But Count Zeppelin was a man of vision. He dreamed of a real ship of the air--a machine that would sail wherever the helmsman chose, regardless of wind and weather. Many years elapsed before he actually began to work out his dreams, and then he met with failure after failure. He believed in big machines and the loss of one of his airships meant the waste of a large sum of money, but he persisted, even though he spent all his fortune, and had to go heavily in debt. Every one thought him a crank until he built his third airship and proved its worth by making a trip of 270 miles. At once the German Government was interested and saw wonderful military possibilities in the new craft. The Zeppelin was purchased by the government and money was given the inventor to further his experiments. That was not the end of his failures. Before the war broke out, thirteen Zeppelins had been destroyed by one accident or another. Evidently the building of Zeppelin airships was not a paying undertaking, although they were used to carry passengers on short aërial voyages. But the government made up money losses and Zeppelin went on developing his airships. Of course, he was not the only one to build airships, nor even the first to build a dirigible. The French built some large dirigibles, but they failed to see any great military advantage in ships that could sail through the air, particularly after the airplane was invented, and so it happened that when the war started the French were devoting virtually all their energies to the construction of speedy, powerful airplanes. As for the British, they did not pay much attention to airships. The idea that their isles might be attacked from the sky seemed an exceedingly remote possibility. RIGID, SEMI-RIGID, AND FLEXIBLE BALLOONS Count Zeppelin always held that the dirigible balloons must be rigid, so that they could be driven through the air readily and would hold their shape despite variations in the pressure of the hydrogen. The French, on the other hand, used a semi-rigid airship; that is, one in which a flexible balloon is attached to a rigid keel or body. The British clung to the idea of an entirely flexible balloon and they suspended their car from the gas-bag without any rigid framework to hold the gas-bag in shape. In every case, the balloons were kept taut or distended by means of air-bags or ballonets. These air-bags were placed inside the gas-bags and as the hydrogen expanded it would force the air out through valves, but the hydrogen itself would not escape. When the hydrogen contracted, the air-bags were pumped full of air so as to maintain the balloon in its fully distended condition. Additional supplies of compressed hydrogen were kept in metal tanks. [Illustration: (C) Underwood & Underwood Giant British Dirigible built along the lines of a Zeppelin] [Illustration: (C) Underwood & Underwood One of the engine cars or "power eggs" of a British Dirigible]