Examining the watch, one has the impression of going into a superbly finished and marvelous small machine which is employs a number of ideas invented at the time the watch was made, for improving timekeeping: Motor power. The reverse fusee arrangement was first employed by Mudge to decrease wear on the center wheel bushing. Berthoud employed it in this watch with a special safety device allowing the balance to be locked when the mainspring power becomes too weak, a device very useful when transporting the piece. The same device, with the help of an additional lever, is utilized for stopping the watch at will, to be able to set it, for instance, exactly at noon. The escapement. After the invention of the dead-beat escapement by Graham, it became clear that the quality of a timekeeper depends to a large extend on the stability and efficiency of the escapement. In the fourth quarter of the eighteenth century it appeared that a chronometer escapement was the most suitable for that purpose. During the period much experimentation was done. By 1771, Berthoud had already invented a pivoted detent escapement with an impulse directly on the balance rim. It is interesting to point out that Arnold, did not patent who his considerably improved version, a clear indication that he considered it a modification of Berthoud?s. Berthoud?s detent, employed in this watch, looks like a pallet fork (similar to Robin?s) but the pallets are just for locking. Both locking and unlocking are done as in the lever escapement, although the impulse is given as in a chronometer ? unidirectional, directly on the impulse roller mounted on the balance staff. The detent, which has circular pallets to avoid recoil, does not have draw, which potentially could cause instability due to unstable banking. Berthoud solved this problem by placing a triangular notch on the free end of the detent which slides on a thin spring, always assuring staying at the banking. The balance. Berthoud decided to employ a plain brass balance, probably due to favorable experience with his famous No. 8, which was performing better than Le Roy?s. He employed large friction rollers to reduce friction as much as possible (jewelling was very costly and known primarily to the English at the time) and installed ruby and diamond flat endstones. He knew, however, that this arrangement would cause small positional errors and advised that when used to determine longitude, it was preferable to always use the watch in a horizontal position. For this same reason he decided to employ a balance amplitude indicator ? the watch performs best when the amplitude is constant, which in this watch is when it is in a horizontal position. The compensation. Since the changes of elasticity of the balance spring with respect to the temperature can amount to several minutes per day, Berthoud had to employ temperature compensation device. He was the first in France, and probably on the Continent, to employ temperature compensation in a watch. This was in 1764, in a form of a gridiron and lever system. For our watch he chose the so-called ?sugar tongs? mostly found on Thomas Earnshaw watches, the earliest in about 1795. Berthoud made several visits to England, the most memorable in 1763 to study the secrets of Harrison?s H.4. During that period, experimentation with bimetallic compensation devices flourished, they were applied both directly to the balance, and to the balance spring. The superbly finished one found on this watch consists of two parallel bimetallic strips with two pins restricting the movement of the balance spring, which close in heat and open in cold. One of the pins is micrometrically adjustable. The strips slide in a steel bracket, which controls their acting length, thus allowing the proper calibration. In addition, the entire contrivance is placed on a sliding rack for easy mean time adjustment, with a small silver sector scale calibrated probably in seconds. This watch was made by Berthoud?s pupil Jean Martin in 1804 following the model of No. 73. Its principles are described by Ferdinand Berthoud himself, in his ?Supplément au Traîté des Montres à Longitudes?, 1807. It is interesting that two years later Martin made the Montre Astronomique No. 3 for Berthoud, which Berthoud used as an example of his ?New explanation of the principles and methods on which the exactness of astronomical watches are based, with some conclusions concerning their use?. He noted that it was superbly made and fulfilled all the conditions necessary to satisfy connoisseurs (The Sandberg Watch Collection, Antiquorum, March 31, 2001, No. 142). Pierre Lamet, Paris, 63 Quai de l?Horloge. A casemaker, his mark was registered in 1803 and 1804. In 1819 the mark was registered as belonging to Girardin, presumably by that time Lamet sold the business.