Riding the wave

Tide Gauge

Tide Gauge

Thomson type tide gage built, as tells the engraving on the instrument, at the Hydrographic Office in 1887.

It is an instrument with mechanical clockwork charged once a week, with a partially ballasted float, a mercury pendulum and a charging weight.

A second weight ensures the tension of recording paper, that is wound onto the charging drum, runs on a second drum and rewinds on a third one.

Measured values are recorded by means of a pen with an ink reservoir, while the ratio between the tidal excursion and the graphical representation is 1 to 5.

Tide gauges of the same type, also built at the Hydrographic Institute of the Navy, are still in use at the tide stations of Genoa and Brindisi, and provide a useful comparison with digital tide gauges, that are operated and read through modems at the Hydrographic Institute.

The collected data allow - besides the development of Tide tables that are essential for the safe conduct of coastal navigation, and other statistical and scientific uses - the determination of mean sea level, on which the Military Geographical Institute in Florence defines altitudes on earth.

The first fundamental tide gauge of the national geodetic network - an original "Thomson" - was put in place in 1883 in a hut located at the western end of the drydock in the harbour. The floating pit communicated with the harbour basin via a short submerged tunnel.

It remained in operation until 1889, when it was replaced by a similar model, built at the same office, and a second sample was put into operation in the basin of Venice that year.

A tide gauge is also visible at the International Maritime Museum of Western Liguria (Imperia).


Sir William Thomson, Lord Kelvin, was a physicist and mathematician born in Belfast on June 26th, 1824, and died in Netherall (Scotland) on December 17th, 1907. He graduated in Cambridge in 1845, and continued his studies in Paris; in 1846 he obtained the chair of general physics at the Glasgow University, and kept it 53 years long.
His first job was a mathematical research of the age of the earth, calculated according to its progressive cooling; he obtained an age between 20 and 400 million years. He published a statement on the dynamic theory of heat and a treaty, still fundamental today, on theoretical mechanics.
He discovered the oscillatory nature of many electric phenomena; suggested the use of stranded conductors in submarine telegraphy; discussed the mathematical theory of propagation in cables and determined the harmful effects of the electrostatic capacity of the cable on signal distortion, that he could greatly reduce using a capacitor at each end; he made the first determinations of the absolute units of electrical forces; invented the galvanometer recorder that allowed the industrial development of submarine telegraphy; understood the distinction between magnetic force and magnetic induction; invented the double bridge for the measurement of low electrical resistances, the compensation of naval compasses, and produced many other practical innovations (including the tide gauge and the sounding wire shown here), which gave him great authority and economic wealth in the industrial-technical world.
For these scientific merits he was elected president of the Royal Society in 1890 and two years later became Peerage of England with the title Baron Kelvin.
(Italian Encyclopedia, XX)
By Paola Presciuttini
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