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Summary

On 16 October 1941 a new 6 in Observation Post for Half Moon Battery was first manned for duty. The two storey building was constructed in the salient of the ravelin on the site of the old Battery Command Post and Electric Light Director's Cell.

Protected Status: None recorded

Other Statuses/Codes: none recorded

Monument Type(s):

• OBSERVATION POST (World War Two to Unknown - 1941 AD)

Full description

On 16 October 1941 a new 6 in Observation Post for Half Moon Battery was first manned for duty. The two storey building was constructed in the salient of the ravelin on the site of the old Battery Command Post and Electric Light Director's Cell.

The two rooms which form the basement, are entered via a flight of steps in an area to the rear. They functioned as the officers' watch shelter and probably contained bunks in the cul-de-sac room heated by a small boiler and messing facilities in the adjacent room from which immediate access to the Battery Observation Post was provided by a floor hatch and wall mounted ladder. Wartime operational arrangements required that all Battery Observation Post personnel slept in the post.

The new observation post consolidated most of the functions serving a modern coast artillery battery, which up until the start of World War Two, had been spread across a wide area in small concrete cells. At the centre of the room, observing the sea through an aperture running the length of the building, was the Battery Commander's position finder which was identified in documents dating from 1943 as a depression position finder '2' Mk II. Beside this and to the east, a depression range finder MkV sat permanently on a post from where it could see the harbour mouth. To the west of the depression position finder were two lecterns, one for the Battery Commander and the other for the Inclination Officer who was also the Battery
Commander's No 2. In front of the depression range finder were the Coast Artillery Searchlight directing stations for lights Nos 1 and 2 at Pendennis Point. The directing stations used remote control gear Mk V.Telephonists sat against the east and rear walls from where they could contact the entire Falmouth Fortress via the Fortress Exchange nearby. There were also dedicated lines which did not pass through the exchange such as the one connecting the post to the Forward Observation Post on Killigerran Head for fall of shot observation. At the rear of the room, a small area was fitted with shelving and a desk where the Battery Standing Orders, and equipment logs and capability charts were kept.

The observation post was painted a dark blue colour internally, probably to reduce the amount of reflected light within, but a small administrative area was delineated with a more common dark painted lower half separated from a lighter upper half by a red line. Painted on the wall was a 'Magslip Transmission Chart.' All of the fire direction and searchlight direction information was passed to the guns and lights by special motors and cables developed by Muirhead Electrical of Beckenham in Kent. This system was known as
Magslip. Target information received by Magslip transmission would arrive from many different sources: eg
radar; Fire Commander's position finder; director No 13 (POINT); Battery Commander's position finder; Battery Commander's depression range finder; depression range finder in ditch; upper position finder cell on keep; miniature range during practice; director No 17 during practice; Army Plotting Room; Battery Plotting Room and could be passed to many batteries: POINT; DENNIS; MOON; MARINES; BOFORS 1; 60 pdr 2; etc. The source, destination and routing of the target data had to be recorded in chinagraph pencil on this chart which helped the Battery Observation Post personnel to set their instruments.

The depression position finder had three main components, the observing telescope, the 'gun arm' and the table. It functioned by an observer 'laying' the cross hairs of the telescopic sights on the bow-wave of a vessel. He would follow the vessel as it moved through the water by rotating handles for horizontal and vertical adjustment. This would simultaneously move the gun arm across the table and a pen attached to the arm would plot the progress and hence the position of the target on the chart of the sea area. The chart was covered with Zylonite plastic which could be wiped clean and reused after an action. The pivot of the telescope and the gun arm coincided with the position of the pivot of A/1 gun, called the 'pivot gun,' on the map. All co-ordinates referred to the pivot gun and had to be corrected for other guns. The semi-circular shape of the position finder reflected the range and arc of influence of the 6 in battery. As well as plotting the route of an enemy vessel, later versions of the position finder including the one in MOON BOP
translated the range and bearing information into an electrical signal using Magslip transmission, where it would simultaneously appear on "follow the pointer" dials for range and bearing at the guns. Any data requiring other correction was sent to the guns via the Battery Plotting Room. Here various corrections were applied for speed and direction of target, state of tide, wind direction and barometric pressure. These
corrections were continuously applied by eight other ranks throughout an action so that the corrected information on the target arrived at the gun at the same moment that it was acquired. This apparatus was known collectively as 'predictor gear'. The source of information and destination corrections were applied in the same way by a co-ordinate converter operated by two other ranks, also in the Battery Plotting Room.

The Inclination Officer used a pair of stereoscopic binoculars fixed to the top of his lectern, which because
of the excellent three dimensional image they produced allowed him to quickly determine when the target changed direction. He would then alert the position finder and depression range finder operatives that this was the case, allowing them to prepare to refocus their instruments. An excellent calibrated panorama, depicting the view from the seaward aperture of the Battery Observation Post, gave an instant rough fix if a vessel was sighted in the offing. The degrees of arc painted on it subtend from the pivot of the position finder.

The Battery Observation Post was geared up to fire-direction for the most likely form of attack - at night. A typical attack might comprise battleships bombarding the fixed defences whilst E-boats made a hit and run raid on vessels in the harbour. Searchlights, called 'Sentry Beams,' at the harbour mouth would be exposed as soon as an enemy vessel was within their effective range. These would enable the observers in the Battery Observation Post to lay on the target and direct the other lights to illuminate it. The incandescent lighting in the Battery Observation Post would be switched off during a raid to prevent it giving away the position of the shore batteries. The cross hairs of the observers instruments were illuminated to allow them to be used at night.

The cams of depression range finders and the position finders had to be cut to mechanically correct their target data relative to the pivot gun. The cams of the depression range finder for MOON Battery Observation Post were returned to the Royal Ordnance Factory at Woolwich in April 1941 because the eight figure correction had been mis-typed and they had been incorrectly cut. New cams arrived in February 1942, once again cut to the old incorrect figures, but by this time the Battery Commander had been without a range finder for 10 months and kept the faulty machine accepting that manual corrections had to be applied in use.

A secondary but very important function of the Battery Observation Post was to direct the fire of MOON in its examination role. Along with the other logs kept within the post were instructions for 'Entry into British Defended Ports' and 'The Handbook of the Examination Service and Local Traffic Regulations.'

In addition to Magslip transmission the Battery Commander could pass orders to the gun captains by loudspeakers, wireless telegraph set No 17, telephone and the orders 'fire' and 'stop firing' by red and green signal indicators in the emplacements.

In 1944 the Post was fitted with a radar set Coast Artillery No 1 Mk II operated by ten other ranks and four telephonists, not all in the Battery Observation Post.

The Battery Observation Post was relegated to Care and Maintenance along with the 6 in battery on 15 January 1945. By 1949 it had been connected by Magslip to the antiaircraft radar. It continued to be used for practice until 1956 when all of the Fortresses technical equipment was photographed and removed by the Ministry of Supply. In 1995 the Battery Observation Post was consolidated and some of the original teak
window joinery discovered and reinstated, augmented by new sashes to make the building waterproof. The roof was re-asphalted and the interior was redecorated in the war time livery established by paint sampling, and the massing of some of the principal instruments within, reinstated in a reversible fashion. The remains of the Magslip transmission chart were revealed and consolidated by a picture conservator. In 1998 facsimile equipment and accoutrements were installed to present the building to visitors (1).

<1> Linzey, R, 2000, Fortress Falmouth. An conservation plan for the historic defences of Falmouth Haven Vol II (2000), Site N14 (Cornwall Event Report). SCO1563.

Sources / Further Reading

Associated Finds: none recorded

Associated Events

• ECO455 - Fortress Falmouth

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