Secular Changes in the Oceanic Tides at Brest, 1711–1936

Summary Long series of sea‐level observations at Brest, extending from 1711 to the mid‐20th century, are analysed by modern methods to search for secular trends in the admittances of the oceanic tides to the gravitational potential, already suggested by results from St Helena. Special attention is paid to the editing and analysis of the historic series of observations of High and Low Waters published in Lalande's Astronomie . The results not only confirm an increase in diurnal phase lead of some 3 degrees per century, but show a smaller but more significant decrease in semi‐diurnal amplitude, about 1 per cent per century, in keeping with some results of Doodson from shorter records from the Bay of Fundy. The possible effects of modern harbour developments are discussed, and found to be probably insignificant, but the question whether the trends are truly oceanic or due to local coastal changes remains open.