ON October 14, the Waterworks Museum in Hereford became the focus for hot-air enthusiasts from across the UK for the annual Stirling Engine Rally.

Members of the Stirling Engine Society brought many hot-air engines to display to museum visitors, ranging from full-scale models, engines that powered model vehicles, through to the smallest of engines that appear to work by magic, powered only by the heat in the palm of your hand.

Developed from the original design of the Rev Robert Stirling in 1816, hot air engines had their heyday in the 1890s, after the age of steam engines and before internal-combustion engines had become widely established. But the Stirling design is not just heritage engineering. The concept is still used today in powering submarines, solar electricity generation and, when operated in reverse to achieve low temperatures, in cooling devices in refrigeration and some MRI scanners.

This year, museum volunteers were delighted to meet Moustafa Kinawy, a stress engineer from Rolls Royce and a Stirling engine 'modern-history' expert, who was overjoyed to find two unique 1967 Harwell Stirling linear-generator prototypes at the Museum.

He knew about their history and told volunteers they are regarded as one of the most important post-1960 UK Stirling engine developments. Fuelled by propane, they were designed to provide electric power for lighthouse lamps and other devices requiring long duration light. However, the design didn't move beyond prototype stage, despite its technical success.