Tide Gauge from Sheerness Dockyard.

Tide gauges are instruments used to measure the height and time of high and low water. The basic elements of a tide gauge are quite simple: a rod attached to a float, a vernier and a measuring scale. Self-registration is however a more difficult task and the Museum conserves, beside the copper float, the innovative mechanism that automated registration. This was added to the main body of the machine to keep track of tide measurements on paper, allowing the tide gauge to automatically register the level of high and low water. Consequently human intervention became unnecessary and the registration of measurements more precise. Moreover, measurements during the night and with bad weather became possible.

The instrument comprises a base in which a large wheel is fitted, coupled to a large sliding bar of brass and two dials. The cylinder on which the sheet of paper should have been rolled is missing, although this was probably held by the two brass poles on the front which also hold the sliding bar. One dial is marked “High Water” on one side and “Low Water” on the other, graded from zero to twelve in Roman numerals. This right-hand dial, which does not appear in the original design published on the Nautical Magazine (vol. 1, 1832), is connected with the large wheel, moved by the float, and reflects the movements of a sliding bar which probably held a pencil. This seems not to have been a clock, but an indicator of the height of water. The dial of the actual clock, which has a movement, is graduated for three one hour periods. It has another small quadrant inside graduated from zero to sixty. This is currently independent from the rest of the registering mechanism. It was probably linked to the missing cylinder by the gear jotting out in front of the dial, but this connection mechanism is missing. The missing pieces for a complete tide gauge, beside those mentioned above, are the house containing it, the wooden rod that connected the float with the measurement system and the original measurement system (a vertical scale and a vernier).

Beside the instrument the cylindrical copper float is also preserved. The float was fixed to a wooden rod. Pieces of the rod are still visible in the upper part of the float. The vertical movement of the rod, which followed the movements of the tides, was, according to the drawing included in the Nautical Magazine, transmitted through a system of wheels to the registering instrument described above.

This is the first self-registering tide gauge, probably an improvement on Henry Palmer's original design (who invented the revolutionary design but never built an instrument) and realised for the Sheerness Dockyard in 1830 by the civil engineer James Mitchell. This mechanism was first brought into use in September 1831, prior to which date an observer had been required to watch for times of high and low water. The first self-registering gauge erected at any Admiralty dockyard, and probably the first in regular use in any part of the world. Originally erected near the south-west corner of Boat Basin with zero of 2.03 feet above O.D.; moved to Cornwallis Jetty in 1856, and zero set to XVIII mark on Basin's index or 2.125 feet O.D. The zero was set to 17 feet 6 inches on Index in 1887