Since the Zeppelin, like all airships, is buoyed up by hydrogen gas—which weighs one and one-tenth pounds per two hundred cubic feet as compared with sixteen pounds which the same amount of air weighs—the dirigible is sent up by the simple expedient of increasing the volume of gas in the envelopes until the vessel rises. 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 rudder or the engines, or both, and the airship was kept on an even keel by use of 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, could climb to 12,000 feet, could carry a useful load of 30 tons, and could remain in the air for 50 hours. Because so many Zeppelins were lost to Germany and because so much time and money were necessary to construct the enormous airships, many people have jumped to the conclusion that the rigid dirigible was an absolute failure even as an offensive war weapon. Yet despite its bulk and the fact that it could not fly faster than seventy miles an hour, and though more than a hundred Zeppelins raided England at some time or another during the war, only two were shot down by aeroplanes and only a few by antiaircraft guns. Most of them were destroyed because they ran out of fuel and consequently became unmanageable and were blown out of their course and forced to land or had to descend so low that they came within easy range of aircraft guns of the land batteries or the naval guns. This record is truly surprising when we stop to consider that the Zeppelin had to navigate entirely by compass and mostly at night over hundreds of miles of hostile sea and land, opposed by the guns of a huge Allied fleet and thousands of antiaircraft guns, without lights or landmarks to aid them and often with untrained and inexperienced pilots to guide them! No wonder that some of these airships met disaster—like the L-49, which had to land in France; or the L-20, which was forced to land on the Norwegian coast near Stavanger; or others, which came down so low over the North Sea that they became easy targets for the British torpedo-boat guns.