A tidal computer, inscribed, 'Giving Greenwich Time of High Water at all important points in the British Isles and the continent of Europe between the River Elbe and Brest. Also times of Turning of Tidal Streams at all important positions in the same area.'

It comprises a card body (305 x 272 mm), which is hinged and doubled. This contains a circular scale marked 1-12 (hours) and divided every 10 minutes. Within is a rotatable card dial of 210 mm diameter with a scale marked from 0 to 100 and inscribed, 'High Water Dover' and an arrow at zero point. Two apertures allow the user to read off mean, spring, neap tides and 'increasing' and 'decreasing'. Instructions for use are printed on the inner dial. On the back are three pages of place names (ports) with a number against each, the number being the tidal difference from Greenwich High Water.