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Rh the envelope, and of which the British military airships are typical. (2) The Parseval type, in which the circular envelope is reinforced against bending under the rigging tension by Parseval trajectory bands passing over the envelope and secured to a girdle to which the car is rigged. (3) The Torres type, made by the Astra firm of Paris, trilobe in section, with riggings led inside the envelope and divided into fans secured to points along the two top ridges.

The two latter systems are intended to decrease the distance between the envelope and the car without producing excessive tendency to bend in a large ship.

At the beginning of the war the French had several non-rigid ships of various types which carried out bombing operations, but no important new ships were built. Germany had a few Parseval airships, which did a little work on the Russian front, but there was no important development of small ships. England had three small non-rigids, also one Parseval and one Astra. It became necessary, however, at the beginning of 1915 to develop the very small non-rigid airship as rapidly as possible as an anti- submarine protection. Extreme simplicity was essential in order to allow of rapid production by firms having no previous expe- rience. For the first 30 ships aeroplane bodies were used as cars, but later special cars far more suitable for patrol work were adopted. Engines of about 90 H.P. were used and a crew of three carried. Some 150 ships of the S.S. classes were built, but at the end of the war it had been decided to adopt a slightly larger ship with twin engines and a crew of five as being more suitable for the longer patrols which became necessary. Later in 1915 a larger type -the Coastal class having greater speed and taking a crew of five, was built. For these the Astra system of rigging was adopted in order to reduce to a minimum the necessary height of the sheds. Thirty-five of these ships and ten of an improved (C*) class were built during the war. These ships later carried a crew of five and had an endurance of 1 2 hours at a full speed of 51 knots. In 1916 the first ship of the North Sea type was flown. This class was intended to work with the fleet and had an endurance of some 24 hours at 50 knots. Sixteen of these ships were built.

The characteristic of these ships, more particularly the N.S. class, was that the petrol tanks and all other weights possible were carried direct on the envelope. In the N.S. class the car was separate from the power unit and the weight distributed over the length of the ship. This gave important advantages over all earlier non-rigids where the loads had been concentrated in the car. The S.S., Coastal and N.S. classes were all designed and built at the R.N. Airship Station, Kingsnorth. They con- stitute a very interesting development from the small supply of ships and experience available at the beginning of the war.

A considerable number of British non-rigid airships were built and supplied to the French, Italian, Russian and American serv- ices, and one Italian semi-rigid was supplied to England for experiment. A large Astra ship of some 800,000 cub. ft. capacity was built in France with two large cars. It is understood that lack of longitudinal rigidity of the envelope gave trouble.

The Italian airship design has favoured the semi-rigid type of construction, their most successful type being one in which the keel girder was not in itself rigid but " vertebrate," consisting of a number of pin-jointed frames capable of taking the longitu- dinal thrust induced by the car riggings, so long as the envelope held the keel in line. This system did not greatly reduce the height of the ship, as the points of attachment of the riggings were necessarily at the bottom of the envelope instead of near the level of its centre line. It did, however, enable a much lower envelope pressure to be used than in the non-rigids of the same size. This enabled a very light envelope fabric to be used and also a system of automatic pressure regulation by air taken at the nose of the airship. These' ships were designed for bombing raids at great heights across the Adriatic Sea. The excellent weather conditions rendered their comparatively slow speed quite satisfactory.

Germany built a few large semi-rigids of the M type and the Parseval type. The two largest, PL26 and 27, were of some

1,120,000 cub. ft. capacity. They embodied many interesting features, including spherical partitions which divided the envelope into sections so that the accumulation of pressure at the upper end of the ship when pitched was avoided. As far as is known, no very thorough trial of these scrips was made, but as far as the experiment was carried it appears to have been satisfactory. The type was not, however, proceeded with on account of the decision to concentrate on the rigid type.

Italy, after the Armistice, built a large semi-rigid " Roma," intended for transatlantic service.

An interesting aircraft which was developed experimentally as a counter to the Zeppelin raids was the " airship-plane " devised by Wing Comm. Usborne. A complete aeroplane was rigged under the envelope of an S.S. airship in such a way that, after patrolling at a great height, the envelope could be released and the aeroplane left free to deliver its attack. After several preliminary flights the first attempt to slip the envelope in flight failed on account, probably, of temporary loss of pressure in the envelope. The machine was partly released prematurely, and was damaged as it fell away; Wing Comm. Usborne and Wing Comm. Ireland were both killed. The former particularly was a most serious loss, as he had up to that time been mainly responsible for the exceptionally rapid airship development.

Kite Balloons. The Drachen kite balloon, in the form origi- nally used by Maj. von Parseval and Capt. von Sigsfeld in 1896, was used by the Germans immediately on the declaration of war for observation of artillery fire. Its value became at once ap- parent, and it was immediately copied by the Allies, very large numbers being made. The stability was, however, so poor that this type could only be used in fair weather, and accurate ob- servation was often difficult. Capt. Caquot of the French army designed an improved arrangement of stabilizers. Three fins, one at the bottom of the tail and two 120 from it, were in the summer of 1916 ultimately adopted instead of the single fin of the Drachen and the string of parachutes which were necessary with it. Considerably improved stability was obtained, ana there was an important increase of the dynamic lift which gave increased height. This type was generally adopted by the Allies for military use and worked well up to 6,000 feet. The same type of balloon was used by the navy, but was replaced by a similar one designed to resist higher wind speeds and capable of only 2,000 feet. This was used extensively by the fleet for gunnery observation and as a look-out for submarines. The balloon, being in continuous telephone communication with the captain of the ship, could transmit information more completely and rapidly than other aircraft. The balloons were also used in the ships protecting convoys, although it was sometimes contended that they acted as buoys to show the position of the convoy to a sub- marine which could thereby keep in touch at a safe distance during the day and deliver its attack at night. These naval balloons were capable of very high wind speeds, in one instance 80 knots being recorded.

An Italian A.P. type of balloon having a considerably smaller length to diameter ratio was adopted to give very great static lift in calm air. These were used for the apron defence against aeroplane attack. A line of balloons lifted to a height of some 15,000 ft. a horizontal cable from which hung thin vertical wires arranged to foul the wings of the hostile aircraft.

Airship Operations. During the early days of the war French airships were employed for bombing behind the German line, but the damage to the ships, usually through gas leakage caused by shell and bullets, was so great that only a limited amount of work was done.

The Italian airships designed specially for bombing raids at very high altitude across the Adriatic obtained considerable protection from their height, and more useful results appear to have been achieved.

The Zeppelin raids over England were an interesting achieve- ment from the airship point of view. So much of the effect of these raids was indirect, in the delays to munition work during raid nights, large amount of personnel and material retained for defence, and also iji the psychological effect produced, that it is