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Shipbuilding engineer John Campbell spent three years constructing his 3ft 3in (99cm) long horizontal live engine.

He is believed to have cast many of the components himself, including the fly-wheel, during his time working for William Arrol & Co at the Parkhead Forge in Glasgow. The remainder of the engine was built by hand including the mahogany plinth and display case.

“Stories about my great-grandfather’s amazing model steam engine had been passed around the family for decades but no one knew its whereabouts,” said his descendant Stephen Campbell. However, while conducting online research into his family tree recently, Stephen found the piece listed on Vavasseur’s website.

“I couldn’t believe it when images of the actual engine itself popped up,” he added.

It was offered for £7500 together with a picture of John Campbell with the engine and the awards of merit it achieved at various science and artisan exhibitions from 1903-14. The six prizes include a silver medal at the 1906 Edinburgh Industrial Exhibition and a diploma of merit from the 1908 Scottish National Exhibition, Artisans Section.

Later, the engine was displayed for many years in the Kelvingrove Art Gallery & Museum in Glasgow where it was demonstrated running on compressed air.

Vavasseur specialises in collectable barometers and scientific instruments. The firm’s Mark Jarrold said: “When I saw this engine, although slightly off-message, it had to be bought.

“The aesthetics together with the accompanying history of this piece set it aside as something truly special.”