John Pratt (Archdeacon of Calcutta)

John Henry Pratt FRS (4 June 1809 – 28 December 1871) 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. Although nominated as Bishop of Calcutta, the decision was rescinded at the last moment with George Cotton appointed to the position. A gifted mathematician who applied his mind to problems of geodesy and earth science, he was approached by the Surveyor General of India to examine the errors in surveys resulting from the attraction of the plumb-line to the mass of the Himalayan mountains. This led him to develop a theory based on a fluid earth of crustal balance which became the basis for the isostasy principle. He died in India of cholera on a visit to Ghazipur.

Pratt was the second son of Josiah Pratt and Elizabeth nee Jowett. His exact date of birth is debated and it is thought that he was born in London as he was baptized on 30 June 1809 at St Mary Woolnoth. His early schooling was at Oakham school under the tutelage of Dr Doncaster. He then went to Caius College, Cambridge and was a student of William Hopkins. He graduated B.A., third wrangler (after Alexander Ellice and Joseph Bowstead) in the Mathematical Tripos 1833, was elected to a fellowship, and proceeded M.A. in 1836. For a while he stayed at Cambridge and gave private tuitions. One of his students was Harvey Goodwin later Bishop of Carlisle. While at Cambridge he wrote a book called Mathematical Principles of Mechanical Philosophy (1836, second edition 1845) which described mathematical applications in gravitational physics.

Pratt was appointed a chaplain of the East India Company through his father's influence on Bishop Daniel Wilson in 1838. He became Wilson's domestic chaplain, and in 1850 was appointed Archdeacon of Calcutta. The leisure allowed during his position in India allowed him to pursue mathematics although he noted that it was difficult to work alone and led to long exchanges in the journals of learned societies in Britain. When Bishop Wilson died in 1858, he was nominated for the position of Bishop. He was approved with the influence of Lord Shaftesbury on Lord Palmerston but it was shortly after decided in the wake of the 1857 uprising that no appointee known for missionary work should be appointed. The chosen appointee was instead Bishop Cotton. They held each other in high esteem. In 1864 an order was passed by the Secretary of State in India to retire chaplains after twenty-five years (earlier unlimited). An exception was made for Pratt and he was extended from October 1867 to March 1869 based on pleas from Bishop Cotton. When Pratt wished to resign in 1869, the Secretary of State extended his service to October, 1872. After the death of Bishop Cotton in 1866, Pratt started a Hill Schools' Nomination Endowment Fund to help support Bishop Cotton's scheme of starting schools in the pleasant climate of the hills for the benefit of the children of poorer English residents in India who could not afford an education in England. Pratt married Hannah Maria Brown daughter of G.F. Brown, a Bengal Civil Servant, at Bhagalpur on 6 March 1854. He was elected Fellow of the Royal Society in 1866. He died from cholera when he was on a visit to Ghazipur, India, on 28 December 1871. At the instigation of Bishop Robert Milman, a memorial to Pratt was erected in St. Paul's Cathedral, Calcutta.

Pratt was the author of ‘Mathematical Principles of Mechanical Philosophy’ (1836), subsequently expanded and renamed ‘On Attractions, Laplace's Functions and the Figure of the Earth’ (1860, 1861, and 1865). This work, known as Pratt's Mechanical Philosophy, had the full title: The Mathematical Principles of Mechanical Philosophy and their application to Elementary Mechanics and Architecture, but chiefly to The Theory of Universal Gravitation, a textbook of some 600 pages. It included the computations of the shape of the earth. The oblate spheroid and its deformation according to the assumption of a fluid being rotated. Part of his work was extensively used verbatim by Isaac Todhunter in his Treatise on Analytical Statics (1853).

Even as he travelled to India in 1838 aboard the Duke of Buccleugh he conducted experiments to examine currents in the ocean and sea temperatures at various depths. While in India, Pratt was approached by Andrew Scott Waugh, Surveyor General of India who 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 Schiehallion in Scotland) but the observed deviations were much less. He propounded a theory that the density of the mountains was less than that of the underlying substrate. It was also noticed that the deviations of the plumbline were greater at the foot of the mountain and reduced as one moved away from the mountain. George Airy came up with the explanation that the roots of the mountain go deep into the earth. He compared it with a heap of logs in water and suggested that when a log juts higher above the water, a greater amount of it must be submerged. Pratt on the other hand suggested that there was a lower density under mountains below sea-level (actually refined later and termed the depth of compensation) and that this is offset by the mass above sea-level. Pratt's explanation assumes a variation in density whereas Airy assumes a constant crustal density. Pratt found fault with Airy's idea as it assumed a thinner crust (10 miles) than that estimated by William Hopkins (800 to 1000 miles) and also that the crust is lighter. Pratt said that the crust cools from the solid interior and should therefore be denser. Airy did not defend his view but Samuel Haughton used the debate to claim that mathematics was a useless tool for speculation. He suggested that the crust would be depressed in the cooler parts of the world and suggested that the plumb deflection was caused by anomalous high density in the oceans south of the Himalayas. Pratt also applied his knowledge of physics and mathematics to a number of applications in India on which he was consulted by engineers. He examined arches, the physics involved in the sudden movement of a mass of water such as in the Indus floods of 1858 and in the bore of the Hooghly river, computed the iron required for cantilever bridges,

Pratt took a great interest in Hindu Astronomy and supported the translation of the Siddhānta Shiromani aiding its publication by the authors Lancelot Wilkinson and Pandit Bapu Deva Sastri in 1863. In response to a question from Professor Edward Byles Cowell on how H.T. Colebrooke had come up with an age for the Vedas. He worked out the calculations based on astronomical references, finding some errors in Colebrooke and came up with the suggestion that it could be either around 1181 or 1229 BC. Pratt however felt that both Indian and Arab astronomy had failed to build a mathematical of physical framework and that their only major achievement was the ability to predict eclipses and even these he considered as being imprecise.

In 1849, Pratt and his brother wrote a memoir on their father. In 1856, Pratt published a book entitled Scripture and Science not at Variance, which went through numerous editions; It was first written to counter a statement by Baden Powell that "all geology is contrary to Scripture" and then went on to counter other scientific theories including of evolution that conflicted with the Bible. and, in 1865, edited from his father's manuscript ‘Eclectic Notes, or Notes of Discussion on Religious Topics at the Meetings of the Eclectic Society, London, during the years 1798–1814. He also published a Paraphrase of the revelation of Saint John (1862).

A Pratt School for girls was founded in his memory in Calcutta in 1876 by Bishop Milman.