© Science Museum / Science & Society Picture Library
The cros-staff was used at sea from the beginning of the 16th century until the first half of the 18th century. It was used to measure the sun's altitude, especially at the meridian pasage. With the knowledge of the sun's declination, the mariner could then obtain his latitude. The staff is original, with the inscription 'Hamon a St Malo 1687'. The cros pieces are replicas. Different cros pieces were used depending on the altitude. A different scale is on each side of the staff.