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Origins
in the First World War
Primitive sound locators were used on the Western
Front to locate artillery and enemy aircraft as early as 1914,
however it was events away from the Western Front that provided the
real impetus for developing means of detecting and tracking aircraft
by sound. In May 1915 Zeppelin and Shutte-Lanz airships of the
German Army and Navy started bombing targets around the Humber and
Thames estuaries. London was attacked for the first time on the 31st
of that month and by 1917 the airships were being replaced by
twin
engined Gotha and Giant aeroplanes. In total 300 tons of
bombs were dropped on Britain during the First World War causing
some 5,000 casualties, a third of which were fatalities. Some form
of early warning
system was badly needed, especially to counter
the night raids.
Following encouraging experiments with a four foot
diameter prototype built by a Professor Mather a 16' mirror was cut
into a chalk cliff face at Binbury Manor between Sittingbourne and
Maidstone in July 1915. The mirror was shaped to form part of a
sphere and a sound collector was mounted on a pivot at the focal
point. The collector was usually a trumpet shaped cone connected to
the ears of the listener with rubber tubes but experiments with
microphones were under way before the end of the war. The listener
would
move the sound collector across the face of the mirror
until he found the point where the sound was loudest. Bearings to
the target could then be read from vertical and horizontal scales on
the collector.
Professor Mather and his colleagues carried out a
series of experiments with this mirror and produced a report which
claimed that it could detect a Zeppelin at a range of twenty miles.
The Army conducted it's own experiments at Upavon which were so
disappointing, something Professor Mather blamed on "the ineptness
of Army personnel", that it wanted to cancel all further work.
Despite this it seems that several mirrors of 15' diameter were
constructed around the South East Coast, Thames Estuary and on the
North East Coast. No information regarding precisely when these
mirrors were built survives but it seems that the ones around Kent
were probably built first. They are similar in construction to the
first mirror at Binbury Manor, being cut out of a chalk cliff, but
were lined with concrete which made a better sound reflective
surface. Later mirrors, such as the ones on the Yorkshire coast,
were free standing and made entirely of reinforced
concrete.
Certainly the mirrors at Fan Bay, Dover (also
identified as Langdon) and Joss Gap near North Foreland saw action
in 1917 and 1918, the Fan Bay mirror detected an enemy raid at a
range of 12-15 miles in October of 1917 and in 1918 both mirrors
were able to detect aircraft heading for London several minutes
before they were audible to the unaided ear. It's worth noting that
late in the war the mirrors reported to a central command centre
which plotted the positions of raiders on a map and organised
defensive measures.
Post-War
Experiments
Despite the early scepticism shown by the Army the
sound mirrors must have performed well enough to warrant further
development. An experimental station to develop sound mirrors and
other sound detection devices was established at Joss Gap before the
end of the war. Work continued there up until 1922 when the research
centre was moved to an area called The Roughs near Hythe. This land
was already owned by the Army and was situated near to the flight
path for commercial aircraft flying between London and Paris.
One
problem the researchers had at Joss Gap was persuading the newly
formed RAF to provide target aircraft. It was hoped that by siting
the new centre at Hythe commercial aircraft could be tracked as they
made their regular flights overhead.
A new 20' mirror and several wooden huts for personnel
and equipment had been constructed at Hythe by the end of 1922 and
the mirror was operational by early 1923. The mirror was built as a
solid slab of concrete set against the face of a cliff with a steel
mast set in a concrete pedestal supporting the
sound collector.
In September of 1923 the mirror detected an aircraft at a range of
12 miles. Further research was carried out with microphones at Hythe
but stethoscope tubing remained the method of choice right up until
the 1930s.
In
1925 Doctor W S Tucker was appointed Director of Acoustical Research
and he was to play a major role in the development of sound mirrors
and locators. In 1927 he put forward a proposal to build a chain of
20' sound mirrors along the south coast. Only two were constructed,
at Lydden Spout (Abbots Cliff) Dover, and at Denge on the Dungeness
peninsula. Like the Hythe mirror they were cast as one solid slab of
concrete but were freestanding and not cut into the face of a cliff.
Both were completed in July
1928 at an estimated cost of œ650
each which included one Nissen hut and a fence around the entire
site.
Even before these mirrors had been finished Dr Tucker
and his team had finalised the design of a 30' diameter mirror.
Incorporating lessons learned from experiments with earlier mirrors
they were angled upwards and of a more sophisticated construction
than the previous slab designs. The listener, previously required to
stand outside in all weathers, was now seated in a booth beneath the
mirror. He rotated the sound collector horizontally with a hand
wheel and vertically by way of foot pedals. The booth was
largely
underground and the upper section glazed with bullet
proof glass. Two were built, one 200 yards west of the Acoustical
Research Station at Hythe and another adjacent to the 20' mirror at
Denge. Both were completed by the spring of 1930.
Although these mirrors did not have a greater range
than the 20' versions they did provide greater accuracy, especially
in the vertical plane.
At the same time as the 30' mirrors were being
constructed work started on something even bigger. In Dr Tucker's
words "Whereas the 30' mirrors are very efficient for (sound) waves
up to 3' or so.....the sounds we wish to deal with have waves of 15'
to 18'... This involves extension of the mirror surface to about ten
times that hitherto employed." As the mirror was intended to operate
only at long ranges elevation angles would be small so the height of
the mirror could be reduced. Even so the eventual design was 200' in
length and 26' high with a curvature of 150'. It's size precluded
the use of a moveable sound collector so it was proposed to use a
line of static listeners and microphones. The mirror was built at
Denge, close to the
existing 20' mirror and was completed by the
middle of 1930. At first the microphones, 20 in all, were connected
to apparatus housed in a hut but in 1933 a two storey listening
chamber was built onto the rear of the mirror with a small window
cut through so that the listeners on the forecourt could be
observed.
Sound
Mirrors in Action
From 1930 until 1935 the mirrors participated in the
annual Air Defence of Great Britain exercises with the RAF. The 200'
mirror was the long range lookout, telling the operators of the 30'
and 20' mirrors where to listen.
They in turn tracked the
incoming aircraft and reported their readings back to a central
control centre which calculated and plotted the raiders' position.
In 1932 the 200' mirror detected aircraft at a range of 20 miles
when the unaided ear could only hear them at 6.5 miles and on
another occasion at 30 miles when an unaided listener could only
hear them at 5.5 miles. Problems were experienced with high winds
blowing across the face of the 200' mirror and unwanted noise such
as ships in the English Channel, however the story about the mirrors
being jammed by a passing milk float appears to be untrue.
Canvas
screens were erected either side of the 200' mirror to reduce wind
noise and to provide some shelter to the listeners on the mirror
forecourt.
The main benefit of the exercises was the development
of a centralised reporting and command structure for air defence
which formed the basis of the system used in World War 2. In the
1935 exercise information was passed to a control centre set up in
central London, Dr Tucker also identified the need for bearings to
be transmitted from the mirrors automatically as transmitting them
by 'phone was too slow. It was also discovered that the job of
listener was very tiring, duty periods of no more than an hour were
recommended, and that great care had to be taken in selecting the
right men.
The
Thames Estuary Scheme and Finale
During
the 1930s it was proposed to build a chain of 200' and 30' mirrors
along the south coast from The Wash to Swanage. The first stage of
this scheme was to be a system to protect the Thames Estuary
consisting of two 200' and eight 30' mirrors. In mid-1935 approval
was given to commence work on the construction of this system, sites
were selected and a budget of œ10,000 was approved. One of Dr
Tucker's team, Percy Rothwell, had designed a 200' mirror
constructed from pre-fabricated blocks which would have been cheaper
and quicker to build than the one at Denge but the end was already
in sight. In February of that year Robert Watson-Watt had performed
his first experiment with Radar at Daventry, with uncharacteristic
speed a Radar research station had been established at Orford Ness
by March and by July aircraft were being detected at ranges of 40
miles.
The Thames Estuary scheme was at first postponed, then
cancelled altogether. In May 1936 the 15' mirrors at Fan Bay, Joss
Gap and Warden Point were abandoned, the 200' mirror followed in
1937 and the remaining
30' and 20' mirrors in 1939 when the
Acoustical Research Station was closed down. The mirrors at Denge
were used for a series of experiments involving the detonation of
small explosive charges at the focus of the mirrors but fortunately
the War Department's order that they were all to be blown up was
never carried out.
What
Remains Today
Of the World War 1 mirrors four still exist on the
east coast at Sunderland, Marske, Boulby and Kilnsea. The two
mirrors cut into the cliff face at Fan Bay, Dover were buried in the
1980s and are not on view although perhaps protected for future
generations (site for a future TT dig perhaps?). Sadly the mirrors
at Joss Gap and Warden Point have been destroyed by coastal
erosion.
The 20' mirror at Lydden Spout still survives although
minus the pedestal for the sound collector. It stands on top of the
cliff about half way between Dover and Folkestone, which would be
about three miles from either. There was an Army rifle range nearby
which was visible from the road, I don't know if it's still there
though. A very approximate Ordnance Survey map reference would be TR
280385.
The 30' and 20' mirrors at Hythe still exist although
the 20' mirror has fallen face down on top of it's pedestal. The
site lies approximately one mile to the west of Hythe on rising
ground to the north of the Royal Military Canal in an area known as
The Roughs. An approximate OS map reference would be TR
141342.
The 20', 30' and 200' mirrors still stand at Denge,
gravel extraction has exposed the foundations and the listening
chamber for the 200' mirror had to be demolished because it was
unsafe. Denge is about half way between New Romney and Dungeness
Point and about two thirds of a mile west of the Romney, Hythe and
Dymchurch Railway. The MoD had a dedicated branch line from this
railway serving the mirrors the track bed of which is now a
footpath. The Southern Railway line to New Romney crossed this line
by means of a bridge which still survives although the line itself
fell to Beeching's Axe. A rough OS reference would be TR 070215.
Alight from the RH&D Rly at the first halt after Greatstone and
head inland towards Lydd. If you can find the point where the former
Southern Railway passed over the RH&D branch line you're very
close.
A second 200' mirror exists at Maghtab,
Malta.
Sources
and Further Reading
Echoes from the Sky by Richard N Scarth, published by
Hythe Civic
Society, IBN 1 900101 30 0
London's Burning by Constantine Fitzgibbon for details
about German air
raids in WW1.
Surfing
If
you want to order a copy of "Echoes from the Sky":-
http://website.lineone.net/~rhsadmirals/
For photographs of the mirrors at Denge and
Dover:-
http://castlekas.freeserve.co.uk/home.htm
Mainly about Radar but some mention of sound
location:-
http://doramusic.com/Radar.htm
A possible future for the
mirrors?:-
http://www.guardian.co.uk/Archive/Article/0,4273,4203067,00.html
Phil Hide, January 2002
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