Ponts et Chaussées — de Prony

Logo for the Ecole des Ponts et Chaussées

Gaspard Clair François Marie Riche de Prony guided the engineering profession in France through the French Revolution and into the 19th century. King Louis XV created the engineering corps of Ponts et Chaussées (bridges and roads) in 1716 to address the critical transportation needs the French national government. Prony entered the corps in 1780 after graduating from the Ecole des Ponts et Chaussées, the school set up to train engineers for this service. While a student Prony was singled out to become a leader in the corps, and in 1783 Prony was appointed as assistant to the director of the corps, Jean-Rodolphe Perronet.

Prony gained universal trust and admiration through competent service over the next several decades, until his death in 1839, a period that spanned the French Revolution, the rise and fall of the First Empire under Napoleon Bonaparte, the period of the Restoration and the revolution of 1830. During this same period, the portfolio of the corps of Ponts et Chaussées expanded to include planning a national system of canals, a sustainable water supply for Paris, and assessing the prospects for building a national transportation system using the novel, new technology of railroads. Napoleon Bonaparte said that he made no major decision on infrastructure projects without first consulting Prony.

Ironically, one of the projects for which Prony is most admired fell short of achieving its desired result. This was the project to produce new tables of trigonometric functions. As part of its program to produce a rational system of weights and measures — the metric system — the French government decreed that the circle should be divided into 400 degrees instead of the usual 360. In the days before electronic calculators, engineers requiring precise values of the trigonometric functions for their calculations had to consult tabulated values of these functions. Every engineer’s work table held a well-thumbed reference book containing these trigonometric tables.

Prony’s task to create tables based on a 400-degree division of the circle required a Herculean effort. Not only did he have to direct the calculation of new values of the functions, a task that employed scores of men and women to calculate products and sums by hand, but he had to check and correct the typeset tables before publication. The calculations were completed, and the results are recorded in seventeen exquisite folio volumes shelved in a storeroom of the Paris Observatory.

Ultimately, the government’s attempt to reform the 360-degree circle proved impractical, and Prony’s new tables were never published. However, Prony’s program of organizing the calculations inspired Charles Babbage to design a programmable mechanical computer, in the 1823. This was a forerunner to the invention of the electronic calculator that has made printed tables of trigonometric functions obsolete.

Gaspard Clair François Marie Riche de Prony is one of the 72 engineers and scientists named on the Eiffel Tower.

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A dead poet, a reformed anarchist, and an earnest engineer celebrate the 19th century revolution in science and technology that transformed Paris and conquered the world — a collection of essays on the theme.

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William Nuttle

William Nuttle

Navigating a changing environment — hydrologist, engineer, advocate for renewable energy, currently writing about the personal side of technological progress

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