Jellett, John Hewitt

John Hewitt Jellett (1817-1888) was the son of the Rev. Morgan Jellett (d.1832) and his wife Harriet, daughter of Hewitt Baldwin Poole (d.1800) of Mayfield, Co. Cork. He was born at Cashel in Tipperary on Christmas Day 1817, and educated at Trinity College Dublin becoming a Fellow in 1840. He graduated BA 1838, MA 1843, BD 1866, and DD 1881. He had been ordained a priest in 1846. In 1848 he was elected to the chair of Natural Philosophy. In 1847 he was appointed to the newly established chair of Natural Philosophy (Applied Mathematics), which he held until 1870. He was a religious scholar of some eminence with publications of theological essays, sermons, and religious treatises, principally An Examination of some of the Moral Difficulties of the Old Testament, 1867, and The Efficacy of Prayer (The 1877 Donnellan Lecture) 1878. In the latter, he suggested the experimental study on the measurable value of prayer and here very much echoed the earlier suggestion of the agnostic John Tyndall.

He was inspired by the Famine shortly after taking up his chair to develop experimental studies on potato blight, leading to his invention ‘On a new optical saccharimeter’, described in a paper read before the Royal Irish Academy, January 26, 1863 and published that year by McGlashan & Gill, Dublin. This invention was based on a polarized optical arrangement to develop a split-field ‘end point eyepiece device’ marking the introduction of the null-balance principle used in Wheatstone’s (S. Hunter Christie actually invented this in 1833) electronic bridges. Tyndall’s celebrated ‘heat’ apparatus for monitoring of gases and vapours and Jellett directly inspired the development in the later Trinity work of Joly’s photometer. Importantly, the saccharimeter was manufactured in Dublin by Spencer and Son and shown in the 1865 Great Exhibition. There was a differentiation of the role of Trinity academics in this 1870 appointment with R.J. Dixon being elected to the Erasmus Smith Chair of Natural Philosophy and becoming, in what was an age of increasing specialisation according to the authorative duo on Trinity history McDowell and Webb, effectively a chair of physics, leaving the more senior man Jellett with mathematics. Certainly the interest in Jellett’s later career moved away from optics and towards the mathematical and educational. He indeed proved to be an able mathematician, and contributed to the important pioneering textbook tradition in Trinity by writing A Treatise of the Calculus of Variations in 1850, and A Treatise on the Theory of Friction in 1872.

Jellett’s greatest contribution to mathematics and physics was perhaps in editing with Samuel Haughton The Collected Works of James MacCullagh (Dublin, 1880) that included the mathematical formulation of the equations of light and led directly to Maxwell’s electromagnetic theory of light. Here the Trinity tradition of mathematics was extended subsequently very significantly by the work of Jellett’s relative George Francis Fitzgerald. In 1870, on the death of Dr. Thomas Luby, he was co-opted by the senior Fellows of Trinity College as a member of their board and in 1868 he received the appointment of Commissioner of Irish National Education. After the disestablishment of the Church of Ireland he took an active part in the deliberations of the general synod and in every work calculated to advance its interests. A year later the Royal Irish Academy elected him president.

Mr. Gladstone’s government in February 1881 appointed Jellett provost of Trinity; in the same year he was apparently awarded a Royal Medal by the Royal Society of London in 1881. Though no reformer, he was notably the only member of Board to support entry of women into college in 1873. He was President of the Royal Irish Academy for five years from 1869 and was awarded an honorary degree from Oxford in 1887. His politics were sufficiently liberal to make him an acceptable candidate to Gladstone who appointed him Provost of Trinity College in April 1881. He died at the Provost's house on 19 February 1888, and is buried in Mount Jerome Cemetery.

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