Flash PatternsThe function of lighthouses is not just to warn sailors of a potentialdanger. They also enable them to find their position. This is doneby not only making sure that the projected beam is highly visiblebut also making it distinct from any other lighthouse beam. Stripesor bands are sometimes painted on the lighthouse to improve theirvisibility during the day when the light may be switched off. Lighthouses on shore are usually painted white; those on low lying land or standing at the foot of cliffs are often striped with red or black. Fog signals which emit a loud noise every 30 seconds or so warn sailors of danger when fog blocks out the light.
However, the most important way of making the beam distinct is to vary the pattern of flashing light so that it is unique to a particular lighthouse. Over two hundred years ago, it was difficult for sailors to distinguish one light from another. One solution to this was to show a different number of lights, whilst another was to build more than one tower, as at the Lizard in Cornwall in 1751.
In the eighteenth century, various devices were invented that covered the light for short periods of time to make a sequence of light and dark. During the nineteenth century, rotating the light became the most favoured method of making distinctive flash patterns.