The remains of the old horizontal mills that were worked in tandem at night. Illustrative picture taken from George Low’s book.

"To the westward is Sound and Gulberwick, two small but beautiful spots, pretty well cultivated in the Schetlandick style. Here I saw the first Schetland mill for grinding Oats and Bear, but this it does in hut a clumsey manner, little better than a hand-mill or quern, only it saves hand labour. It consists of a very simple set of machinery ; a small horizontal wheel for the water to play on, the top of its axis runs thro’ the lower and supports the upper stone as in other water-mills ; a hopper and shoe with a leaver to level the upper stone completes the apparatus."
George Low
A Tour Through Schetland and Orkney - CONTAINING HINTS RELATIVE TO THEIR ANCIENT MODERN AND NATURAL HISTORY COLLECTED IN 1774