While most of us might despair such a fate, Mitchell learned to use his hands to examine and learn and invent. He became a tinkerer and borrowed money to start a brick company in Belfast. He oversaw construction of about twenty buildings and was quickly acknowledged for his brilliant ideas. He designed several useful methods and tools for his company. During this time he also married and had a family.
Living near the sea, Mitchell loved all things nautical. His son, John, became his eyes, assisting him in his experiments and travels. The family had a boat, and Mitchell designed a special sail for it. In 1832, while on Belfast Lough, he tested the sail. It was attached to a pole on which a large flanged screw had been mounted to make it swivel.
It was windy that day, so Mitchell thrust the screw of the sail's pole into the sand while he positioned the boat in order to affix the sail to it. He told his son to hold the sail, but to their surprise, the wind caught the sail and whirled the pole around and around, screwing it firmly into the sand. When Mitchell tried to remove the screw he found it so tightly anchored in the sand it wouldn't budge. It took great effort to free it.
This got Mitchell to thinking about screws. He had always been fascinated with them. Even the corkscrew that opened a wine bottle at dinner interested him. He began to think about larger screws that might anchor iron piles for wharves, piers, bridges, even lighthouses. He set to work getting drawings prepared and had a few examples of his invention made. He called it the screwpile.
In the spring of 1833, he and his son, then aged nineteen, took one of the fabricated screwpiles out to Belfast Lough in their boat. Together father and son screwed the pile deep into the mucky sands of the lough until only a few feet of it remained above water. They returned home, and in the night a storm passed through Belfast. The next day, they rowed out to the lough and found the screwpile still firm in the seafloor.
Mitchell knew his design would not only anchor lighthouses firmly, but also that wind and waves could pass through the iron piles unhindered.
|Mitchell's proposed method for inserting screwpiles into the sea floor.|
A second test of a screwpile in Belfast Lough gave similar results. Confident in his idea, Mitchell had his screwpile patented later in the year. At first, no one seemed interested. But in 1838 the Corporation of Trinity House--the lighthouse authority of England--contacted Mitchell and asked if he would design the foundation for a lighthouse at Maplin Sands in the Thames Estuary.
Mitchell's design called for eight iron legs to be screwed twenty-two feet into the mucky estuary floor. The legs were heavily braced and then a house for the lightkeeper was mounted on top. The lighthouse was first lit in February 1841 and was a great success.
In the years that followed, Mitchell supervised the construction of many projects involving screwpiles, including the lighthouses at Wexford, Cobh, Soldier's Point, and Spit Bank. Perhaps he was happiest when a screwpile lighthouse went into service in his home harbor at Belfast Lough, where he had first experimented with screwpiles.
|Belfast Lough Lighthouse|
|A photo of the Belfast Lough Lighthouse years after its construction. Today, Blackhead Lighthouse and other beacons around Belfast have replaced it.|
|Remains of the Wyre Lighthouse a few years ago at Port Fleetwood. The superstructure was destroyed by fire in 1948, but enough of the foundation remained to mount a small light. It still stands but is crumbling. A group has formed to save it.|
Lighthouse authorities around the world quickly adopted Mitchell's design. Brandywine Shoal in the Delaware Bay was the site of the first screwpile lighthouse in the United States. The image below shows the old lighthouse next to its new, caisson-style replacement.
Eventually, over one-hundred of these designs would be built in the nation. Among the more famous ones are the tall, iron screwpile lighthouses of the Florida Strait. Lt. George Gorden Meade, who would later distinguish himself as the Union commander at the Battle of Gettysburg, oversaw construction of these lighthouses. When he discovered that coral reefs were hollow, he designed a spinoff of Mitchell's invention and called it the diskpile. It had special shoes that anchored the screwpile to the underside of the crust of a coral reef.
|Sand Key Lighthouse, which successfully replaced a traditional masonry lighthouse that was washed away by the sea.|
|Carysfort Reef Lighthouse off the Florida Keys.|
|Fowey Rocks Lighthouse, southeast of Key Biscayne, Florida|
Today, few of Mitchell's screwpile lighthouses remain. Many have been replaced by caisson lighthouses, which sit on large concrete tubes filled with riprap. Some stand on heavy tubular legs similar to those of marine oil platforms. But you will find several screwpile lighthouses preserved in museums and a few that remain standing.
|Thomas Point Lighthouse is the last Chesapeake Bay screwpile lighthouses standing and operating on its original site.|
|Seven Foot Knoll Lighthouse, now on display at Baltimore's Inner Harbor.|
Alexander Mitchell died in 1868 at the age of 88. He is buried in Belfast.