Smeaton's Tower (third Eddystone Lighthouse)

Classic Projects: Smeaton's Tower lighthouse

Date: 1759Engineer: John SmeatonCost: £40,000 (£4.2m in today’s money)

There can be few waterways around Britain more feared by sailors or more treacherous to shipping than the approach to the English Channel near Plymouth. This is where the notorious Eddystone Rocks break the sea's surface 12 miles SSW of Plymouth Sound. Placing warning lights at Eddystone was one of the great construction challenges of the 17th and 18th centuries, due to their inaccessibility, the huge swells and the reef's partial submersion.

In the late 17th century the first of a series of four lighthouses was built, but Winstanley's Lighthouse was washed away in the Great Storm of 1703. The second tower, named after the silk merchant John Rudyard who designed it, burnt down in 1755, probably as a result of a stray spark from the candle lanterns.

Both Winstanley's and Rudyard's constructions were made largely from wood, based on shipbuilding techniques. This was a mistake that the father of civil engineering John Smeaton was not to repeat, although he didn't abandon the wood theme entirely.

As the first engineer to be given the job of constructing a lighthouse at sea, Smeaton decided his structure was to be modelled on the shape of the trunk of an English oak tree – considerably wider at its base that at the top – the tower would be built of granite blocks with smooth concave sides, designed to withstand anything the storm-riven waters of the south coast could throw at it.

Smeaton used interlocking masonry, in the form of nearly 1,500 dovetailed granite blocks, and set the building into the bedrock using a form of concrete he had invented that could set under water. Further strength was added to the design by the integration of 'endless chains' at each level set into grooves in the masonry and then sealed with molten lead, a design feature inspired by Christopher Wren's dome of St Paul's Cathedral.

Originally illuminated by 24 candles, the lighthouse was first lit in October 1759. By 1810 oil lamps with reflectors had replaced the candles, and by 1845 the reflectors had been replaced by lenses, giving the tower a much better light intensity.

For more than a century the third Eddystone Lighthouse stood its ground. But as early as 1818 it was noted by Robert Stevenson (grandfather of Robert Louis Stevenson) that there were geological problems endangering the structure. And while periodic work was done internally to strengthen the lighthouse, by the 1870s it was being affected greatly by wave impact and was found to be rocking from side to side. Smeaton's tower was being undermined by erosion of the rocks on which it stood and was becoming unsafe. It was taken out of service in 1887, partially dismantled and re-erected in Plymouth Hoe in 1882.

Victorian engineers charged with decommissioning the lighthouse soon discovered that the foundations were too strong to remove and so left them where they stood (and can still be seen today). The relocated structure was built on a new base and opened to the public in 1884. Meanwhile work was under way on the fourth iteration: Douglass's Lighthouse, first lit in 1882 and still in operation.

Today the iconic red and white painted lighthouse is an architectural icon recognised the world over and a memorial to its designer. Smeaton coined the expression 'civil engineer' to distinguish the discipline of public works engineering from military engineering. He went on to establish the Society of Civil Engineers in 1771, which decades after his death changed its name in 1830 to the Smeatonian Society of Civil Engineers in honour of its illustrious founder.

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