Each gas-bag is mounted above an air-bag, and when the gas-bag is inflated to the maximum the air-bag is almost empty. The ship is then at its most buoyant stage. To reduce this buoyancy the air pumps are put in motion, and they force air under pressure into the air-bags. This pressure, acting on the gas-bags, forces out the hydrogen through pipes and non-return valves to the storage tanks. If at any time it is required to make the vessel ascend, the air-bags are deflated and the gas supply pipe with its pump is employed to force more hydrogen into the gas-bags. One thousand cubic feet of hydrogen have a lifting power of nearly 75 lbs. at sea-level, and this lifting power acts very quickly. Thus a Zeppelin changes its altitude rapidly when the weight is altered, and at the same time there is automatic control whereby the vessel can be kept at the same level if necessary. When a Zeppelin drops a bomb it suddenly becomes lighter, and it rises in consequence. This circumstance is very disconcerting to gunners, for if, say, a 200 lb. bomb were dropped, the ship would leap up nearly 200 feet in the air, unless the captain desired to check the ascent. The discharge of water ballast produces the same rising effect, and with almost equal suddenness the ship can sink by using its powerful air pumps to press out the hydrogen. Moreover, when the Zeppelin is in motion it can use its elevating planes for changing altitude in the manner of an aeroplane. Thus, in addition to its power of steering from left to right in the same plane, and of climbing and descending along an inclined path by the use of the elevators, the Zeppelin can rise and fall vertically, and by its system of storage tanks these manœuvres can go on for a long period. [Illustration: SECTIONAL VIEW OF ZEPPELIN AIRSHIP, SHOWING THE ARRANGEMENT OF THE HYDROGEN AND AIR BALLONETS WHICH CONTROL THE WEIGHT OF THE AIRSHIP, THUS ENABLING IT TO RISE AND FALL AS REQUIRED. (1) Section of one of the eighteen ballonets. (2) Hydrogen gas-bag partly inflated. (3) Air. (4) Rear gondola. (5) Outer covering of fabric. (6) Metal work. (7) Air space between gas-bag and frame. (8) Hydrogen gas-bag fully inflated. (9) Flexible gas-pipe. (10) Inner ballonet deflated. (11) Metal gas tank into which hydrogen is pumped under pressure. (12) Forward gondola. (13) Flexible pipe from pump to ballonet. (14) Keel cabin. (Diagram from a photograph taken from a point at the forward part of a Zeppelin Airship.)]