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tress are given free way in attempting to land. At night suitable markings are required on all fixed obstacles dangerous to flying within a zone of 500 metres of an aerodrome.

The London terminal aerodrome at Croydon, Sur., may be taken as typical of a modern air-port for commercial traffic. It consists of a level grass field 900 yd. long by 800 yd. wide, and is equipped with a continental arrival and departure station, a customs office, repair shops and stores, aeroplane hangars and the private offices of companies engaged in air and road transport. An indicator, consisting of a conical linen bag, painted in conspicuous colours and attached to a mast, shows the direc- tion of the wind by day; and the movements of machines are directed from a control tower. Along the south side of the aerodrome the name Croydon is let into the turf in chalk letters of 30 ft., legible from a height of 10,000 feet. For the assistance of night flying an aerial lighthouse shows the position of the aerodrome, while a searchlight distinguishes the aerodrome from its surroundings and illuminates the path of the machines. Electric lights are sunk into the ground to indicate the direction of the wind for landing. A wireless transmitting and receiving station is installed capable of telegraphic communication with ground stations within 400 miles and aircraft within 200 miles, and of telephonic communication within 200 and 100 miles respectively.

(b) Coastal Stations. A sheltered stretch of water, usually an inland lake, was selected by the pioneers of hydro-aviation, a sloping beach, a rough shed and one or two small boats being the only other requirements. The equipment of the English station at Lake Windermere, the scene in 1911 of the first take- off and landing on water by a British aeroplane, was almost negligible, and it was not until 1913 that the first organized seaplane stations came into existence. The management of these stations is very similar to that of an aerodrome, with the excep- tion of slipways up and down which aircraft are moved on leav- ing and entering the water, mechanical power for hauling heavy machines, and wheeled trucks to move them about on shore. At most of the early stations, however, man-power was sufficient to move machines, which were small and light, up and down the sloping beaches, while the pilot was carried to and from his sea- plane while it was still afloat.

The first British flying-boat was produced in 1912, but it was not until 1915 that the larger boats were sufficiently developed to enable them to stay out on the water for days at a time. This development caused a corresponding expansion in the organization of seaplane bases. Launches and rowing-boats, used previously to assist machines in difficulties, became ferry- boats for taking fuel, stores, and personnel to and from the large flying-boats which were moored out to buoys in sheltered waters adjacent to the coastal stations. The organization and management of these depots, until the formation of the Royal Air Force in 1918, was modelled on that of H.M. ships.

Calshot, Hants., was in 1921 the most up-to-date coastal station in Great Britain ; the sheltered area of Southampton Water provides ample sea room for craft getting off and alighting, while the narrow promontory on which Calshot Castle stands, almost surrounded by water, allows of numerous slipways for the handling of machines in and out of the sea at most states of the tide. Repair shops, sheds and living-quarters occupy a large area ashore; boat seaplanes, which are gradually replacing float seaplanes for all but special purposes, are moored out in a backwater; launches and rowing-boats are moored alongside a small pier, and trucks of special construction are held in readiness on the beach to move craft about on, when they have been hauled up the slipways by electric power capstans.

(c) River Stations. The value of river stations lies in the fact that they can be located in the centre of many large cities, and passengers by air can thus save the time, now lost, in reaching aerodromes necessarily situated on the outskirts. River stations were still in 1921 in an experimental stage, but stations on the Thames, the Seine and the Spree will probably be developed to serve the three capitals London, Paris and Berlin which are already important airline termini.

(d) Airship Harbours. In the early days of airships any convenient open space, such as a parade ground or moorland, was utilized, but as their size increased stations were selected so as to afford shelter from the wind, accessibility by air and

road, suitable accommodation for personnel, and privacy. In 1909 the Royal Aircraft Factory, then called the Balloon Factory, -Farnborough, was used for the first airship flights in England, and in 1912-3 it was provided with an elementary mooring mast. This station was abandoned in 1915. After the outbreak of the World War large airship harbours and construc- tion stations were erected in many parts of the United Kingdom, thus following on the far greater development in Germany.

The first sheds for the housing of airships were comparatively small and constructed of various materials, such as canvas, wood or corrugated iron. As the development of the airship progressed these were superseded by sheds about 750 ft. long, built of cor- rugated iron on iron girders, and capable of accommodating two large rigid airships and several smaller non-rigid types. The annexes of the sheds contained all the requisite workshops for engineering, carpenter and fabric work as well as stores for general equipment and laboratories for research.

The development of the airship, however, was so rapid that it was not possible to keep pace with the construction of airship stations, which entailed considerable labour and expense. For this reason other schemes for housing had to be devised.

The first method for small airships was a reversion to the early one of " housing " them under natural shelter, but it had the disadvantage that the airship fabric rapidly deteriorated by constant exposure.

Owing, however, to the length of rigid airships it was im- possible to dock them in this manner. Experiments were accord- ingly made for mooring them in the open by the three-wire system (see AIRSHIPS, Section 9). This was superseded by reversion to the mooring mast, which proved so successful that a large mast was erected at Pulham, where the first attempt was made in England to organize an airship harbour for com- mercial traffic, and the largest airships have been moored to it for long periods and in high winds. The adoption of the mooring mast has enabled the sheds to be mainly used for the housing of airships for the purpose of overhaul and repair, and has re- duced the personnel required for handling airships on the ground from an average of about 200-350' to an average of eight men.

Airship harbours have facilities for gassing airships with hydrogen, either from steel bottles or by manufacture on the spot by the water-gas process.

(ii) Signals. The methods for effecting communication with aircraft are ground signals, such as flags, pennants and ground strips; smoke signals-, rockets, flares, flash lamps and search- lights; and wireless telegraphy (latterly also wireless telephony).

Visual signals for indicating wind direction and landing- grounds date from the birth of flying, while flash lamps, flares and rockets have long been used at night, or in fog. Ground strips of cloth or canvas, which are generally white on one side and black on the other so as to show up against, dark and light back- grounds respectively, were placed in varying positions, according to a pre-arranged code. The flash lamp using Morse code was a little used prior to the World War, whilst the flier dropped written messages in a weighted bag attached to coloured streamers or a white parachute; early in the war the signalling- lamp (involving a knowledge of Morse by both operators) was employed.

Annex D of the International Convention prescribes that an air- craft in the air, or stationary upon land or water, but not anchored, shall carry forward and at the rear a white light, on the right side a green light, and on the left a red light. These lights, which are visible at varying dihedral angles and distances, are fixed so that only one can be seen at a time. On airships all lights are doubled. An aircraft when on the surface of the water, and not under control, dis- plays two red lights visible all round the horizon. When moored, but not near the ground, the airship, the mooring-cable and the object to which it is moored are marked by lights or streamers.

An aircraft wishing to land at night on an aerodrome fires a green Very's light or flashes a green lamp, and makes by international Morse code the letter group forming its call sign, permission being given by the repetition of the same call sign from the ground fol- lowed by a green light. It is forbidden to land by the firing of a red light or the display of a red flare. If it is compelled to land, a red light is fired from the aircraft and a series of short flashes made by the navigation lights. When an aircraft is in distress it gives one or more of the following signals: the international SOS, the inter-