 Large airships were also experimented with for military purposes, notably the American construction of two airborne aircraft carriers, but their large size made them vulnerable and the idea was dropped.This period also saw the introduction of non-flammable helium as a lifting gas by the United States, while the more dangerous hydrogen continued to be used since the United States had the only sources of the gas at that time, and would not export it.In 1919 the British airship "R34" flew a double crossing of the Atlantic and in 1926 the Italian semi-rigid airship, "Norge" was the first aircraft confirmed to fly over the North Pole.