Died: 29 July 1839 in Paris, France

**Gaspard de Prony**'s family name was Riche, the de Prony title having been bought by his parents. In fact de Prony's younger brother was always known by the name Riche. De Prony was educated at the Benedictine College at Toissei in Doubs. From there he entered the École des Ponts et Chaussés in 1776 where he studied engineering. He graduated in 1779 as the top student and remained for a further year in Paris, doing as the head of the École des Ponts et Chaussés told him:-

M de Prony ... concern yourself with acquiring a deep knowledge of your art, for you are destined to become head of the École des Ponts et Chaussés.

In 1780 he became an engineer with the École des Ponts et Chaussés and after three years in a number of different regions of France he returned to the École des Ponts et Chaussés in Paris 1783. This was the same year he published his first major work in the Académie des Sciences on the forces on arches. Monge was impressed with this paper and realised that de Prony was someone of great potential.

In 1785 de Prony visited England on a project to obtain an accurate measurement of the relative positions of the Greenwich Observatory and the Paris Observatory. Two years later he was promoted to inspector at the École des Ponts et Chaussés. Around this time he was involved with the work on the Louis XVI Bridge in Paris which is now called the Pont de la Concorde.

Further promotion in 1790 was followed the next year by his being appointed Engineer-in-Chief of the École des Ponts et Chaussés. This promotion was as a result of the opening of the Louis XVI Bridge.

Also around 1791 de Prony was working on geometry with Pierre Girard. Then in 1792, de Prony began a major task of producing logarithmic and trigonometric tables, the *Cadastre.* With the assistance of Legendre, Carnot and other mathematicians, and between 70 to 80 assistants, the work was undertaken over a period of years, being completed in 1801. The tables were, see [2]:-

... vast, with values calculated to between fourteen and twenty-nine decimal places. Each copy consisted of eighteen folio volumes together with another volume of mathematical procedures.

Getting such a massive work published was another matter. Negotiations went on over a number of years until, in 1809, it seemed they would appear. The publisher wrote:-

The present generation would never have witnessed the end of this monumental work if M de Prony had not had the fortunate idea of applying the powerful method of division of labour, conceiving methods to reduce the long and laborious part of the production of the tables to simple additions and subtractions...

However the tables were never published in full and it was near the end of the century before even a part appeared. It was just too expensive to print at a time when France was not in the best of financial states.

In 1794 the École Centrale des Travaux Publics was founded by and was directed by Carnot and Monge. It was renamed the École Polytechnique in 1795 and de Prony was certainly one of the main lectures by this time. He is listed among the first teachers at the university as:-

Prony, lecturer in analysis, director of the Cadastre, member of the Institute. Annual salary6,000francs. Accommodation within the school...

De Prony's lectures given at the École Polytechnique were published, including his lectures on hydraulics.

In 1798 de Prony refused Napoleon's request that he join his army of invasion to Egypt. Fourier, Monge and Malus had agreed to be part of the expeditionary force and Napoleon was angry that de Prony would not come. It did mean that de Prony was to fail to receive the honours he deserved from Napoleon but de Prony's wife was a close friend of Joséphine and this probably saved de Prony from anything worse.

In 1798 de Prony achieved his ambition of being appointed director of the École des Ponts et Chaussés. His desire for this post was almost certainly a main reason for his refusing to join Napoleon. As director he began producing a number of important texts on mathematical physics. He became a member of the Bureau de Longitude and, in 1810 and 1811, he produced two further major texts from his lectures at the École Polytechnique, namely *Leçons de Mécanique Analytique* and *Sommaire des Leçons du Cours de Mécanique.*

After Napoleon was defeated the reorganisation in France included a reorganisation of the École Polytechnique which was closed during 1816. De Prony lost his position as professor there and was not part of the reorganisation committee. However, as soon as the school reopened, de Prony was asked to be an examiner so he continued his connection yet only had to work one month per year.

One of de Prony's most important scientific inventions was the 'de Prony brake' which he invented in 1821 to measure the performance of machines and engines. It was based on ideas of Hachette and was a considerable improvement on a method which Pierre Girard had used two years earlier.

The last part of de Prony's career was more involved with education rather than administration. One battle he fought, without success, was against Cauchy's more towards pure mathematics and away from the more applied mathematics which de Prony firmly believed in. In [2] Margaret Bradley writes:-

... there had long been an increasing demand for the reform of the École des Ponts et Chaussés and his lack of attention to this attracted severe criticism. He was now showing even less interest in the day to day running of the school, in favour of science. He was disillusioned by the failure of his attempts to reform mathematics teaching at the École Polytechnique, where he had made energetic and determined efforts to combat the emphasis on theory of A-L Cauchy ... Prony seems to have lost heart for the continuing struggle and to have been less conscientious with regard to his duties as examiner.

**Article by:** *J J O'Connor* and *E F Robertson*

**April 1997**

[http://www-history.mcs.st-andrews.ac.uk/Biographies/De_Prony.html]