it and the one on langness were built c.1811 by Thomas Brine (architect then in employ of British Gov - he later stayed on in Castletown and designed many fine buildings still there today) - their purpose was to enable seamen to distinguish the hills. On reflection I'm not sure re Langness one being used as a beacon - a small tower with light on was built within the old Derby fort on St Michael's island and that used until the light on Langness was built.