Portland Bill Lighthouse

The two original lighthouses, now known as the Old Higher Lighthouse and Old Lower Lighthouse, operated as a pair of leading lights to guide ships between Portland Race and The Shambles sandbank.^[1] They were constructed in 1716, both rebuilt in 1869, and decommissioned following the completion of the present lighthouse.^[4] At the turn of the 20th-century, Trinity House put forward plans for building a new lighthouse at Bill Point. They acquired the required land in 1903.^[5][6]

The builders, Wakeham Bros. of Plymouth, began work on the foundations in October 1903.^[7] Chance & Co of Birmingham supplied and fitted the lantern.^[8] A pressurised vapour paraffin lamp was used, placed at the centre of a large (first-order) revolving optic; weighing 3.5 tons, this was made up of four asymmetrical catadioptric lens panels and a concave prismatic reflector.^[9] The lighthouse was completed in 1905 at a cost of £13,000, and the lamp first lit on 11 January 1906.^[4] A red sector light was provided in addition to the main light, shining from a window in the lower part of the tower, to indicate the position of The Shambles.^[1] The light was electrified in the mid-1950s.^[10]

In 1940 the lighthouse was provided with an F-type diaphone fog signal, sounding from a window part-way up the tower. Compressed air was provided to six cylindrical storage tanks by a pair of Reavell compressors, all located (together with a standby generator) within the base of the tower.^[9] These were connected at a higher level to the sounding tanks, which fed the compressed air to the diaphone itself, mounted behind its trumpet-like emitter which protruded through the window. Admission of air into the diaphone was controlled by a clockwork (later electric) coder, which caused the diaphone to sound a 3.5-second blast every 30 seconds. The 180 Hz note had an audible range of 7 nmi (13 km; 8.1 mi) (which could be doubled under favourable conditions). The diaphone remained in regular use as an aid to navigation until 1995, when it was replaced by a high-frequency electric fog signal (sounding from another window, further down) in readiness for automation.^[9]

On 18 March 1996, Portland Bill Lighthouse was demanned, and all monitoring and control transferred to the Trinity House Operations & Planning Centre in Harwich.^[11] The original Type F diaphone was decommissioned in 1996, but in 2003 Trinity House restored it to occasional use for the benefit of visitors;^[12] (it was sounded regularly for half an hour on Sunday mornings, except when foggy, until 2017).^[13]

In the early 21st century the lighthouse used a 1 kW MBI lamp together with the same rotating lens system that had been in use since 1906. (It flashed four times every 20 seconds with an intensity of 635,000 candelas and a range of 25 nautical miles.) The fog signal was used in times of bad weather; it gave a four-second blast every 30 seconds with a range of 2 nautical miles.^[1]

In November 2018 Trinity House applied for (and obtained) planning permission to remove the lamp and optic from the lantern room as part of a programme of modernisation.^[14] It proposed relocating the lens array to the base of the tower,^[15] which led to the removal of the historic diaphone fog-signalling equipment, installed there in 1940 and still in working order, on the basis that this was 'the only available [space] for retaining the historic optic on-site'.^[16]

In 2019-2020 a new non-rotating LED light source was installed in the lantern room^[16] and a new omnidirectional fog signal was installed on the exterior lantern gallery (replacing the electric emitter installed in the 1990s).^[17] The two LED lanterns (one of which is used, the other kept on standby) have a reduced range of 18 nautical miles (33 km; 21 mi).^[18]

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  The lighthouse and the surrounding ex-quarried area.
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  Lamps in Portland Bill Lighthouse
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  Trinity House flag on Portland Bill Lighthouse, Dorset
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  The diaphone foghorn emitter of Portland Bill Lighthouse
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  Fresnel lenses and prismatic mirror

• List of lighthouses in England