John Henry Pratt FRS was a British clergyman, astronomer and mathematician. A Cambridge Apostle, he joined the British East India Company in 1838 as a chaplain and later became Archdeacon of Calcutta.
Sketch of John Henry Pratt in 1839
Great Trigonometrical Survey
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The Great Trigonometrical Survey of India was a project that aimed to carry out a survey across the Indian subcontinent with scientific precision. It was begun in 1802 by the British infantry officer
John Pratt (archdeacon of Calcutta)
…India who had succeeded George Everest, to examine the gravitational anomaly caused by the Himalayas on the plumbline which resulted in errors in the Great Trigonometrical Survey. Pratt estimated the deviation of the plumbline that would be expected by the mass of the mountains (based on density estimated in 1772 at Mount…
Index to the Great Trigonometrical Survey
The first triangulations across the Peninsula
Measurement of the Calcutta baseline in 1832 based on a sketch by James Prinsep. This shows surveyors stretching a chain on coffers supported on pickets. The chain is housed under shade to reduce errors due to thermal expansion, and is aligned using a boning telescope.
A zenith sector was an upward-facing telescope with accurate angle measurement scales. A star close to the zenith of known declination from the Pole Star was used to determine latitude, as a direct measurement of the Pole Star could be affected by refraction.