Keeping the lighthouse clock on time

I found this outside the museum at Port Charlotte on Islay. It was from a local lighthouse. The lighthouse was provided with a clock and was expected to keep regular hours. But what if the mechanism ran fast, ran slow or stopped? They were provided with a sundial to give the correct time to set the clock. When a measuring instrument gives the true value, it is said to be ACCURATE. This sundial makes the lighthouse clock accurate.