pressure is called the pressure height: if the airship climbs beyond this it is necessary to vent gas to prevent rupturing the gasbags. History Early history By 1874, several people had conceived of a rigid dirigible (in contrast to non-rigid powered airships which had been flying since 1852). The Frenchman Joseph Spiess had patented a rigid airship design in 1873 but failed to get funding. Another such individual was the German Count Ferdinand von Zeppelin, who had outlined his thoughts of a rigid airship in diary entries from 25 March 1874 through to 1890 when he resigned from the military. David Schwarz had thought about building an airship in the 1880s and had probably started design work in 1891: by 1892, he had started construction. However, Schwarz's all-aluminium airship would not perform any test flights until after his death in 1897. Schwarz had secured help in its construction from the industrialist Carl Berg and the Prussian Airship Battalion; there was an exclusive contract in place between Schwarz and Berg, thus Count Zeppelin was obliged to reach a legal agreement with Schwarz's heirs to obtain aluminium from Carl Berg, although the two men's designs were different and independent from each other: the Schwarz design lacked the separate internal gasbags that characterise rigid airships. Using Berg's aluminium, von Zeppelin was able to start building his first airship, the LZ 1, in 1899. First practical rigid airships During July 1900, Ferdinand von Zeppelin completed LZ 1. Constructed in a floating shed on Lake Constance, it was 128.02 m (420 ft) long, 11.73 m (38 ft 6 in) in diameter with a volume of 11,298 m3 (399,000 ft 3) and was powered by a pair of 11 kW (14 hp) Daimler engines. The first flight, lasting 20 minutes, was made on 2 July, but ended with the airship being damaged. After repairs and modifications, two further flights were conducted in October 1900. However, these initial experiments failed to attract any investors, and Count Zeppelin did not complete his next design, LZ 2, until 1906. This performed only a single flight on 17 January 1906, during which both engines failed and the zeppelin was compelled to conduct a forced landing in the Allgäu mountains; it was subsequently damaged beyond repair by a storm. Undeterred, another zeppelin with a largely similar design, the LZ 3, was quickly completed and put into flight. LZ 3 proved to