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 Although nominated as Bishop of Calcutta, the decision was rescinded at the last moment with George Cotton being appointed to the position. A gifted mathematician who worked on 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 while on a visit to Ghazipur. Biography Pratt was the second son of Josiah Pratt and Elizabeth née Jowett. His exact date of birth is debated and it is thought that he was born in London as he was baptised 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