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A Longitude Timekeeper
Figure 1. The recently discovered longitude timekeeper by
Isaac Thuret, signed Thuret A Paris, 1675-1680.
The recent discovery of an extremely interesting clock signed Thuret A Paris reveals that this clockmaker was much more closely involved in the development of a clock to be used for finding longitudes at sea than has generally been assumed. Isaac Thuret was the clockmaker who made for Christiaan Huygens the first watch with a spiral spring regulating the movements of the balance wheel in 1675.
The conflict about his contribution to the invention is usually seen as to have ended their cooperation. However, a closer study of Huygens' correspondence strongly suggests that this was not the case. This longitude timekeeper, provided with the pirouette as published by Huygens but abandoned by the makers of pocket watches, supports this view. Its close correspondence in design with the later marine chronometers (no remontoire neither fusee) confirms that Huygens and Thuret, apart from the pirouette, were on the right track. However, it would take a century before the reliability and accuracy of the mechanic timekeeper reached the level required for measuring longitudes.
The recent discovery of an extremely interesting clock signed Thuret A Paris reveals that this clockmaker was much more closely involved in the development of a clock to be used for the measurement of longitudes at sea than has generally been assumed. Isaac Thuret (about 1630-1706) was the most distinguished Paris clockmaker of the second half of the 17th century, Horloger Ordinaire du Roi and clockmaker of the Académie Royale des Sciences and its observatory. He made the first watch with a spiral spring attached to the balance wheel invented by Christiaan Huygens in 1675, to be considered as the most important step forward in the development of accurate mechanic watches.
However, it was not known that Thuret ever had used this invention to construct a timekeeper specially designed for finding longitude at sea, a true precursor of the marine chronometers developed in the 18th century. The clock illustrated in Figure 1 shows that he actually made such a clock. Restudying Christaan Huygens' Œuvres Complètes in the light of this discovery showed that his invention of the spiral balance spring was much more aimed at the problem of measuring longitude than is usually realised. The significance of Thuret's clock can only be understood fully against the background of his close relation and cooperation with Huygens. Therefore, I will deal with this relation prior to giving a description of the clock.
2. The early contacts between Christiaan Huygens and Isaac Thuret
The earliest reference to Thuret in the preserved correspondence of Huygens is in 1662. Apparently, Christiaan Huygens had not met Thuret during his second visit to Paris in 1660-61. Shortly after his return in May, 1661, his father Constantijn Huygens 1 was sent to Paris to negotiate the restitution of the principality of Orange to the ten-year old Prince Willem, the later stadholder of The Netherlands and King William III of England, Scotland, and Ireland with his wife Queen Mary. A letter from Christiaan in The Hague to his brother Lodewijk, who accompanied his father to Paris, strongly suggests that Constantijn had brought with him a pendulum clock as a present to King Louis XIV.2 We know that the successful application of the pendulum as a clock regulator by Huygens in 1656-7 had been highly enthousiastically received in Paris and that several clocks made in The Hague had already found their way to that city.3 So, it is perfectly understandable that the proud father presented one to the French king.
In these contacts with the court, Constantijn Huygens may have met Isaac Thuret. This clockmaker was born in Senli and, as many of his colleagues, a Protestant from origin. As Augarde has recently explained, Thuret had close relations with eminent court artists; his daughter Suzanne married Charles-François de Sylvestre, Maître de dessin des Enfants de France, and his son Jacques, who succeeded him as Horloger du Roi, married Louise, daughter of Jean Bérain, Dessinateur des Menus-Plaisirs du Roi.4 Constantijn Huygens saw clocks made by Isaac Thuret and was greatly impressed. We know this from another letter to Lodewijk, dated 12 April 1662.5 In this letter Christaan asks his brother to describe how the clocks of Thuret are made. His father had written to prefer Thuret's clocks strongly above his own; if Christiaan could know the 'form', it could be used to instruct the clockmakers in The Hague. Unfortunately, Lodewijk's reply is not preserved, so we do not know in which respects Thuret's clocks were considered to be better than the clocks made by Salomon Coster and his successors (Visbach, Oosterwijck, Pascal, van Ceulen).6
The next references to Thuret are in letters sent by Christiaan during his fourth visit to Paris in 1663-4. In a letter dated 18 January 1664, he complains to Lodewijk about a clock by the Hague clockmaker Claude Pascal ordered from Paris through Christiaan.7 The owner is very dissatisfied because the clock goes so badly that Christiaan is inclined to return it to the maker. We learn from subsequent letters that the clock has been taken back by Christiaan and brought to Isaac Thuret for repair.8 Just after having sent the last letter, three more clocks made by Pascal arrived. Immediately, Christiaan informs Lodewijk about how badly they were damaged during the journey.9 He is very angry about this and assures his brother that he will never mediate again in ordering Dutch pendulum clocks. Again, Thuret is mentioned as the clockmaker in charge of the repairs.10
From these letters we know that the first personal contacts between Christiaan Huygens and Isaac Thuret resulted in a close cooperation in the care of early pendulum clocks sent from The Hague. Although we have no direct letters to Thuret, he is mentioned again and again in Huygens' correspondence in the period between the latter's return to Holland in June, 1664, and his move to Paris two years later. Pierre Petit11 gives an obscure description of some construction by Thuret.12 A later letter reveals that Thuret had designed a remontoire13 to increase the accuracy of a timekeeper, which illustrates the inventiveness of this maker.14 Huygens played down this achievement, obviously because he himself was involved in a similar endeavour.
Huygens had developed his remontoire for a more precise version of his pendulum clock to be used for measuring longitude at sea. This had been one of his main aims from the beginning.15 Thanks to the mediation of his father, again in Paris, Christiaan Huygens obtained early 1665 from Louis XIV a French patent for his new remontoire clock.16 According to a notarial act drawn up in The Hague, Huygens authorized Jean Chapelain17 to dispose of the patent as his representative. Chapelain writes to him on 13 March 1665, that
......that excellent clockmaker Monsieur Thuret, of whom you yourself have said me much good, visited me yesterday and asked me to offer you his service for the construction of clocks to be used on ships and for their sale and distribution.18
Within two weeks, Christiaan Huygens agreed.19 We do not need to follow the negotiations about the financial arrangement, and may confine ourselves to Chapelain's view that Huygens will be served by Thuret 'incomparably better and with more capacity [. . .] and intelligence than by any other'.20
In the next few months, two clocks with a remontoire ('after your new invention') made in The Hague arrived safely in Paris.21 In agreement with Huygens' instruction, Thuret was called upon to install the clocks. As Chapelain wrote to Huygens, Thuret had noticed that Huygens' 'secret' was quite similar to his own but that the little chains in your construction testify of a less simple skill than in his own and more subjected to arrest as has happened with the one of Monsieur Carcavi and the one of Monsieur de Montmor.22
In June 1666, Christaan Huygens moved to Paris to become the most distinct founder member of the Académie Royale des Sciences.23 This move can explain why the name of Isaac Thuret is not mentioned in the correspondence of the next eight years. The only exception is in a letter by Christiaan to his brother Lodewijk, dated 6 January 1668, in which he mentions to have heard from Thuret that the Dutch admiral Michiel de Ruyter had a pendulum clock with him on his expedition to Guinee and Terra Nova in 1665.24
This does not mean, however, that Huygens and Thuret did not meet each other frequently during these years. We have, on the contrary, clear evidence that they did indeed. Huygens had brought with him from The Hague two clocks with remontoires to be tested at sea.25 These remontoires appeared to cause great difficulties in practice, as Thuret had already noticed. Huygens was obliged to order new clocks without this feature, which where at sea in March, 1669, as he writes to Robert Moray in London.26 This letter also tells that, as a substitute of the remontoires, Huygens had perfected the clocks by other means. We may assume that Thuret was intensively involved in this development. We can conclude from another letter that the clocks were provided with one-second pendulums.27 From Richer's report of the observations during the journey to America in 1672-3, we know that these clocks, one marking seconds, the other half-seconds, were made by Thuret, who by his exactness and the delicacy of his works has surpassed up to now everyone involved in the production of watches and pendulum clocks.28
For this expedition, Huygens even had proposed a new pendulum construction less susceptible to ship movements but, due to his illness, this clock was not ready in time.29
At the same time, Huygens was equally strongly involved in improving clocks to be used in astronomy. In 1673 he published his major book Horologium Oscillatorium with a drawing of a clock evidently designed for astronomical purposes.30 It is clear that, again, Thuret had constructed this clock. The Museum Boerhaave in Leiden has a clock signed THURET A PARIS, with a one-second pendulum, almost exactly corresponding to the drawing.31 This is most probably Huygens' own clock brought from Paris when he returned. Two other clocks after the same design are known: one also signed by Thuret in a private collection in The Netherlands and one in the Ole Rømer Museum in Taastrup, Denmark, provided with the coat of arms of this astronomer.32 In his dedication of the Horologium Oscillatorium to Louis XIV, Huygens remarks that many clocks made after his invention are found in the king's private rooms in the palace as well as in the recently founded Observatory. Without doubt, these clocks were made by Thuret. His name is mentioned in account books from 1669 onwards for works made for the Académie Royale des Sciences and, three years later, also for the maintenance of all pendulum clocks of the observatory and the academy.33
Returning to the marine clocks, it is significant that Huygens discovered, additional to the very difficult problem of keeping a pendulum clock going at sea with the required accuracy, a quite unexpected other drawback of his construction. The America trip of 1672-3 revealed that the length of a one-second pendulum is not a universal measure but depends on latitude.34 Huygens had to face the fact that the pendulum as a timekeeper at sea had not only practical but also more fundamental disadvantages.
This knowledge may have prepared his mind to look in quite another direction for regulating clocks for finding longitude at sea. His interest in using a spring in combination with the traditional balance, resulting in the spiral spring universally applied after 1675, should be seen against this background.
3. Huygens and Thuret in the controversy of the balance-spring construction
The idea of applying a spring to regulate a clock was not new for Huygens. As early as 1665 Robert Moray had announced that Robert Hooke35 had lectured for the Royal Society about providing the balance of a clock with a spring rather than a pendulum in order to avoid the effects of movements both at sea and on land.36 According to Hooke, there were many different methods to apply the spring. In his reply, Huygens had mentioned that already during his visit to Paris in 1660, Artus Gouffier, duke of Roannes, had brought him to the clockmaker to whom the duke and Blaise Pascal had communicated such an invention.37 Apparently, Huygens had not been much impressed; he expected that the ship movements would introduce small irregularities in the movements of the balance wheel difficult to eliminate and that it was not known whether temperature changes would effect the vibrations. According to Huygens' preserved journal of his trip to Paris, the clockmaker visited by Huygens can be identified as (Giles) Martinot.38 In his next letter to Moray, Huygens pointed out again the disadvantages of a spring compared with a pendulum.39
Nevertheless, ten years later Huygens reconsidered the merits of the spring and, as we know, came to a quite different conclusion. It seems apparent that this change of mind was initiated by his theoretical demonstration of the isochronism of string vibrations in 1673-5.40 From the view that a spring, clamped at one side, vibrates in equal times independent of its amplitude41 to the spiral spring as a regulator of the oscillations of a clock balance is only a single, but brilliant, step. In his letters as well as from his notes we have a very elaborate, be it one-sided, report of this application that changed the mechanic watch forever. Let us follow his day-by-day report.42
The notes begin with the first sketch of Huygens' idea of how to provide the balance of a clock with a spiral spring in order to get a more accurate timekeeper, dated Sunday, 20 January 1675, see Figure 2. The next day, Huygens tries two times to find Thuret in order to have his idea converted into a model, but Thuret is not in his workshop. A third visit on the morning of 22 January is more successful and Huygens communicates his invention confidentially to Thuret who reacts very enthousiastically. Immediately, Thuret starts making a model according to Huygens' instruction and, without a lunch break, the model is finished at about three o'clock in the afternoon. We might expect that this model was a watch provisionally provided with a spiral spring. Huygens takes this model with him to his room.
However, Thuret does not stop after Huygens' departure, but tries out what may have been a slightly different, and improved, application of the invention on a watch of his own. He is so proud of the result that on the next day, 23 January, he calls Huygens away from a meeting to show the construction, asking to keep it secret. Subsequent versions are seen by Huygens on the next two days.
With the intention to get a patent, Huygens shows his model on 31 January to Colbert, the important minister to Louis XIV and promotor of the sciences. On the afternoon of the same day, he visits Thuret and tells him to have been to Colbert who, indeed, had promised a patent. Thuret expresses his hope to be considered as a participant in the invention, but Huygens refuses. He responds that Thuret will profit more from the invention than Huygens himself and that he alway will testify that Thuret had contributed so much skill to the execution of the invention. It is understandable that Thuret was not very happy with this answer. As Thuret was finishing a watch after the new invention to be presented to the king, he asks Huygens not to make haste in requesting a patent in which Huygens agrees.
The next day, Huygens hears that, according to some people, Thuret has told around to have contributed significantly to the invention. In view of this, he decides to hasten the patent procedure and to remonstrate Thuret about his pretentions. For various reasons, this meeting does not find place until 4 February.
The bomb bursts really on 8 February when Huygens discovers, as his own words read, 'that Thuret had shown my invention to M. Colbert eight days before me'. Without having told to Huygens, Thuret had gone to Colbert as early as January 23 with a watch in which the balance was provided with a spiral spring. The next day, 9 February, in the presence of some witnesses who knew Huygens' role in the invention, Thuret tries to deny the accusition but, in a subsequent interview, he has to admit. In the margin of this confession in his notes, Huygens added to have given the watch to Gaudron in order to get it finished.
As Huygens also tells in these notes, some mutual acqaintainces tried to hush up this conflict. Unfortunately, we do not know in which way Thuret improved the construction proposed by Huygens but, apparently, others valued the clockmaker's contribution much higher than Huygens did. They tried to persuade him to acknowledge Thuret's part in the development of the invention, but Huygens did not yield. Probably the most open and long discussion of the opponents took place on 25 February. During this conversation, Thuret told that the watch was nearly finished and that he would present it to Huygens to dispose of it. Thuret saw this watch as the result of their combined efforts in the invention but Huygens, although he expressed to be happy to have the watch, was not prepared to accept it in another way than by paying. This demonstrates that Huygens annexed the exclusive right for the invention, considering Thuret just as a very able craftsman who had materialized his idea but nothing more.43 In the end, and in spite of important advocates as Madame Colbert and Ch.H. d'Albert de Luynes, duke of Chevreuse and married to a daughter of Colbert, Thuret had to capitulate. He signed a letter, dated 10 September 1675, drafted by others, in which he plainly admitted to consider the spiral spring as the exclusive invention by Huygens, but not after having suggested that the displeasure of Huygens might have had its origin in the fact that Thuret had recently succesfully made some clocks provided with a pendulum attached to a straight spring instead of the usual thread.44
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