An authoritative account of this benchmark was given by Bruce Hamon in a paper titled "Early Mean Sea Levels and Tides in Tasmania" published in Search magazine in 1985. There has been a lot of scientific interest in this bench mark as it is believed to be the earliest bench mark installed anywhere in the world for the scientific study of changes in sea level. Bruce Hamon's paper was reproduced in the Tasmanian Surveyor in 1986.
The tide gauge was operated by Thomas Lempriere, the Deputy Assistant Commissary General at Port Arthur 1837 - 1848. Lempriere was a keen amateur scientist and he set up a meteorological station and conducted observations while he was stationed there. Information about the nature of these observations is given in his own description of buildings located at Port Arthur:
"The same building comprises the Commissariat Office and a museum in which specimens of the animal, vegetable and mineral production of the Peninsular are preserved; the meteorological registers are also kept here, they contain the height of the barometer, attached and external thermometers, direction and force of the wind, weather, also a tide gauge and pluviometer, the whole observations except the two last being taken at 8 a.m., 2 p.m., and 8 p.m."
Captain Sir James Clark Ross visited Tasmania during his voyages to Antarctica and the Southern Ocean and travelled to Port Arthur in 1841 to visit Lempriere. Ross was keen to implement a proposal of scientist Baron von Humboldt that bench marks be struck at suitable locations around the world with a view to assisting future generations of scientists monitor earth crustal moment relative to sea level.
Ross chose Port Arthur as a suitable site because of Lempriere's continuing observations and because "the tides in the Derwent were too irregular, being influenced greatly by the prevalence of winds outside and freshes from the interior, so that we could not ascertain with the required degree of exactness the point of mean sea level".
A bench mark coinciding with the height of the tide was cut into a near-vertical sandstone cliff on the Isle of the Dead on 1 July 1841. It is a standard Survey Ordinance bench mark, and consists of a horizontal line about 500 mm long surmounted by a broad arrow A plaque describing the time and the height of the water in the Port Arthur Tide Gauge was fixed to the cliff nearby. The plaque was reported missing about 1913 but the bench mark is still clearly visible.
When Bruce Hamon published his paper in 1985 the only sea level records associated with the benchmark were two conflicting descriptions of the inscription on the plaque and some "range of tide" values for each month of 1842. The meaning of these values, which contain some obvious typographical errors, is obscure and they were unlikely to provide any indication of mean sea level at that time.
Searches of British Admiralty and Tasmanian State Archive records had not unearthed any further information. Hamon was able to resolve the conflicting plaque inscriptions by hindcasting the tide at Port Arthur using tidal observations from Hobart, but the absence of any other useful data led him to sadly remark "we must admire the industry and foresight of men like Ross and Lempriere - and regret that so much of Lempriere's effort was in vain". . . . ."The position would of course be different if Lempriere's original observations ever came to light".
Now this has happened and there is a good possibility that useful scientific information can be obtained from these observations. The type of tide gauge used by Lempriere remains a mystery. The inscription on the benchmark plaque refers to the "height of water in tide gauge" and this suggests some form of stilling well was used. The question of whether the gauge was self recording is intriguing. The first self recording tide gauge is believed to have been installed in the UK in 1831 and one was installed in Williamstown, Victoria, in 1858. It is just conceivable that the Port Arthur gauge was self recording. If so it would have been installed only 6 years after the UK gauge and would have preceded the Williamstown gauge by about 20 years.
The Lempriere data is currently being analysed for both tidal motions and mean sea level, and Dr John Hunter of the CSIRO Division of Oceanography has proposed that another program of tidal measurements be conducted at Port Arthur. Such a program would indicate any significant change in mean sea level or tidal range over the 150 years that has elapsed since the bench mark was installed, and provide information about sea level changes caused by global warming. It would certainly serve as a fitting tribute to "the industry and foresight of men like Ross and Lempriere".
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