This picture is of the Point of Ayre Windvane ventilator,but many Scottish lighthouses have a similar ventilator.

As built in 1818 Point of Ayre had a Ball Vent,same as Calf of Man lighthouses in above photograph,replaced by the Windvane Vent in 1890.

The whole Vent turns in the wind and at some lighthouses a pointer was fixed to the internal shaft and
the compass points marked on the underside of the dome to give visual indication of wind direction.