Thomas Sprague was the dominant actuary of the second half of the 19th century, his supremacy unrivalled.
Dr Sprague was the main person behind the International Actuarial Notation which was adopted unanimously at the 2nd International Congress of Actuaries in 1898 . This established an international actuarial notation.
Dr Sprague was the main person behind a mortality study covering the experience of twenty U. K. life offices. This study resulted in the Institute of Actuaries Life Tables (the so-called Twenty Offices Table) which was published in 1869 . From this study, he produced, in 1879, the first Select Tables of Mortality  which were the first two-dimensional mortality tables ever published (the two dimensions being 'insured duration' i.e. the 'select period' and 'age attained'). The 'select period' was five years.
Dr Sprague pioneered the important 1870 Life Insurance Companies Act  which was introduced following the notorious insolvencies of both the Albert and the European life assurance companies. The 1870 Act required:-
... an investigation into the financial condition of a life insurance company to be made regularly by an actuary,required a separate "long-term fund" and required the:-
... preparation of a revenue account and balance sheet every year in prescribed form to be filed with the Board of Trade,the latter being a public document. Dr Sprague was one of the foremost advocates of the principle of 'Freedom with Publicity' (i.e. documents available to the public) and was opposed to there being any Government regulation prescribing the manner of valuation of policy liabilities. He wrote the major 19th century work on the preparation of life office accounts in conformity with the 1870 Act .
Dr Sprague published some eighty papers in the Journal of the Institute of Actuaries and, including articles, Presidential Addresses, translations, comments and reviews for this and other actuarial journals (Transactions of the Faculty of Actuaries, Transactions of the Actuarial Society of Edinburgh), his published contributions to actuarial science must number over one hundred. In addition, he published eight pure mathematical papers:  and  in the Transactions of the Royal Society of Edinburgh and six papers in the Proceedings of the Edinburgh Mathematical Society: ,, , ,  and .
A particularly memorable translation, from Danish, is Sprague's translation of Dr T N Thiele's 1871 paper on mortality . Thiele was from 1872 to 1910, among other appointments, the chief actuary of the Danish insurance company Hafnia. Thiele stated, for the first time, a mathematical formula expressing the force of mortality throughout the whole of life. The Thiele 1871 mathematical formula was not bettered until the 1980s . Thiele's law, where x is the age and μx is the force of mortality, is:-
Dr Sprague was the only person to have been President of both the Institute of Actuaries (1882-86) in London and the Faculty of Actuaries (1894-96) in Edinburgh. He was also President of the Actuarial Society of Edinburgh on three occasions (1874, 1882 and 1891) and did much to mould the futures of all these bodies.
He was elected a Fellow of the Royal Society of Edinburgh (FRSE) in 1874  and of the Edinburgh Mathematical Society (EMS). The University of Aberdeen made him an LL.D. in 1893.
Dr Sprague was the actuary to the Equity and Law life insurance company from 1861-1873 when he was appointed to be Chief Executive (1873-1900) of the Scottish Equitable Life Assurance Society in Edinburgh (not to be confused with the Equitable Life Assurance Society in London) retiring from business life at age 70. When President of the Institute of Actuaries, from 1882-86, he was resident in Edinburgh but travelled to London regularly to fulfil his role as President of the Institute. He was on the Council of the Institute of Actuaries for thirty-eight years (1863-1900) and editor of the Journal of the Institute of Actuaries for 16 years (1867-83) .
Dr Sprague was invited by Clerk Maxwell (the latter being scientific editor, along with T H Huxley, of the 9th Edition of the Encyclopaedia Britannica) to write the article on 'Annuities' . This article was a most thorough analysis of the contributions, including those of de Witt, Halley, and de Moivre, to the advancement of actuarial science.
At the 1874 meeting of the British Association in Belfast (at which occasion Clerk Maxwell wrote his poems entitled 'Notes of the President's Address' and 'Molecular Evolution' ), Sprague gave a paper on the solvency of life offices  which the 1870 Life Insurance Companies Act (see above) did much to ensure.
Dr Sprague's one peculiarity was his advocacy of spelling reform which caused some difficulties with the Editorial Board of the Journal of the Institute of Actuaries as to whether his special brand of phonetic spelling should be permitted in the papers which he submitted e.g. 'treatis', 'anser', 'articl', 'coud', 'giv', 'conclusiv', 'hav', 'ov' (e.g. the wording in reference  is not a mis-spelling). However, such was the eminence of Dr Sprague that the Journal had no alternative but to permit it!
In Maclaurin House (the home of the Faculty of Actuaries, named after Maclaurin, who, in 1744, acted as consultant to the first pension fund in the world established on sound actuarial principles, namely the pension fund for the widows and children of the 'Ministers of the Church of Scotland and the Professors at the Scottish Universities') his bust  stands alongside that of William T Thomson (one of the founders of the Institute of Actuaries in 1848 and the Faculty of Actuaries in 1856). A bust  of him also stands in the main hall of Staple Inn, the home of the Institute of Actuaries.
A prize for the best performance in actuarial science, finance, insurance, operational research, probability and statistics in Part III of the Cambridge Mathematical Tripos is awarded each year in honour of Dr. Sprague.
His son, Alfred Ernest Sprague, MA, DSc, FFA, FIA was also President of the Faculty of Actuaries from 1919-21.
Article by: David O Forfar Heriot-Watt University.