Manuel Johnson's tide record at St. Helena

David E Cartwright,Philip L Woodworth,Richard D Ray

doi:10.5194/hgss-8-9-2017

Abstract

Abstract. The astronomer Manuel Johnson, a future President of the Royal Astronomical Society, recorded the ocean tides with his own instrument at St. Helena in 1826–1827, while waiting for an observatory to be built. It is an important record in the history of tidal science, as the only previous measurements at St. Helena had been those made by Nevil Maskelyne in 1761, and there were to be no other systematic measurements until the late 20th century. Johnson's tide gauge, of a curious but unique design, recorded efficiently the height of every tidal high and low water for at least 13 months, in spite of requiring frequent re-setting. These heights compare very reasonably with a modern tidal synthesis based on present-day tide gauge measurements from the same site. Johnson's method of timing is unknown, but his calculations of lunar phases suggest that his tidal measurements were recorded in Local Apparent Time. Unfortunately, the recorded times are found to be seriously and variably lagged by many minutes. Johnson's data have never been fully published, but his manuscripts have been safely archived and are available for inspection at Cambridge University. His data have been converted to computer files as part of this study for the benefit of future researchers.

Highlights

The rapid rise to scientific fame of the young Manuel John Johnson (1805–1859), from Second Lieutenant in the East India Company’s artillery at St
It is inconceivable that Johnson would have employed two conventions for time in adjacent columns on the same page, so we conclude that local apparent time must have been used for the times of high and low waters
There is an intriguing postscript to be added to this discussion, in that if Johnson’s recorded times of high and low waters had been given in Greenwich Mean Time (GMT), with no necessity for the equation of time and 22.9 min adjustments, they would have been in better overall agreement with the synthetic times

Summary

Introduction

The rapid rise to scientific fame of the young Manuel John Johnson (1805–1859), from Second Lieutenant in the East India Company’s artillery at St. We know little about Johnson’s educational background, other than that he was trained in England at Addiscombe Military College, which prepared young men for service in the HEIC Even assuming that he acquired there a first class knowledge of the theory of astronomy, this does not automatically qualify a person to set up an observatory or deploy its measuring instruments. The suggested measurements on the windward coast were never carried out, and would have been almost impossible with the instrument available After his visit to Fallows, Johnson returned to St. Helena in March 1826; he was to pay another visit to the Cape Observatory in 1828. A striking feature is the straight rise of the railed stairway of 699 steps, known to the islanders as Jacob’s Ladder This stairway was built by the HEIC militia in 1828–1829 to facilitate transport of men and materials between Jamestown and various military buildings, including the Observatory. Jacob’s Ladder remains largely intact, and is still used by the more agile members of the island’s population

Description of Johnson’s tide gauge and its records

Ambiguity of time convention

Times of high and low tide

Accuracy of tidal ranges and high and low water heights

Conclusions