Building of the Month - November 2009

Located in Cork Harbour, to the south of Cobh, Spitbank Lighthouse is set at the end of long mud bank, marking a ninety degree turn in the shipping channel.  Its peculiar form and design make it a striking addition to the maritime heritage, as it differs greatly from the more traditional, stone-built lighthouses which are found along the south coast (fig. 1).

The man behind the design of this curious structure, Alexander Mitchell (1780-1868), was quite extraordinary (fig. 2).  Born in Dublin, his family moved to Belfast while he was still a child and he was educated at the Belfast Academy, where he showed great mathematical aptitude.  His eyesight failed throughout his teenage years, and he was blind by the age of twenty-three.  Amazingly his blindness did not prevent him from becoming a pioneering self-taught engineer.

A paper he wrote for the Belfast Literary Society in 1856 provides insight into his inquiring mind.  As a child, ‘the bellows, though my first love, did not possess my undivided affection, for I flirted occasionally with a pair of scissors, a corkscrew and many other mechanical contrivances’.  As a school boy, ‘I gave my mind with ardour to arithmetic, Euclid, and all the mathematics that I thought applicable to my favourite study of mechanics’.  Putting theory into practice, ‘My first grand work was a clock, the wheels of wood, toothed with wire bent into the form of staples’.  And later ‘I succeeded in making a sail that enabled a boat to sail in the teeth of the wind – this by means of a spiral in the air and another in the water.  I next tried this in the earth, and this is the origin of the screw mooring and the screw pile, with which my name has become so associated that Admiral Francis Beaufort has fixed on me the sobriquet of “the apostle of screw piles”‘.

Mitchell patented the “Mitchell Screw-Pile and Mooring” in 1833, a cast-iron support system which allowed for construction in deep water on mud and sand banks (fig. 3).  Apparently inspired by the domestic corkscrew, its helical screw flange could be used for difficult shifting foundations on a broad range of structures and its potential was realised in a broad range of projects: lighthouses in Britain and Ireland as well as more than 150 lighthouses in North America (fig. 4); piers such as in Courtown, County Wexford, and the impressive pier at Madras (fig. 5); bridges and viaducts on the Bombay, Baroda and Central India Railway; and also for the telegraph network in India.  Mitchell went on to apply the same technology to propellers and patented the screw propeller in 1854.

Lighthouses using this innovative system were built under Mitchell’s supervision at Maplin Sands in the Thames estuary in 1838; Wyre in Lancashire in 1840; Belfast Lough in 1848; Spitbank in 1853 (fig. 6); and Dundalk in 1855.  The foundations for the first lighthouse, Maplin Sands, were sunk in the incredibly short period of nine days.  Before Mitchell’s wonderful invention, floating lights had been used where traditional lighthouse construction was not possible.  Floating lights were not ideal, as the movement of the light ship caused great variance in the light’s location during storms, and floating lights could break from their mooring, causing havoc for mariners.

An unlighted buoy had previously marked the eastern point of the Spit Bank but Cork Harbour Commissioners required a more notable structure to take its place.  Mitchell won the commission to construct the new lighthouse for £3,450, and moved with his family to Cobh – then Queenstown – in 1851, renting Belmont House overlooking the harbour.  He immediately set about engaging workmen, testing the ground and examining the iron for the piles and the wood for the house.  His son and grandson laid the piles for the lighthouse, with regular inspections from Mitchell, while he oversaw the construction of the timber house on shore.  During his fifteen months at Cobh, Mitchell took trips into Cork city, during which he met with academic staff at the university and forged a friendship with the great mathematician, Boole.  The light was exhibited for the first time two years later, while a fog horn was added in the 1890s.  Set out at sea, with no room for living accommodation, a principal and an assistant keeper lived in rented accommodation in Cobh.

Incredibly, accounts survive of Mitchell personally overseeing construction, taking trips out to his lighthouses in small boats, even on rough seas and on occasion falling overboard, going up and down ladders, crawling along planks and examining the wood, iron and rivets.  At times he rallied the workers’ spirits, leading them in sea shanties.  Through touch he checked the quality of the iron work, sometimes noting flaws which had escaped the workers’ and foreman’s eye.  One worker is recorded as exclaiming ‘Our master may say what he pleases, but I’ll never believe that he can’t see as well as thee or I’.  Mitchell was made an associate of the Institute of Civil Engineers in 1837, and was elected a member in 1848, at which time he received the Telford silver medal for the invention of the screw pile.  He was awarded the Napoleon Medal from the Paris Exhibition in 1855.

Spitbank Lighthouse remains an iconic structure in Cork Harbour (fig. 7).  Described by some as a giant spider in the sea, its curious form and design has attracted much comment and curiosity for the past 150 years.  Thanks to the endeavours of this truly gifted inventor and dedicated engineer, countless lives have been saved at sea.

Figures 1 and 7 photographed by Andy Mason for the National Inventory of Architectural Heritage publication An Introduction to the Architectural Heritage of East Cork (2009)

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