A cat walk—an arched passage with a footway nine inches wide—running along the keel enabled the crew, which consisted of twenty-two men, to move about the ship and get from one gondola to another. The gondolas, made of aluminum alloy, were four in number: one was placed forward on the centre line; two were amidships, one on each side, and the fourth was aft, again on the centre line. The vessel was propelled at 60 miles an hour in still air—by means of six Maybach-Mercedes gasoline engines of 240 horse-power each, or 1,440 horse-power in all. Each had six vertical cylinders with overhead valves and water cooling, and weighed about a thousand pounds. They were connected each to a propeller shaft and also to a dynamo used either in lighting or for furnishing power to the wireless installation. One of these engines with its propeller was placed at the back of the large forward gondola; two were in the amidships gondolas, and three were in the after gondola. In the last case one of the propellers was in the centre line of the ship, and the shafts of the two others were stayed out, one on either side. The gasoline-tanks had a capacity of two thousand gallons, and the propeller shafts were carried in ball bearings. Forward of the engine-room of the front gondola, but separated from it by a small air space, was first the wireless-operator’s cabin and then the commander’s room. The latter was the navigating platform, and in it were concentrated the controls of the elevators and rudder at the stern, the arrangement for equalizing the levels in the gasoline and water tanks, the engine-room telegraphs, and the switchboards of electrical gear for releasing the bombs. Nine machine-guns were carried. Two of these, of half-inch bore, were mounted on the top of the vessel, and six of small caliber were placed in the gondolas—two in the forward, one each in the amidships ones, and two in the aft one. The ninth was carried in the tail. The separate gas-bags were a decided advantage over the free balloon and earlier airships, which carried all the gas in one compartment; for if the latter sprung a leak for any reason it had to descend, whereas the Zeppelin could keep afloat with several of the separate compartments in a complete state of collapse.