Tuesday, 3 December 2013

Touching, But What's the Point?

by Jenny Bulstrode

Astrolabes and Stuff is proud to present its first ever guest post!  For this we must thank Jenny Bulstrode, a student in the Department of History and Philosophy of Science at Cambridge.

Quadrants, sextants, octants, none of them really meant much to me until last year. They’re all descendants of astrolabes. They’re all based around a circle with a scale marked on it. They’re all just fans of metal with bits on. The only way I could see the Navy using one was as an elaborate bottle opener.

No, I'm still not sure what it is
If anyone asks all you generally need to know is this versatile bit of kit was a tool for measuring the angle between two distant objects. This made it handy for everything from telling the time to surveying.  Last year I found out a bit on how they were used but rather more on how one particular eighteenth century instrument maker, John Bird, made them, and why we should care. It’s these last two points I want to share with you.

‘Now use your hands to warm the metal.. except you Bob,
your hands are too clammy’ (not Bird’s original words)
Metal expands when heated. Enough to get the lid off that gherkin jar by running it under the hot tap, more than enough to mess up the precision process of marking a scale on a circle. John Bird used the warmth of his hands to standardise the expansion of his tools and the circle he was working on. As if that doesn’t seem odd enough, he then wrote about it, in his instruction manual.

Temperature control was so important Bird wouldn't allow a fire to warm the room. Even candles were forbidden because they gave out too much heat. (Actually my mum has a similar policy for controlling utility bills). With little light to work by Bird detected points and traced guideline scratches using his fingertips. Magnifying lenses were widely used by eighteenth century instrument makers, including Bird, but what he really had faith in was his sensitive touch.   

Almost as though fingertips were designed
to move tiny puncture marks in metal…
Finally, when checking back revealed a point slightly out of position, Bird would ‘coax’ it into place. Extraordinary as it sounds, through fingertip pressure on the brass, he would shuffle the point along. His fingertips worked to control, detect, and even correct the scale marking process.

All very touching (see what I did there?) but what’s the point? Three things stand out to me. Firstly, John Bird was one of the last circle dividers working exclusively by hand. Towards the end of his life new, automated machines started taking up the job. That’s the old story of mass-production taking over, right? Except for nearly a century after his death these ‘automated’ machines used handmade circles, including Bird’s, to cut copies from. The action of the machine had to be guided, by hand. In fact even when the machines made errors, which they did, often, the points were still ‘coaxed’ into place. Bird might’ve died but his hands were still very active...

Not a bomb, just a highly controlled loaf
Secondly, handmade does not just mean surprisingly expensive bread. Of course it means those elements of delicacy, care, and exclusivity we associate with a walnut and fig pavé but it can also mean industrial quality control. When Bird used his hands to control the expansion of the metal he was setting an industrial standard. Granted it was one based on his body temperature but the principle for control was the same.

Finally, and above all: Bird achieved an unprecedented level of precision in his instruments. In fact, he divided circles so precisely he was commissioned to make the Mural Arc at Greenwich. This Arc set the standard for British Mean Time. I think if there is a point to be taken from all this it is that a huge idea like ‘Time’ can be traced back, defined even, to something as human as the pressure of a man’s fingertip.

While instrument makers were busy constructing time with their hands,
Time the avatar went to all the good parties

15 comments:

  1. Rob Barton Captain RN4 December 2013 at 17:01

    What a fascinating subject. I have used Sextants many times in the Royal Navy but did not know anything about the history of their development.

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    1. Hi there! Thanks for commenting, i think this is such an important point: historians need to be reminded that the navy continued using sextants until actually very recently. In fact these bits of kit are still big parts of the navy culture. The heroic story of John Harrison and his chronometer as the solution to the longitude problem is so exciting, particularly because Dava Sobel's book is such a good read, that we forget what really happened i.e. the sextant persisted as the principal method in the RN for a long while after the invention of Harrison's watch. It's a great wake up call to historians to look at what's there rather than just what makes a good story - thanks for commenting!

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    2. The blog manager, Seb Falk, has reminded me that Harrison's chronometric method relied on the use of sextants as well. I see this as an important reminder to historians like myself to go look at their notes before commenting on blog posts :) If i can redeem myself, when Bird was dividing there were two principal methods of establishing longitude: lunar distance and chronometric method. Both required a sextant in order to take local time with. The astronomer royal Nevil Maskelyne was Bird's and backed the lunar distance method. In a sense this put Bird in competition with Harrison's chronometric method. So while both methods required sextants, it could be said that only one made Bird a necessity. There is a sort of version of the story where Harrison is seen to have resolved the problem of longitude with his timepiece and it worth remembering the history of longitude and of technology generally just isn't that simple. (i look forward to the next set of corrections!)

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  2. Thanks! It's really a fascinating subject. Of course the principle underlying sextants hasn't changed for centuries, but those centuries saw a huge range of attempts develop better ways of measuring angles of elevation, while standing on a moving deck, without hurting your eyes. The cross-staff, back-staff, mariner's astrolabe, sextant and so on are all different versions of the same thing - if you're interested in the history of technology, it's fascinating. Jenny's telling a part of the story I know very little about - how instrument makers dealt with the problem of dividing a circle precisely. Thanks Jenny!

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  3. It is fascinating that that the pressure of a finger-tip could move a mark on brass. Do we have any idea how far he was able to move the marks. Is it microns? It is equally fascinating that the finger-tip can detect errors as small as this. As doctors we check the integrity of the nerve supply to the ends of the body by testing two-point discrimination, but we only check down to one millimetre or so. This must have been an order of magnitude more accurate. Perhaps he could get more accuracy by stroking his finger across the marks, and detecting the change in frequency rather than measuring the actual distance. This would then have worked almost like a Vernier scale.

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    1. Hi there! What an interesting question, thank you! I’m sorry for the slow reply. I think just as you say Bird was detecting what he refers to as ‘sensible inequality.’ This meant the tangible difference in position between points marked and those made in the process of checking back. Given this I imagine it would be a question of sensing two points where only one should be felt. I’m afraid I don’t know how far it was possible to move the points. Although I imagine it could be calculated from Bird’s notes on error and correction, Bird’s notes and instructions are not very intelligible (another story in itself). Possibly this calls for a keen re-enactor and some metal work skills! What I can say with confidence is that through his idiosyncratic process of coaxing and temperature control Bird was able to achieve the unprecedented half-degree of accuracy stipulated by The Board of Longitude. You can read more about The Board at http://blogs.rmg.co.uk/longitude. In short they commissioned Bird to construct new and renovate old instruments to half a degree’s accuracy corresponding to 30 geographical miles. – Thanks for the great question!

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    2. I am not sure how big Bird's sextant was, but a modern sextant has a radius of around 30cm, which gives (by my rough calculations) a circumference of 180cm. If that is divided into 360 degrees, then that is 5mm. per degree. However the mirror draws the horizon down two degrees for every one degree it moves (it bisects the angle), so each degree of altitude should be reflected by 2.5mm (2500 microns) of movement on the scale. A minute is one sixtieth of that and half a minute 1/120. So we are talking twenty microns as the space between each mark if they are half a minute apart. Obviously, if his sextant was twice as big then the distance would be forty microns etc.

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  4. I was recently on a merchant ship and discussing with a senior officer who said they still train using sextants (and enjoy the challenge) just in case everything stops working. he didn't say that they had to make his own, though, and it is great to understand some of the history behind it, so thanks.

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  5. Fascinating account of the handmade precision needed for accurate navigation. Ties in nicely with the solving of the longitude problem by creating accurate timepieces.
    Good work Jenny

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    1. Hi there, thanks for reading! The link you make to John Harrison’s timepiece is a really interesting one. As I’m sure you already know Bird and Harrison were both under commission by The Board of Longitude, a bureaucratic award giving body set up to pursue the problem of longitude. One interesting aspect of the story is that The Board treated Bird much more amicably than they did Harrison. I can thoroughly recommend an essay by historian Jim Bennett titled ‘The Travels and Trials of Mr Harrison’s Timekeeper’ in the collected works ‘Instruments, travel and science.’ In the essay Bennett describes Harrison’s difficulty in meeting The Board’s very particular idea of ‘discovery.’ I guess red-tape never changes. One suggestion among longitude historians is that Bird was commissioned to publish his odd method as a way of showing Harrison what he needed to do if he ever wanted the ‘discovery’ reward. It seems for The Board part of ‘discovery’ was to master and then reveal craft secrets. Thanks for reading and commenting!

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  6. For two point discrimination see
    Johnson, KO; Phillips, JR (1981 Dec). "Tactile spatial resolution. I. Two-point discrimination, gap detection, grating resolution, and letter recognition.". Journal of neurophysiology 46 (6): 1177–92. PMID 7320742.

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  7. Dear Jenny, I was fascinated by your article. I am intrigued by navigation, engineering, metals and naval history, and your work brought all those elements together in one work. I have a question; my sextant has a silver scale, as does my Gunter's Quadrant. I have always assumed the silver was used not for ostentation, but that it was a suitable metal in a maritime environment and was harder that brass. I wonder if silver was in use in Bird's day, and if it was, could it be 'coaxed' in the same way as brass? Its all a bit Yuri Geller.......
    Of course the other fascinating thing talking about precision clock makers ( if you can use such a crude word as 'precision' to describe someone of Harrison's ability! ) is that they were doing the same thing, ie, having to very accurately divide circles.

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    1. Silver has the same co-efficient of expansion as Brass so it cannot have been used to reduce temperature errors. It cannot have been to avoid corrosion as both metals are attacked by sea-water. I just wonder if a scale etched on silver is more easily seen in poor light than one on brass. Certainly the modern sextants all have a scale etched onto a silver surface (presumably stainless steel), though the new Plath sextants have white etched on a black background. I am sure that this is something to do with the human eye finding it easier to determine the exact edge of a line when moving from black to white rather than white to black, but I can't find the reference. For example a star is more visible on a black background than a black speck of dust would be on a white background.

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    2. Hi there! Thanks so much for reading and commenting, and what a wonderful collection! As I understand it silver was good for working with, particularly for processes such as graduation. Just as Sea Bee says it made for a good scale because it was clear to read from. It polishes up well, and apart from looking good that also makes it good to look with. On the downside silver would blacken with use and was heavy, as well as obviously expensive. Towards the end of the nineteenth century aluminium alloys were increasingly used. However, it is interesting to note that these alloys widely retained a silver component. For example, using aluminium: silver ratio in the proportions 95:5 was a recommended composition. This seems to have been to maintain that rigidity and workability, even some of the polish, at the cost of weight. Of course the scale was one of the most valuable components of many instruments because of the level of skill its manufacture represented. This legitimised using the most expensive materials on this particular component. I have always been interested in the materials used by instrument makers – for example Bird specifies using ‘white fir’ for the limbs of his beam compass, but I’m not sure why. It’s a tricky subject to research but it’s one I’d love to find out more on.

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    3. I also love the second part to your comment. The point you make about precision in relation to someone with the really unique genius of Harrison is a fantastic one. When we think about it, precision is whatever the group of people we’ve decided are the experts agree it is. Bird’s Mural Arc is an example of a consensus that sets a standard to work against. When someone like Harrison comes along and raises the bar, our ideas of precision change again. In fact precision has different meanings in each of military, naval and industrial contexts.. it even has religious connotations! Further to this you make the great point that clockmakers also have to divide circles. William Ludlam rewrote Bird’s method just a few years later to make it more intelligible to gentlemen of the Royal Society. He also added in a bit of heroic biography in Bird’s background that might or might not be fiction. Ludlam has Bird rising from humble origins to “accidentally” discover “the art” of “dividing and engraving” because he was “offended” by a clockmaker’s “coarse and irregular divisions.” Ludlam even says Bird “amused himself” correcting them before becoming an instrument-maker. This sounds like some elaborate CV gilding, but it does demonstrate how closely related Bird’s and Harrison’s skill sets were and how tense the competition was between the two of them. Thanks for the fantastic contributions!

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