Conference 'Who needs scientific instruments?', Museum Boerhaave, Leiden, The Netherlands.
Date: October 19th - 22nd, 2005
Conference topic of the 22nd: Innovation:
An important aspect of the development of instruments is innovation. Propagation of instruments has an important part to play in the innovation process. Who brings the new instruments to wider notice, so that they will be used and modified? Users want specific qualities in their instruments. Can their demands be met by the instrument makers or do users make their own adaptations? And do users or manufacturers change the initial purposes of an instrument over time?
Papers in this session will deal with:
Author: N. de Hilster, independend scholar
© 2005, All rights Reserved. No part of this publication may be reproduced or used for other publications without the prior permission of the author.
The instrument mentioned is the spiegelboog, or mirror-staff in English. Invented in 1660 by the Dutch Joost van Breen3, the spiegelboog is regarded as the first navigational instrument using a silvered glass mirror for its observations. Perhaps it even was the first scientific instrument at all using a glass mirror for its functioning. Please note that I am mentioning mirrors made of silvered glass, not of metal. The instrument was invented 71 years before Hadley invented the double reflecting octant in 17314, still regarded by many people as the first reflecting navigational instrument. Between 1660 and 1731 two other instruments using glass mirrors were invented, but never came into production. These were a single reflecting instrument by Robert Hooke in 1666 and a double reflecting instrument by Sir Isaac Newton in 16995.
In those days the use of glass mirrors was not common practice in The Low Countries. Research on old patents learned that the first patent on producing high quality glass (crystal) mirrors in The Low Countries dates from 16656, 5 years after Van Breen put one on his spiegelboog, so a truly innovative step. Before that time high quality glass mirrors were mainly made in Italy and, closer by, in Germany and Flanders7.
Through time variations on the cross-staff were made like the back-staff, invented by John Davis at the end of the 16th century9. It allowed navigators to take observations with their backs turned towards the sun by casting a shadow on a horizon vane.
John Davis' described the back-staff in 1595 in his book called Seaman's Secrets. In it another staff is shown (second from the left) with a single cross on which two vanes are mounted10. Complemented with a horizon vane it can be found on the front page of Van Breen's Stiermans Gemack (second from the right)11. This is not yet a spiegelboog as the staff protrudes from the cross which is not possible with a spiegelboog12. So Van Breen probably took this instrument, modified the cross by adding an adjustment screw, added a mirror to the horizon vane and he had the spiegelboog.
Well, Joost van Breen thought all navigators did. The two principal period instruments used for celestial navigation were the cross-staff and the back-staff. Although both instruments allowed for backward observations, neither of them was capable of doing so with weak celestial bodies and only the back-staff was capable of sighting a weak sun. By using a mirror, the spiegelboog was not only capable of back sighting a weak sun at a hazy sky, but also performing back sights on the stars13. In addition to that the spiegelboog could be used without the mirror for taking back sights using a bright sun14 and for forward observations when the celestial body would be too low above the horizon to be measured backwards in which case the observer's head would simply block the sight on them15.
The reason that the spiegelboog was mainly used in the Zeeland Chamber of the VOC can be explained. Van Breen lived in Zeeland in the town of Middelburg19 and had contact with the highest deputies of the VOC, the Heren XVII. In 1665 he requested them to supply the ships on the route to Asia with his book and instrument. The instrument was examined by map maker and examiner of the Amsterdam Chamber of the VOC, Joan Blaeu, and in 1668 it was decided that the Chambers were allowed to use it, but still had to decide themselves whether or not to do so. In 1670 it was even strongly approved20.
The reason for this strong approval probably has its origin in a conflict between the Zeeland Chamber and the same Joan Blaeu over the bill for navigational charts that had risen to 21.135 guilders for the year 1668 (almost enough to buy two 85 feet ships21). The Zeeland Chamber searched for an alternative supplier and ended up with local chart makers Arent Roggeveen and Joost van Breen22. Shortly before 1670 Arent Roggeveen and Joost van Breen were appointed examiners of the mates of the Zeeland Chamber23. In 1670 the Heren XVII asked several experts whether the default list of equipment used on board of the VOC vessels needed an update. Among these experts where several captains and the examiners of the Chambers of Amsterdam and ... Zeeland24.
I rather rephrase it: 'Who needs scientific reconstructions?'
or even more specific: 'Who needs a reconstruction of the spiegelboog?'
Apart from the fact that I wanted one for my collection of navigational instruments, people involved in research on navigational instruments, like archaeologists and curators, would like to see one as the reconstruction can tell them how to recognise parts when found.
As already mentioned no spiegelboog survived, but we are not 100% sure of that. Over the years the world leading expert on cross-staffs and former curator of the Dutch Maritime Museum in Amsterdam, Willem Mörzer Bruyns, tried to track down as many as possible surviving cross-staffs all over the world. With his latest article on the cross-staff published in 200425 his count stopped at 124 staffs, but not all of those are complete. A lot of them do not have the original vanes or no vanes at all. Some of them are even just fragments of a staff. It is well possible that some of these incomplete staffs are in fact spiegelbogen, but as the staffs look quite identical it is hard to distinguish them. Making a reconstruction might solve this problem.
About five years ago I started building 17th and 18th century wooden navigational instruments. I obtained my first drawings of a back-staff through Peter Ifland, a collector of navigational instruments and author of several books on celestial navigation. The curators of the Dutch Maritime Museum in Amsterdam and the University Museum in Utrecht helped me out with filling in the details by allowing me to study several original instruments. The knowledge built up over the years making back-staffs and cross-staffs made me confident enough to start this reconstruction.
The knowledge of the instrument mainly comes from one source: the navigational book Stiermans Gemack written by Joost van Breen in 1662 in which he describes the use of the instrument in chapter 13. This chapter also contains the only detailed drawing of the instrument known so far.
The only other depiction of the instrument I know of can be found on a print in the book De Nieuwe Groote Ligtende Zee-Fakkel, dated 1734, and shows a mirrored version of the instrument (here at right).
Although the reconstruction looks like the sketch Van Breen made 350 years ago, it was this same sketch I did not rely on in the first place when I started my research on the instrument as the sketch looks quite odd with its strange perspective and slanted horizon.
So I examined his book and only used the sketch when it was referred to in the text. One of the first indications of size and scale was the remark that the instrument could not be used when the altitude of the celestial body would come below 10 to 12 degrees as my own head would block sight on it26. This single remark told me that the staff would not have a scale for backward observations lower than 10 degrees and that with the smallest vane setting I would see my own head in the mirror.
Measuring the height of my skull above my eyes I found approximately 110 millimetres. This is a direct indication for both the length of the staff and the distance of the smallest vane setting. In Van Breen's sketch I found 54 millimetres for the smallest vane setting, roughly half the size measured on my skull. Other measures in Van Breen's drawing confirmed this 1:2 scale. Even the perspective was not arbitrary, but came out at a 1:3 scale. So the drawing looks quite odd, but it is to scale.
In Zeeland two of the old local measures used are the 'Goesse Voet'27, named after the town of Goes which is close to Middelburg where Van Breen lived, and the 'Wynroeyers Voet'28. They both measure 289 millimetres and being a Wynroeyer29 (Wine Gauger, a civil servant who measures the contents of wine barrels in order to determine the amount of tax due) Van Breen must have been familiar with this measure. One Goesse Voet was divided in 12 duim, making a duim about 24 millimetres. Multiplied by 4.5 this gives 108 millimetres which is exactly twice the measure found for the smallest vane setting in Van Breen's sketch. In this way quite some measures in Van Breen's sketch can be found that could easily be made using this 24 millimetre duim, by dividing it in halves, quarters, eighths etc.
Final reason to believe that his sketch must have been to scale was his connection with Arent Jansz. Roggeveen, his associate map maker30. If anyone should have been able to, or explain how to, create a scaled drawing it must have been Roggeveen.
One part of the sketch however proved to be not an accurate depiction of what Van Breen made. In retrospective it is not surprising that this concerns the mirror vane. As Van Breen had the privilege for 15 years to be the only spiegelboog maker, he was probably not too keen in sharing his invention other than selling it, and therefor reluctant to show the mirror vane in detail in a book. But as one could purchase the instrument in order to make a copy a more plausible reason why he simplified his sketch of the mirror vane vane was an economical one: he wanted to explain the whole instrument using one sketch only, while the mirror vane has two different functions.
In the sketch a mirror vane can be seen with a clear reflection of the shadow vane and the sun. Also a rectangular hole is shown, through which the staff can be seen. Reading Van Breen's book it becomes clear that the hole is in the vane, not in the mirror, so what is shown in the sketch is the combined use of the mirror vane with and without the mirror. When the mirror is in place observations had to be taken as close to the left edge of the mirror as possible31, but when used without the mirror, so for bright sun observations using shadows, the observations had to be taken on the right side of the hole in the mirror vane32. These remarks by Van Breen clearly tells us that the mirror had no hole nor a clear part in it.
The mirror vane also contains the solution to an old problem: how to distinguish a spiegelboog from a cross-staff, even when all is left is the staff or even a part of it. The solution lies in the way the scales are read on both instruments. On a spiegelboog this is done on the side of the block on the mirror-vane33, while on a cross-staff this is done on the opposite side. For this the scales of the spiegelboog need to be shifted by about 25-30 millimetres towards the eye-end of the staff in order to get an accurate reading. But on the spiegelboog only three out of four scales are used in combination with the mirror-vane, the fourth one is used in combination with the shadow vane for forward observations34. For this the fourth scale does not have the offset necessary for the other three scales. As the scales can be calculated from the vane lengths using simple math, the opposite can be done as well. When calculating the vane-lengths from the scale intervals one can calculate not only the vane lengths, but also the offsets of the scales. A difference in offset for one of the scales of about 25-30 millimetres is a good indication that the staff belonged to a spiegelboog. You can check it yourself here.
Back in 1661 Van Breen tested the spiegelboog against a cross-staff and a back-staff on Fort Den Haeck in Zeeland35. On October 11th, 2005 3 skilled navigators and a survey specialist (Nico Duijn, Jan Jonker, Jaap Ypma and Ad Pieters) joined me on a similar exercise in IJmuiden.
This time we not only used the cross-staff and back-staff as reference, but also a sextant and a theodolite. Recording the accurate time of the observations made it possible to compare the measurements against the calculated altitude of the sun. From this exercise we now know that the spiegelboog performs almost as well as a cross-staff, but is more difficult to handle. When used without the mirror the spiegelboog performs just a bit better than a back-staff.
In comparison to the results of Van Breen's test back in 1661 our results are quite similar. Back in 1661 differences were found up to 7 minutes between two spiegelbogen and as well as between a spiegelboog and a cross-staff. In our test we found differences up to 11 minutes , but then we were all inexperienced in using these 17th century navigational instruments (Click on the graph for a larger image).
In addition to that we now know that Van Breen's depiction of the staff was quite accurate, which might help to identify any spiegelboog parts when found on archeological sites.
We also know how it is to use one, how it compares to contemporary instruments like the cross-staff and back-staff and that the accuracy given in Van Breen's book is realistic.
1 Answers.com. Instrument. Available from http://www.answers.com/topic/instrument-4 (last accessed November 2nd, 2005)
2 Davids, Dr. C.A., Zeewezen en Wetenschap. De wetenschap en de ontwikkeling van de navigatietechniek in Nederland tussen 1585 and 1815, Amsterdam/Dieren: 1986, p. 291
3 Notulen van de Staten-Generaal, September 25th, 1660, Nationaal Archief, inv. No. 112311, folio 432 left
4 Mörzer Bruyns, W.F.J., Schip recht door zee, Amsterdam: 2003, p. 28
5 Ifland, P. Taking the Stars. Celestial navigation from Argonauts to Astronauts, Malabar: 1998, p. 13-15
6 Doorman, G., Octrooien voor uitvindingen in de Nederlanden uit de 16e - 18e eeuw, 's Gravenhage: 1940, G487, p. 236
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8 Mörzer Bruyns, W.F.J., The Cross-staff, History and development of a Navigational Instrument, Zutphen: Vereeniging Nederlandsch Historisch Scheepvaart Museum, 1994, p. 14
9 Mörzer Bruyns, W.F.J., Schip recht door zee, Amsterdam: 2003, p. 8
10 Hastings Markham, A., The voyages and works of John Davis, London: The Hakluyt Society, 1880, p. 329
11 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, front page
12 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 9-10
13 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, title page
14 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 16
15 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 11
16 Davids, Dr. C.A., Zeewezen en Wetenschap. De wetenschap en de ontwikkeling van de navigatietechniek in Nederland tussen 1585 and 1815, Amsterdam/Dieren: 1986, p. 175
17 Notulen van de Staten-Generaal, September 25th, 1660, Nationaal Archief, inv. No. 112311, folio 432 left
18 Davids, Dr. C.A., Zeewezen en Wetenschap. De wetenschap en de ontwikkeling van de navigatietechniek in Nederland tussen 1585 and 1815, Amsterdam/Dieren: 1986, p. 175
19 Missive d.d. November 30st , 1661 written by the Staten van Zeeland to the Staten van Holland and West-Friesland
20 Davids, Dr. C.A., Zeewezen en Wetenschap. De wetenschap en de ontwikkeling van de navigatietechniek in Nederland tussen 1585 and 1815, Amsterdam/Dieren: 1986, p. 175
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23 Broeze, drs. F.J.A., dr. J.R. Bruijn and drs. F.S. Gaastra, In Maritieme geschiedenis der Nederlanden, Deel 3: Achttiende eeuw en eerste helft negentiende eeuw, van ca 1680 tot 1850-1870, Edited by dr. G.Asaert, drs. Ph. M. Bosscher, dr. J. R. Bruijn and dr. W.J. van Hoboken. Bussum: 1976-1978, p. 213
Davids, C.A., Zeewezen en Wetenschap. De wetenschap en de ontwikkeling van de navigatietechniek in Nederland tussen 1585 and 1815, Amsterdam/Dieren: 1986, p. 399-400
24 Davids, C.A., Zeewezen en Wetenschap. De wetenschap en de ontwikkeling van de navigatietechniek in Nederland tussen 1585 and 1815, Amsterdam/Dieren: 1986, p. 349-350
25 Mörzer Bruyns, W.F.J.. The Cross-staff, Ten Years Later. Bulletin of the Scientific Instrument Society, no. 80 (2004): 18-23
26 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 11
27 Pouls, H.C., De Landmeter van de Romeinse tot de Franse tijd, Alphen aan den Rijn: 1997, p.179
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29 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, title page
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31 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 11
32 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 16
33 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 14
34 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 11
35 Van Breen, J., Stiermans Gemack, 's Gravenhage: 1662, Chapter 13, p. 20
© 2005, All rights Reserved. No part of this publication may be reproduced or used for other publications without the prior permission of the author.