This is one of highest grade tourbillons Girard-Perregaux ever made. It features the famous gold bridges and Pellaton cage while others are based on Grether; it features spring detent escapement; and finally it employs a Guillaume balance. Only one other similar tourbillon regulator is known; this is No. 284409 illustrated in ?Le Tourbillon? by Reinhard Meis, Paris, 1990, p. 133. The cuvette of the watch is inscribed : ?Chronomètre Tourbillon, Bulletin d?Observatoire de 1ère Classe ?Hors Concours? Membre du Jury Exposition Universelle Paris 1900?. There is no doubt that Girard-Perregaux paid special attention to the finishing of this watch. Their efforts were rewarded by the First Prize at the Observatory Contest. The carriage and the escapement: There is no doubt that the carriage construction was entrusted to Albert Pellaton-Favre (1834 -1914), at the time the most esteemed tourbillon maker in Switzerland. He fitted it with a spring detent escapement, his specialty for his best tourbillons, with a Guillaume balance and a special feature preventing overbanking, consisting of a perpendicular pin fastened to the hairspring, which stops the balance when the amplitude is too great. To arrange all this into an 18.7 mm. carriage required a master such as Pellaton. The ?Three Gold Bridge? design: On March 25, 1884 a patent (No. 14919) was filed in the USA patent office stating: ?Be it known that I, Girard Perregaux of Chaux-de- Fonds, Republic of Switzerland, have invented and produced a new and original Design for a Watch-Movement, of which the following is a full, clear and exact description: ?In a watch movement the design for a bridge, consisting a bridge having a central annular portion, spread-out ends and bar-like portions between the said annular portions and ends, as shown?? The design proved to be one of the most successful watch movement designs ever. It has been used by Girard-Perregaux up to now. Even today, their most prestigious watch, the 13??? wrist tourbillon uses the same design. Guillaume balance: Anibal (acier au nickel pour balanciers), an alloy invented by Dr. Charles Edouard Guillaume, exhibits unusual properties, both in terms of thermal expansion and in changes in elasticity. These properties are very different from those of two other famous alloys invented by Guillaume, Invar and Elinvar. At the end of the 1800?s, Guillaume attempted to eliminate the so-called Middle Temperature Error caused by the fact that the change of rate in a timekeeper with a steel-brass bimetallic balance is approximately a linear function of temperature, while the change of rate caused by change in elasticity of a balance spring is approx-imately a quadratic function. Thus, it equals zero at only two temperatures, causing secondary error. Countless attempts were made to eliminate Middle Temperature Error, usually by means of auxiliary compensation devices. In 1899, Guillaume noticed that steel with an addition of 44.4% nickel had a negative square coefficient of thermal expansion. This alloy, combined with brass in bimetallic lamina, makes its expansion close to quadratic. Balances with bimetallic rims made of anibal and brass are usually called Guillaume balances, or, as their inventor called them, inte-gral balances. When combined with special balance springs, they exhibit remarkable temperature stability, on occasion not excee-ding 1/50 second per day at 1oC. This watch was clearly meant to stand out, even among the select group of Girard-Perregaux tourbillons. We thank Mr. Willy Schweiser of Girard-Perregaux for the information he has kindly provided.