far behind the lines; the short reconnaissance with enemy activities near the front. The photographic reconnaissance consisted of taking aerial photographs of everything of military importance within flying radius. These photographs pieced together showed the enemy defences along the entire British front and their changes from day to day. Wireless apparatus was soon attached to aeroplanes, and this enabled an aviator to communicate with people on the ground many miles away; and so what was called artillery observation was developed. Roughly speaking, this is the direction of the fire of our batteries against enemy targets; but, just as specialization came in reconnaissance and fighting, so now machines specialized in artillery observation. To-day the efficiency of the artillery depends largely upon its direction from the air. For instance, when a battery takes over a new area the gunners may be called upon to fire at certain targets, such as cross-roads or houses used as infantry headquarters or ammunition and stores dumps, at a moment's notice. Consequently, if these targets are registered by aeroplane, all the gunners have to do when called upon to open fire is to refer to their registration book which will give them the necessary angles to use on their sights, then, by allowing for the temperature of the day and the direction and velocity of the wind, their shooting is certain to be far more accurate than it would be if the target had not been previously registered. The registration of targets to-day without the use of areoplanes is very often impossible.