The Tide Gauge
1885 · Automatic Recording · Work 644 of 1,871
A tide gauge in a small timber hut on a harbour wall, a float riding in a stilling well below the floor connected by a fine wire over a pulley to a counterweight, the wire moving a pen across a clock-driven paper drum, the drum wrapped in a sheet of ruled graph paper carrying a single drawn curve rising and falling in slow waves over several days, the brass clockwork case beside the drum, damp floorboards, a coil of rope in the corner, a single oil lantern hung from a hook throwing its light on the drum, the harbour dark through the open door.
- What it was used for
- The Marégraphe of Marseille is a float gauge that does not merely record sea level but continuously averages it. The copper float is 0.9 metres across and drives, through a 10:1 reduction, a mechanical integrator, the "totalisateur", which yields mean values directly. The instrument was built by Dennert & Pape of Altona to a design by F. H. Reitz. The two buildings on the Corniche were put up in 1883, the instrument was installed between November 1884 and January 1885, and observations began in February 1885. Its purpose was not navigation but national surveying: the gauge was to fix the zero of the French levelling network. The fundamental benchmark is a bronze bolt with a platinum-iridium cap, set in a cylindrical block of granite in an underground chamber.
- What was new
- The distinctive part was the integrator. Other self-registering gauges drew a curve that then had to be measured off; this one added up the water level mechanically and gave out the mean itself. According to Wöppelmann and co-authors (2014) only three instruments of this type were operating at the time, and only Marseille has been in operation ever since. Charles Lallemand took the value towards which the mean curve tended as the new zero of French heights; at the end of 1896 the curve was evaluated from the start of measurement, and in 1897 the fundamental benchmark was fixed at 1.660 metres; the new zero lies 71 millimetres below the old Bourdalouë zero. The barb: the instrument was built to supply a fixed reference height, and it has shown that there is none. The Marseille record, extended back to 1849, gives a relative rise of 1.08 plus or minus 0.04 millimetres per year for 1849 to 2012. The 1885 gauge is still maintained and read weekly.
What the sources leave open: The work's title names no place; the identification with the Marseille gauge rests on the year 1885, since the work's page on fromsteamtosilicon.art could not be retrieved for this run. The sources disagree on the start of measurement: IGN gives 1 February 1885, Wöppelmann and co-authors (2014) give 3 February 1885. Analogue curves were drawn until August 1988.