This watch \vas described by Ferdinand Berthoud himself, in his Supplément au Traité des Montres à Longitudes, 1807, pp. 29-31. It vvas also described in : Ferdinand Berthoud- 1727 - 1807, Horloger Mécanicien du Roi et de la Marine, published by the Musée International d'Horlogerie de la Chaux-de-fonds, in the catalogue of the Exhibition, pp. 224-225. This watch was made by Jean Martin in 1806 following the model of No. 73, but at a reduced scale. Berthoud had originally intended to use watch No.72 as his model but eventually settled on No.73 which has centre-seconds with eccentric hours and minutes. In No. 3, the wheels have the same tooth count and even the same diameter, but the watch was made smaller by removing the rollers, the compensation mechanism and the stop pallets of the balance. Thus the dial plate became smaller than in watch No. 73. The balance beats four to the second with arcs of 260e, and the pivot holes are in ruby. It has temperature compensation by four bimetallic weights for which the strips are formel in arcs of a circle. and carries four small weights for adjustment in the different positions. The whole train is jewelled. Berthoud used this watch as a reason to offer a ' New explanation of the principles and methods on which the exactness of astronomical watches is based, with some conclusions concerning their use'. He notes that it is very well made and fulfils all the conditions necessary to satisfy connoisseurs. He specifies however that when used to determine the longitudes it is preferable to use the watch always in its horizontal position, like the watch No. 73. For this reason, instead of the ruby end-stone of No. 73, he used one of diamond and made a suspension so that it could always be horizontal. IIe notes that these changes worked perfectly as in the horizontal position the watch had a constant rate. With this arrangement, watch No. 3 could be worn vertically as a pocket watch, or placed horizontally to become a small longitude timekeeper.