Since the Zeppelin, like all airships, is buoyed up by hydrogen gas which is .008 lighter than air, the dirigible was sent up by the simple expedient of increasing the volume of gas in the envelope until the vessel arose. This was done by releasing the gas for storage-tanks into the gas-bags. In order to head the nose up, air was kept in certain of the rear bags, thus making the tail heavier than the forward part, which naturally rose first. Steering was done by means of the rudder or the engines, or both, and the airship was kept on an even keel by use of the lateral planes. The airship could be brought down by forcing the gas out of the bags into the gas-tanks, thus decreasing the volume and by increasing the air in the various compartments. This airship had a flying radius of 800 miles and could climb to 12,000 feet, and could carry a useful load of four tons and could remain in the air for fifty hours. Without a doubt it is one of the largest rigid dirigibles ever built. [Illustration: _Courtesy of Flying Magazine._ Observation balloon about to ascend. These balloons were stationed at intervals along the battle-fronts.] Owing to the great amount of material used, the immense cost, and the time necessary to construct a Zeppelin, under the urgent demands of war, the British built and developed a small rigid dirigible measuring between 200 and 250 feet in length, buoyed up by two balloonets, one front and back, and carrying a fuselage and one aeromotor, and propeller situated directly under the cigar-shaped airship. These vessels made about fifty miles an hour, carried two men, were fitted with wireless, and made excellent scouts over the North Sea and waters contiguous to allied territory, looking for submarines. These air-vessels were called Blimps. The kite balloon was cigar-shaped and non-rigid, with only a basket suspended underneath. It was attached to a rope and was lifted by the gas and the wind which passed under the fins, which extended from the sides near the rear. It combined the principle of the free balloon and the man-lifting kites. These balloons were used very extensively in the Great War for observation purposes. Suspended at the end of a cable attached to a donkey-engine or a windlass at an altitude of 3,000 feet, they afforded the best observation for artillery-fire, and by means of the telephone in the basket the observer could keep headquarters well informed of troop movements within a radius of many miles. Naturally it was the special delight of the aeroplanes to dive down on these stationary balloons and by means of incendiary bullets to ignite the gas. It was dangerous work for the heavier-than-air machines, for all the way down the antiaircraft guns blazed away. It was also dangerous work for the observers in the imprisoned balloon, who often had to jump with their parachutes in order to escape.