OBSERVER CORPS IN AIR DEFENCE. At the be ginning of the World War the requirements of air defence had hardly been considered, but very soon the importance of knowing the movements of the hostile aircraft became evident. The infor mation was required both for purposes of active defence, and also for the system of air raid warnings. At first, in Great Britain the defences were in the hands of the Admiralty, who arranged with the police to telephone reports of any aircraft heard or seen within 6o miles of London. Early in 1915 this system was extended to in clude East Anglia, Northampton, Oxfordshire, Hampshire and the Isle of Wight ; and in April, 1915, chief constables were asked to telegraph similar messages to the War Office; there was much overlapping. The system was then extended to cover England and Wales, the reports to come to the Admiralty, who would inform the War Office. Direct interchange of intelligence was also ar ranged between the chief constables. In 1916 the War Office took over, and a more regular system of observers was introduced, cor dons were organized 3o miles outside vulnerable areas. London was provided with two cordons, and coastal posts were organized.
The above system was in force when the London air defences were re-organized under General Ashmore in the autumn of 1917. The aeroplane patrol work was improving at the end of 1917, but information from the cordons, etc., was not quick or certain enough for the use of the pilots. Early in 1918 therefore a new system was inaugurated; the German activity was then confined to London and the south and east of it, and this area was well covered with the various units of the defence, coastal posts, squadrons, guns, searchlights and balloon aprons. A system of reporting from all these units was arranged by which their information came through certain centres, and so on to a big map in the headquarters. During operations the lines were always through, and no ringing up was required. The necessary telephone construction work was heavy, and the system was not ready until after May 19, 1918, the oc casion of the last raid.
On the revival of air defence in the beginning of 1924, it was plain that unless special measures were taken, large areas would be blank so far as any air craft intelligence was concerned. The requirements were simple ; no hostile aircraft must be allowed to move over any part of the country without its movements being known constantly and in stantly at the headquarters of the air defences. To obtain the in
formation it was necessary to cover the country, within range of bombing, with a series of posts, six to eight miles apart, provided with suitable watchers and a complete telephone organization for quick reporting. The first experiment was made in 1924. A few posts were organized at the required interval round Cran brook, which was the reporting centre. The experiment went to show that the scheme was feasible, and valuable experience was gained.
In 1925 the system was ex tended to two zones covering the whole of Kent and Sussex; each zone consisting of a network of observer posts connected by direct telephones to an observation centre, the centres again being con nected up to the headquarters of the air defences. The posts and centres were manned by special constables, enrolled for this ob ject by the chief constables of counties and boroughs. The 1925 lay-out was well tested by day and night work with the R.A.F. After the 1925 work, the organization received the official sanction of the Home Office and the War Office, and the general terms of service for the special constables were fixed. In 1926, two further zones were organized, so that by 1927 the system was in working order from the west of Hampshire, through Hampshire, Sussex, part of Surrey, Kent, Essex and half Suffolk. The administration under the chief constables and regular police works smoothly. The observers, who do the work without pay, are keen and effi cient. Each group of about 25 posts communicates direct to a centre.
In the centre is a large table map on to which the observer lines are laid. Plotters sit round the table with telephone headsets on, three posts to each plotter. As the reports come in, they are plotted on the control map, and an independent recorder keeps a record. A teller overlooks the control map, and reports to the air defence headquarters the courses as they appear. Periodical exer cises are held in which large numbers of bombing and fighting aeroplanes fly over the posts. The system transmits the courses with certainty and with great rapidity, and has proved itself an indispensable part of the air defence organization. (E. B. A.)