Now as regards rising and falling. To many people the manœuvring of a Zeppelin in the air is still a matter of mystery. It is certainly not easy for the lay mind to grasp and hold the fact that a monster vessel made of metal, and weighing nearly 20 tons, can float in a medium through which a feather falls. The Zeppelin, in effect, is lighter than a feather, volume for volume, and this lightness is obtained by creating an enormous space within the carcase of the ship and filling this space with hydrogen gas, which is about fifteen times lighter than air. If we imagine that a steel boiler 50 feet long has the same width and height as a Zeppelin and weighs 20 tons, it is easy to understand that if this were filled with hydrogen gas it would not float in the air. But imagine the boiler to be drawn out until it was 500 feet long, and one gets some idea of the lightness of the Zeppelin structure. Each plate of metal in the boiler would be increased to ten times its normal length, and thus would become exceedingly thin. Of course, in the Zeppelin lighter materials are used, with the result that for a small weight we get an enormous volume. Then, by filling this space with hydrogen the ship displaces its own volume of air, but this volume of air is so much heavier than the ship’s weight that the vessel rises. The most remarkable feature of the Zeppelin is the ingenious manner in which the volume of hydrogen is controlled, and through this control the altitude of the ship is regulated. In principle the method resembles that of the air bladder of a fish. When the eighteen gas-bags of a Zeppelin are filled with hydrogen the ship is at its maximum of buoyancy or lightness. It then has a lifting power which unless restrained by heavy weights would take the vessel high up into the air until a thin atmosphere was reached, where the ship would float motionless in a medium of less density. But if we replace the hydrogen with air when the ship is held to the ground, we increase the weight of the vessel so much that it will not rise. Thus in the Zeppelin, by the alternative use of light hydrogen and heavy air, we can so alter the weight that the vessel can be made to rise or sink. By a highly-developed system of tanks, pumps, and valves the relative volumes of hydrogen and air can be controlled with wonderful accuracy. In the older system of airships the hydrogen was allowed to escape when it was desired to make the ship heavier, but the modern Zeppelin, when it takes hydrogen from the gas-bags, is able to store the gas in metal tanks under pressure, and it also has a reserve supply to make up for unavoidable leakage.