FIG. 90–BASKET, MOTOR, AND PROPELLER OF THE BRITISH ARMY AIR-SHIP "NULLI SECUNDUS"
A balloon rises for the same reason that wood floats on water. The wood is lighter than water, and the water holds it up. The balloon is lighter than air and the air pushes it up. The upward push of the air is just equal to the weight of the air that would fill the same space the balloon fills. The balloon can support a load that makes the whole weight of the balloon and its load together equal to the weight of the air that would fill the same space. For the balloon to rise the load must be somewhat lighter than this. A balloon may be made lighter than air by filling it with heated air or coal-gas. Hydrogen, however, is used in the better balloons and in air-ships of the "lighter than air" type.
The air-ship must, of course, use a very light motor. A steam-engine cannot be made light enough. Neither can an electric motor, if we add the weight of the storage battery that would be required. Air-ships have been propelled by both steam-engines and electric motors, but with low speed because of the weight of the engine or motor. The only successful motor for this purpose is the gasolene motor, which is a form of gas-engine using gas formed by the evaporation of gasolene.
The first air-ship that could be controlled and brought back to the starting-point was made in France, in 1885, by Captain Renard, of the French army. It was a cigar-shaped balloon, with a screw propeller run by an electric motor of eight horse-power. The ship attained a speed of thirteen miles an hour.
A more successful air-ship was that built by Santos Dumont. With this ship, in 1901, he won a prize of $20,000, which had been offered to the builder of the first air-ship that would sail round the Eiffel Tower in Paris from the Aerostatic Park of Vaugirard, a distance of about three miles, and return in half an hour.
The balloon part of this air-ship was 112-1/2 feet long and 19-1/2 feet in diameter, holding about 6400 cubic feet of gas. The car was built of pine beams no larger in section than two fingers and weighing only 110 pounds. This car could be taken apart and put in a trunk. A gasolene automobile motor was used, and thus it is seen that the automobile aided in solving the problem of sailing through the air. It was the automobile that led to the construction of light and powerful gasolene motors. The car and motor were suspended from the balloon by means of piano wires, which at a short distance were invisible, so that the man in the car appeared in some mysterious way to follow the balloon. The ship was turned to the left or right by means of a rudder. It was made to ascend or descend by shifting the weight of a heavy rope that hung from the car, thus inclining the ship upward or downward.
Count Zeppelin, of Germany, constructed a much larger dirigible balloon than that of Santos Dumont. The balloon of the first Zeppelin air-ship was 390 feet in length, with a diameter of about 39 feet. It was divided into seventeen sections, each section being a balloon in itself. These sections serve the same purpose as the water-tight compartments of a battle-ship. An accident to one section would not mean the destruction of the entire ship. Within the balloon is a framework of aluminum rods extending from one end to the other and held in place by aluminum rings twenty-four feet apart. The balloon contains about 108,000 cubic feet of gas, and it costs about $2500 to fill it. One filling of gas will last about three weeks. There are two cars, each about ten feet long, five feet wide, and three feet deep. The cars are connected by a narrow passageway made of aluminum wires and plates, making a walking distance of 326 feet—longer than the decks of many ocean steamers. A sliding weight of 300 kilograms (about 600 pounds) serves the same purpose as the guide-ropes in the Santos Dumont air-ship. By moving this weight forward or backward the ship is raised or lowered at the bow or stern, and thus caused to glide up or down. Anchor-ropes are carried for use in landing. The ship is propelled by four screws, and guided by a number of rudders placed some in front and some in the rear. The first Zeppelin air-ship carried four passengers. The work of Dumont and Zeppelin has led the great powers to manufacture dirigible balloons for use in time of war. Fig. 91 shows one of the Zeppelin air-ships sailing over a lake.
FIG. 91–A ZEPPELIN AIR-SHIP