Blind brick-maker's idea to fix lighthouses in sand
An Irishman helped improve marine safety with some inspiration from corkscrews. Mary Mulvihill reports.
Strange but true, it was a blind Irish brick-maker who invented the first successful technique for building lighthouses in treacherous, sandy areas. What's more, his inspiration came from an ordinary corkscrew.
From these bizarre beginnings came a successful system that was used around the world, not just to anchor lighthouses and so save lives at sea, but also to erect harbour piers, telegraph poles and even railway bridges.
The problem with sandbanks was that, first, they were a hazard to shipping, and second, it was difficult to anchor a warning lightship there, never mind get a firm foundation for a permanent lighthouse. The problem gave Alexander Mitchell many sleepless nights, starting in 1831, as he worried about the dangers to sailors.
It is not clear why Mitchell (born Dublin, 1780, died Belfast, 1868) was interested in this problem. He was blind, had no formal training in engineering or lighthouse building, and indeed, for the previous 30 years had run a brick-making business near Belfast.
Yet in 1832 he retired from brick-making, and the following year, aged 53, patented what would become known as Mitchell's screw pile. And the rest, you could say, is civil engineering history.
Mitchell's simple, yet ingenious, solution was to use long iron rods, or piles, for his foundations. But instead of being hammered in, which would give no purchase in a sandy area, they were screwed in place. Each pile, measuring perhaps six inches across and 20 feet long, had a threaded drill-bit end, and just above that, a wide helical screw flange, perhaps four feet across.
The piles could be firmly screwed into soft or sandy ground to provide a secure mooring, and Mitchell found that just one screw turn was enough to support 60 tons. Erect a cluster of these screw piles, and you could mount a lighthouse on top, creating a leggy, but stable, structure.
Within five years Mitchell had convinced Trinity House, the body then responsible for British and Irish lighthouses, of the merits of his system, and the first screw pile lighthouse was erected at Maplin Sands in the Thames estuary in 1838. The problem of how to build lighthouses in sandy areas had finally been solved, and hundreds of screw pile lights soon followed. There were some 150 in North America alone by 1910.
Several were built in Ireland, most of which still stand, notably at Dundalk Harbour, the Spit Bank in Cork, Duncanon in Waterford harbour, and at Belfast Lough. Mitchell built the latter at cost (£1,200) as a goodwill gesture to his adopted town.
Once screw-piles had been successfully proven at sea, they were adapted for other structures, especially in Japan, after that country was opened to Western trade in the 1860s, and in India where they were used to anchor the Indian telegraph system and the bridges of the Bombay-Baroda railway.
Mitchell's brick-making business had been commercially successful, but his screw pile invention brought him fame, wealth and international honours. Despite being blind, a result of childhood smallpox, this talented and ingenious thinker would draw up the detailed structural plans for his lighthouses (with the help of modified drawing implements), and he supervised all the marine work, using his acute sense of touch to detect flaws that others missed. Where hot rivets were used he could reputedly test their temperature by holding his hand above them - a skill he acquired, perhaps, in his brick-making days?
Alexander "Screw Pile" Mitchell is buried at Belfast's Clifton Street cemetery.