Owing to the fact that skilful landing is the most difficult thing for a flier to acquire, and because more accidents occur to the novice when he brings his machine to the ground than at any other time except, perhaps, when stunting too near the ground, night flying is especially hazardous. With properly lighted landing-fields in peace-times much of the peril of landing after dark can be eliminated, provided the night is clear and no fog or mist has settled over the aerodrome since the aviators set out. If a mist has settled over the landing-place the flier must take his chances and come down by guesswork, unless his machine is equipped with wireless telephone, for the compass and other instruments cannot tell him exactly where he is with regard to hangars or take-off on an aviation-field. Indeed, if the telephone operator on the ground cannot exactly locate the flier, it is exceedingly difficult to direct the airman to the exact corner of the field in which he should come down. On a clear night, however, with flambeaux, search-light flares, etc., a pilot has little trouble in landing, for the straightaway can be as illuminated as it is in broad daylight. Nevertheless, when the aircraft is high in the sky, owing to the vast distances of infinite space, the speed at which an aeroplane moves, and the drift out of its regular course, due to the wind, it is often difficult for the flier to keep his bearings. For that reason aviators try at night to locate the lights on a railroad-track, the reflection of light on a river or stream, and follow them to their destination. The Germans in their raids on London usually tried to locate the Thames River, which they then followed until they reached the metropolis, which they usually succeeded in doing on moonlight nights despite the British long-rayed search-lights, swift-climbing Sopwith Camels, and the barrages formed by the thousands of anti-aircraft guns. As a matter of fact, no adequate means of preventing aeroplane raids was developed by any of the countries involved in the Great War, for the simple reason that there is no way of screening off a metropolis so that those modern dragon-flies cannot fly around, over, or through the screen. That is another reason why a huge commercial aerial fleet will always be a tremendous danger and perpetual threat to any contiguous country or neighboring city, because these aerial freighters can be loaded with inextinguishable incendiary bombs as easily as with passengers, and 10,000 such aeroplanes could drop on a city within a hundred miles of its border enough chemical explosives to raze it by fire.