John Pratt attended Gonville and Caius College, Cambridge receiving his B.A. in 1833. He was third Wrangler, an excellent result, and he was elected to a Fellowship. In 1836 he received his M.A. from Christ's and Sidney Sussex College.
John's father was secretary of the Church Missionary Society and John left university with two strong drives inside him, one coming from his exceptional scientific ability, the other from the missionary zeal coming through his father.
Pratt obtained a chaplaincy appointment with the East India Company in 1838. He remained in India for the rest of his life, becoming chaplain to the Bishop of Calcutta in 1844. Then in 1850 Pratt was appointed Archdeacon of Calcutta.
His first work, published in 1836, was The mathematical principles of mechanical philosophy. He revised the work in 1842, then expanded and republished it under the title On attractions, Laplace's functions and the figure of the Earth in 1860.
This work of Pratt's was on the shape of the Earth. He assumed that the Earth behaved as a fluid and showed, as Newton had done, that the resulting shape would be an oblate spheroid. He gave 26.9 miles as the difference between the equatorial and polar axes, quite a good result.
In 1855 he postulated density differences in the Earth's crust, lower densities under mountains, higher densities in lowlands, to explain the (too nearly constant) values obtained for gravity at a given latitude. This is included in the rewrite of his work in 1860.
In the same year, 1855, Airy offered a different explanation of the gravitational data. Both Pratt's and Airy's proposals have their merits but are oversimplifications of the actual situation.
Pratt also published religious works; Scripture and science not at variance (1856) was a popular work which ran to many editions. He also edited his father's Notes of discussion on religious topics at meetings of the Eclectic Society, London, during 1798 - 1814.
He was elected a Fellow of the Royal Society in 1866.