The frame of the hull is formed of sixteen longitudinal beams, or girders, of trellised metal work running from prow to stern and riveted at regular intervals to cross bridges of similar trellised metal work, each cross bridge being a sixteen-sided wheel with trellised rims strengthened by radial rods running inward to a central flange of sheet aluminum. Thus the body of the vessel is divided into many compartments, each bounded by two wheels, and the surrounding longitudinal beams. Each compartment contains a hydrogen balloon, or sac, which fairly fills it and exerts a lift against the longitudinal beams and against a netting formed of ramie cords stretched from wheel to wheel, diagonally between beams at their inner corners. Similarly the outward corners of the beams are joined by strong diagonal wires for the purpose of rigidity, and the whole external frame is covered with a heavy fabric which forms the outer skin, or wall of the hull. Between this skin and the hydrogen bags are air spaces, as also between bag and bag. Thus the whole vessel is buoyed up by numerous thin hydrogen sacs, protected by the frame and outer skin from the direct sun, from foul weather, and from external shocks. The gas bags are also separated from each other by the bridge work and flanges of aluminum. PLATE X. [Illustration: _GROSS III._] [Illustration: ZEPPELIN AIRSHIP STRUCTURE.] Obviously there is a material advantage in having many gas cells and two propelling plants; for if one fails it may not prevent navigation. The tandem arrangement of bags separated by the wheel-like cross bridges also allows the balloon to rear any amount without material displacement of the gas, or dangerous increase of pressure; for it must be remembered that a single hydrogen sac extending the full length of an up-ended balloon of such length, would have an outward pressure of about thirty pounds per square foot greater at the top than at the bottom. The poise of the vessel is maintained by shifting weights, and also by use of fins, or rudders, when driving through the air; but those arrangements vary in the different machines. So much for the general features of these wonderful ships, of which four were built during the decade from 1898 to 1908, and several more since that period.