The one thing above all others that the Zeppelin commander feared was the attack of airplanes. In the early stages of the war, it was considered unsafe for airplanes to fly by night because of the difficulty of making a landing in the dark. Later this difficulty was overcome by the use of search-lights at the landing-fields. The airplane would signal its desire to land and the search-lights would point out the proper landing-field for it. So that after the first few months of the war Zeppelins were subjected to the danger of airplane attack. Of course, on a dark night it was very difficult for an airplane to locate a Zeppelin, because the huge machine could not be seen and the throb of its engines was drowned out by the engines of the airplane itself. Nevertheless, Zeppelins were occasionally located and destroyed by airplanes. [Illustration: Photograph by Kadel & Herbert Enormous Range-finders mounted on a Gun Turret of an American Warship] The danger of the Zeppelin lay in the fact that it was supported by an enormous volume of very inflammable gas and the airplane needed but to set fire to this gas to cause the destruction of the giant of the air. And so the machine-guns carried by airplanes were provided with explosive, flaming bullets. A burst of flame within the gas-bag would not set the gas on fire, because there would be no air inside to feed the fire, but surrounding the gas-bag there was always a certain leakage of hydrogen which would mix with the air in the compartment and this would produce an explosive mixture which needed but the touch of fire to set it off. The Zeppelin was provided with a ventilating-system to carry off these explosive gases, but they could never be disposed of very effectively, and, as a consequence, a number of Zeppelins were destroyed by the tiny antagonists that were sent up by the British and the French. To fight off these assailants the Germans provided their Zeppelins with guns which would fire shrapnel shell. It is difficult for a Zeppelin to use machine-guns against an airplane because the latter would merely climb above the Zeppelin and would be shielded by the balloon itself. And so the Germans put a gun emplacement on top of the balloon both forward and aft. There was a deck extending along the top of the balloon which was reached by a ladder running up through the center of the airship. But it was impossible to ward off the fleet little antagonists, once the dirigible was discovered. True, a Zeppelin could make as much as seventy miles per hour, but the fastest airplanes could travel twice as fast as that. SUSPENDING AN OBSERVER BELOW THE ZEPPELIN