Pierre Le Roy (; 1717–1785) was a French

clockmaker A clockmaker is an artisan who makes and/or repairs clocks. Since almost all clocks are now factory-made, most modern clockmakers only repair clocks. Modern clockmakers may be employed by jewellers, antique shops, and places devoted strictly t ...

. He was the inventor of the

detent escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to t ...

, the temperature-compensated balance and the isochronous balance spring. His developments are considered as the foundation of the modern precision

clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...

. Le Roy was born in

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...

, eldest son of Julien Le Roy, a clockmaker to

Louis XV Louis XV (15 February 1710 – 10 May 1774), known as Louis the Beloved (), was King of France from 1 September 1715 until his death in 1774. He succeeded his great-grandfather Louis XIV at the age of five. Until he reached maturity (then defi ...

who had worked with Henry Sully,''Encyclopedia of time'' Samuel L. Macey p.348
/ref> in which place Pierre Le Roy succeeded his father. He had three brothers:

Jean-Baptiste Le Roy Jean-Baptiste Le Roy (; 15 August 1720, in Paris – 20 January 1800, in Paris) was an 18th-century French physicist and one of the major contributors to the ''Encyclopédie'' by Diderot and d’Alembert for technology. The son of 18th-century P ...

(1720-1800), a physicist; Julien-David Le Roy (1724–1803), an architect; and Charles Le Roy (1726–1779), a physician and encyclopédiste.

Invention of the detent escapement

Description of the Le Roy detent escapement mechanism 14 August 1748

In 1748, he invented a pivoted detent type of escapement,''Britten's Watch & Clock Makers' Handbook Dictionary & Guide Fifteenth Edition'' p.12

/ref> or detached escapement, which makes him the inventor of the Escapement#Detent escapement, detent escapement: "The invention of the detached escapement belongs to P. Le Roy".''A Journal of natural philosophy, chemistry and the arts'' p.159
/ref> This should not be confused with the detached ‘lever’ escapement which was invented by Thomas Mudge circa 1755. He was distinguished principally in his mastery and improvement of the

and

chronograph A chronograph is a specific type of watch that is used as a stopwatch combined with a display watch. A basic chronograph has hour and minute hands on the main dial to tell the time, a small seconds hand to tell that the watch is running, and ...

, above all of the

marine chronometer A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time (GMT), and the time at t ...

, in which he carried forward the pioneering work of

John Harrison John Harrison ( – 24 March 1776) was an English carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the History of longitude, problem of how to calculate longitude while at sea. Harrison's sol ...

. He took a different approach from that of Harrison, believing that the way to achieve seaworthiness was to detach the escapement from the balance.''Biographical dictionary of the history of technology'' Lance Day, Ian McNeil p.42

/ref> He also differed from Harrison regarding his temperature compensation method, which used the variation of the rotation radius of the balance by modifying the diameter of the balance through bi-metallic components, a method which would become a standard in chronometers. His technique for temperature compensation was highly efficient in that it worked without changing the length of the spiral balance spring, which he had discovered to be

isochronous A sequence of events is isochronous if the events occur regularly, or at equal time intervals. The term ''isochronous'' is used in several technical contexts, but usually refers to the primary subject maintaining a constant period or interval ( ...

only at a precise given length (i.e. when frequency is independent of amplitude, so that a

mechanical clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, and the ye ...

watch A watch is a timepiece carried or worn by a person. It is designed to maintain a consistent movement despite the motions caused by the person's activities. A wristwatch is worn around the wrist, attached by a watch strap or another type of ...

runs at the same rate regardless of changes in its drive force, so it keeps correct time as the

mainspring A mainspring is a spiral torsion spring of metal ribbon—commonly spring steel—used as a power source in mechanical watches, some clocks, and other clockwork mechanisms. ''Winding'' the timepiece, by turning a knob or key, stores energy in ...

unwinds).

Development of the modern marine chronometer

After having designed plans in 1754, he constructed his first chronometers by 1756, and accomplished his masterpiece in 1766. This remarkable chronometer incorporated a detached escapement, a temperature-compensated balance and an

balance spring, innovations which would be adopted in subsequent chronometers. Harrison demonstrated a reliable chronometer at sea, but these developments by Le Roy are considered by

Rupert Gould Rupert Thomas Gould (16 November 1890 – 5 October 1948) was a lieutenant-commander in the British Royal Navy noted for his contributions to horology (the science and study of timekeeping devices). He was also an author and radio personality. ...

to be the foundation of the modern chronometer. Pierre Le Roy's chronometer had a performance equivalent to that of the Harrison H4 chronometer. In 1769, after his chronometre underwent testing aboard the corvette ''Aurore'', he was awarded the double prize offered by the

Académie française An academy (Attic Greek: Ἀκαδήμεια; Koine Greek Ἀκαδημία) is an institution of tertiary education. The name traces back to Plato's school of philosophy, founded approximately 386 BC at Akademia, a sanctuary of Athena, the go ...

for the best method of measuring time at sea. He succeeded in giving his instruments the greatest possible regularity by the discovery of the isochronous spiral spring, in which he was in competition with

Ferdinand Berthoud Ferdinand Berthoud (; born 18 March 1727, in Couvet, Plancemont-sur-Couvet, Principality of Neuchâtel; died 20 June 1807, in Groslay, Val d'Oise), was a scientist and watchmaker. He became master watchmaker in Paris in 1753. Berthoud, who held ...

, but which he published first. He was the author of several valuable publications on the art and science of clock-making and chronography, among them the ''Étrennes chronométriques'' of 1760. He also became ''Horloger du Roi'' in 1760. The work of Le Roy was not fully recognized in France however, and his contemporary

became more famous, obtaining the prestigious title of ''Horloger de Marine'', which left Le Roy disillusioned and led him to retire. He died in Vitry in 1785.

References

Sources and external links

hautehorlogerie.org: Longitude at Sea

Bibliography

* Dorange, Auguste Jean, 1880: ''Notice sur Julien Le Roy, horloger'' (p. 3). Tours: Rouillé-Ladevèze {{DEFAULTSORT:Le Roy, Pierre 18th-century French inventors French clockmakers Members of the French Academy of Sciences French scientific instrument makers Scientists from Paris 1717 births 1785 deaths Le Roy family