A captive balloon in war serves as an observation tower from which to observe the enemy. It is connected to the ground by a cable. This cable is wound on a drum carried by the balloon wagon. The balloon can be lowered or raised by winding or unwinding the cable.
The gas-bag is sometimes made of oiled silk, sometimes of two layers of cotton cloth with vulcanized rubber between. The cotton cloth gives the strength needed, and the rubber makes the bag gas-tight.
The most convenient gas for filling balloons is heated air, but the air cools rapidly and loses its lifting power. Coal-gas furnished by city gas-plants is sometimes used. This gas will lift about thirty-five pounds for every thousand cubic feet. A balloon holding thirty-five thousand cubic feet of coal gas will easily lift the car and three persons. The lightest gas is hydrogen. This gas will lift about seventy pounds for every thousand cubic feet. Hydrogen is made by the action of sulphuric acid and water on iron. If a bit of iron is thrown into a mixture of sulphuric acid and water bubbles of hydrogen gas will rise through the liquid. This gas will burn if a lighted match is brought near.
A balloon without propelling or steering apparatus is not an air-ship. It may be raised by throwing out ballast or lowered by letting out gas, but further than this the aeronaut has no control over its movements. The balloon moves with the wind. No breeze is felt, for balloon and air move together. To the aeronaut the balloon seems to be in a dead calm. It is only when he catches sight of houses and trees and rivers darting past below that he realizes that the balloon is moving.
If a balloon has a propelling apparatus it may move against the wind, or it may outspeed the wind. A balloon with propelling and steering apparatus is called a "dirigible" balloon, which means a balloon that can be guided. Figs. 89 and 90 are from photographs of a "dirigible" used in the British army. Such a balloon is usually long and pointed like a spindle or a cigar. It is built to cut the air, just as a rowboat built for speed is long and pointed so that it may cut the water. The propeller acts like an electric fan. An electric fan drives the air before it, but the air pushes back on the fan just as much as the fan pushes forward on the air, and if the fan were suspended by a long cord it would move backward. So the large fan or screw propeller on an air-ship drives the air backward, and the air reacts and drives the ship forward. In the same way the screw-propeller of an ocean liner drives the vessel forward by the reaction of the water.
FIG. 89–BRITISH ARMY AIR-SHIP "NULLI SECUNDUS" READY FOR FLIGHT
