two or three other metals—its exact content was kept a secret for years). Early Zeppelins used rubberized cotton for the gasbags, but most later craft used goldbeater's skin, made from the intestines of cattle. The first Zeppelins had long cylindrical hulls with tapered ends and complex multi-plane fins. During World War I, following the lead of their rivals Schütte-Lanz Luftschiffbau, the design changed to the more familiar streamlined shape with cruciform tail surfaces, as used by almost all later airships. They were propelled by several engines, mounted in gondolas or engine cars, which were attached to the outside of the structural framework. Some of these could provide reverse thrust for manoeuvring while mooring. Early models had a comparatively small externally-mounted gondola for passengers and crew which was attached to the bottom of the frame. This space was never heated (fire outside of the kitchen was considered too risky) so passengers during trips across the North Atlantic or Siberia were forced to bundle themselves in blankets and furs to keep warm and were often miserable from the cold. By the time of the Hindenburg, several important changes had taken place: the passenger space had been relocated to the interior of the overall vessel, passenger rooms were insulated from the exterior by the dining area, and forced-warm air could be circulated from the water that cooled the forward engines, all of which made traveling much more comfortable. This did prevent passengers from enjoying the views from the windows of their berths though, which had been a major attraction on the Graf Zeppelin. On both the older and newer vessels, the external viewing windows were often open during flight. The flight ceiling was so low that no pressurization of the cabins was necessary, though the Hindenburg did maintain a pressurized air-locked smoking room (no flame allowed, however—a single electric lighter was provided, and could not be removed from the room). Access to Zeppelins was achieved in a number of ways. The Graf Zeppelins gondola was accessed while the vessel was on the ground, via gangways. The Hindenburg also had passenger gangways leading from the ground directly into its hull which could be withdrawn entirely, ground access to the gondola, and an exterior access hatch via its electrical room; this latter was intended for crew use only. On some long-distance units, Blau gas was used to run the engines of the Zeppelin airships. This