The Zeppelin Outwardly a Zeppelin may be described as a long cylinder with ogival ends and a V-shaped keel running the length of its bottom. From afar the cylinder and pointed ends appear circular in cross section, but they are 16-sided. About one-third of the distance from either end of the great ship a small boat is suspended from the hull so closely that at those places the keel is omitted to make room. These two boats are rigidly connected with the hull and support it when the vessel rests on or is towed along the water. Within them are the crew and petroleum engines, while above them and outward on each side of the hull and fas tened to it by outrigging are two pairs of screw propellers, so placed as to exert their united thrust along the line of resistance. In some cases the crew can walk through the V-shaped keel from one boat to another, the passageway being illuminated here and there by transparent covering. Again an observer may climb up through the hull and take observations of the sky from above. Telephones, electric bells and speaking tubes serve to transmit intelligence from one part of the vessel to another.
The frame of the hull is formed of 16 or more 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 16-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 compart ments, each bounded by two wheels and the sur rounding longitudinal beams. Each compart ment contains a hydrogen balloon or sac; which fairly fills it and exerts a lift against the longi tudinal 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 modern Zeppelin is supported by about 18 separate gas bags, each of which contains within itself a small air bal loonet, in which the air pressure is maintained at a given point by means of an automatic air pump and relief valve. If, therefore, the tem perature of the hydrogen in the gas bags be comes high enough to expand the gas, the pressure set up by it on the outside of the balloonet forces the air out of the balloonet through the relief valve, and the correct pressure is in that way adjusted without the loss of hydrogen. The converse happens in case the temperature of the hydrogen falls, and its bulk for that reason becomes less. When it becomes necessary to reduce the buoyancy of the machine hydrogen can be pumped from the gas bags and stored in a compressing cylinder, this hydrogen being available for use later when desired to increase the buoyancy again. By the simul
taneous manipulation of horizontal rudders and °trimming° devices and the discharge of ballast a Zeppelin can shoot up to a higher level at the rate of 2,000 feet per minute, the engines in this case assisting the lifting power of the gas.
The size of the Zeppelin has changed con stantly since 1912, as shown by the following table : Cubic Length Diameter metres metres metres Type 1912 19.500 141 14.80 Type 1913 22,000 156 14.80 Type 1914 27.000 158 16.58 Type 1915 30,000 which should give a length of about 160 metres.
Other Lebaudy airship em ployed by the French is an example of the semi rigid type of dirigible. In this design a sub stantial keel extends to the full length of the vessel; and to this keel is attached the machin ery, together with the accommodation arrange ment for the crew. The keel is securely fixed to the gas bag above it to which it gives very considerable support. In the German service the Gross airship is another of this type. The envelope of a non-rigid airship keeps its shape only by the pressure of its gas, the car carrying the machinery etc. The German ParsevaL French Clement-Bayard and the Astra Torres are all examples of this type.
The Parseval gas bag is provided with an air-balloonet in its forward end and another aft, the amount of air in each of these bal loonets being controlled by a pump in the car. When it is desired to alter the trim of the ves sel, air is transferred from the container in the forward end to the other one, with the result that the after end of the ship becomes the heavier. A cord attached to the exterior of the balloonets passes through the hydrogen to a valve in the exterior of the gas bag. If the hydrogen expands to such an extent that the combined capacity of these vessels is insuffi cient to relieve the excessive pressure, their collapse causes a strain on the cord which opens the relief-valve in the gas bag and allows some of the hydrogen to escape.
During the first few months of 1916, the British authorities developed a dirigible of con siderable importance. Small of structure, its range is naturally limited, but it is capable of great speed which makes it a particularly de sirable instrument. A unique feature of this dirigible is its car, which constitutes an ordinary fusilage of a British army aeroplane complete with its engine tractor air-screw or propeller. Even the landing gear wheels are retained. This is particularly practicable for small struc tures where it advantageously does away with the heavy car, which together with the elaborate engine mounting and propeller transmissions greatly reduces the load and also reduces head resistance which greatly retards the speed of the vessel. During the year 1916 only one dirigible was constructed in the United States — a small vessel of a type of construction which embodied no departure from European practice. See MILITARY AERONAUTICS.