The plaque on it reads -
The Trust engineers were David and Thomas Stevenson, from the famous engineering family. Their father, Robert was the builder of the Bell Rock lighthouse and many more. His son, Alan, famous as the builder of the Skerryvore lighthouse among others, wrote of his father -
"There is scarcely a harbour or navigation in Scotland about which at some time he did not give valuable advice." Biographical Sketch of the Late Robert Stevenson (1861).
David and Thomas designed and built lighthouses as well as inventing aids to navigation like the flashing buoy pictured, which used compressed gas to light the lamp. They were an inventive and hard working family. The next generation also produced engineers.
The writer, Robert Louis Stevenson was the son of Thomas and you can read his account of the history of this remarkable family here.