Ferdinand, chronometer-maker to the French admiralty, member of the Institute of France and of the Legion of Honour, was born near Neufchâtel in 1727. His father was an architect, and the son was intended to be bred to the church; but, having shown a taste for clockwork, an experienced workman was engaged to instruct him in its principles, and young Berthoud was afterwards sent to Paris to improve himself in the knowledge and practice of the art. He settled in Paris in 1745, and applied himself to the making of chronometers, an art which was then in its infancy. A chronometer is an accurately-made watch, whose chief peculiarity consists in a piece of mechanism intended to render the number of vibrations of the balance equal in equal times, at all the degrees of temperature to which the instrument is exposed; and the chronometer being a portable instrument, which can be used on ship-board, is by this mechanism made to move at a constant rate—say at the rate of mean solar time—so that it shows what hour it is at the meridian of Greenwich, if the chronometer, at the commencement of the voyage, was set to Greenwich time; whilst the observation of the height of the sun or of a star gives the hour, angle, and the hour at the place where the ship is; and the difference between these two times is the longitude of the ship. Fleurieu and Borda, by order of the French government, made a voyage from La Rochelle to the West Indies and Newfoundland for the purpose of trying the chronometers of Berthoud, and found that they gave the longitude with only a quarter of a degree of error, after a cruise of six weeks. Satisfactory results were also obtained in the expedition of Verdun, Borda, and Pinard, which was appointed to try his chronometers and those of Le Roy. An account of this expedition is published.

Sully, an English watch-maker established in Paris, was the first who in that city attempted the construction of chronometers for finding the longitude; and this he did in 1724. In 1736 the chronometers of the English artist Harrison were tried at sea. In France there were no chronometer-makers of note, from the first attempts of Sully, till Pierre le Roy and Ferdinand Berthoud, between whom there was some discussion about the priority of their discoveries and improvements. Ferdinand Berthoud's chronometers were long the most esteemed of any in France. Louis Berthoud, his nephew and successor, improved upon the machines of his uncle, and made them generally of a smaller size, so as to become more portable. Many further improvements have been made by the English chronometer-makers.

Berthoud was regular in his habits, and retained the use of his faculties to the last. He died of hydrothorax, at his country house, in the Valley of Montmorency, in 1807, at the age of eighty. The principal of his published works are *Essai sur l'Horlogerie*, 1786, 2 vols. 4to; two Tracts on Chronometers, 1773; *De la Mesure du Temps*, 1787, 4to; *Les Longitudes par la Mesure du Temps*, 1775, 4to; a Tract on Chronometers, 1782, 4to; *Histoire de la Mesure du Temps par les Horloges*, 1802, 2 vols. 4to; *L'Art de conduire et de regler les Pendules et les Montres*, 1760, 12mo. This tract contains directions suited to general readers for regulating clocks and watches; it has gone through several editions.