Small blue crosses represent the position of enemy balloon barrages and their height. The position of these barrages must be known accurately, for to run into them is fatal and at night they are very apt to trap the unwary. Roughly, they are a series of balloons supporting a huge wire net or cable streamers. The balloons, anchored to the ground and carrying the nets with them, are sent up to a considerable altitude about large cities and important industrial centres. They are to the night aviators what the spider's web is to the fly. Another conventional sign of this map which is always puzzling to the uninitiated is a series of small pins with streamers attached. These streamers are marked with green dots. One streamer will have one green dot, another two green dots, another three, etc., while others will have different spaces between the dots. These pins mark the position of what is called the "Hun green-ball batteries," and these green balls, fired up to a height of about six thousand feet, direct the Hun aviators to their respective aerodromes when returning from a night raid. A better system than this for directing aviators at night has never been devised, for low clouds or mist cannot obliterate the signal and they are visible to the aviator for over fifty miles. In fact, this type of signal was so very excellent that our knowledge of the exact positions of the various batteries was of great assistance to us in our raids over Germany. On our side of the lines this map was marked with conventional signs similar to those which marked the position of enemy anti-aircraft batteries, aerodromes, and balloon barrages; but on our side of the lines there were large areas marked in red to indicate what was called "prohibited areas"; i.e., areas over which no aeroplane, Allied or enemy, could fly without being subjected to the fire of our anti-aircraft batteries. There were also white drawing-pins, each bearing a letter, placed at irregular intervals. These located accurately the position of small lighthouses which are usually about fifteen miles apart and from three to ten miles back of the front-line trenches; the letter marked on each drawing-pin designates the letter flashed in Morse code by that particular lighthouse. This system of signals, used by the British to direct their night aviators to their aerodromes when returning from a raid, had but two great faults. In the first place, the signal was obliterated by low clouds and mist. In the second place, the flash of the light only carried a few miles even under the best conditions. On the other hand, the letters which the lighthouses flashed could be readily changed and consequently were of very little assistance to Hun aviators. On the third wall of the map-room are aerial photographs of enemy aerodromes, railway stations, sidings, etc., and large-scale plans of German towns and factories.