Split Seconds [Clock]

[Michael GilliganParticipant @michaelgilligan61133]

My brother has unknowingly presented me with another mystery !

They are visiting St Andrews, and he thought I might be interested in the little wall clock that they have in the museum … It was purchased from Joseph Knibb, by James Gregory.

My ‘research’ so far identifies this as a split-seconds timer, but I can find no real technical detail about it.

I know [conceptually, at least] how split-seconds Chronograph watches work … but can anyone please explain what Knibb contrived to perform this magic in an early wall clock ?

This snapshot, taken  through the display case, tells us very little … and the one published by St Andrews may be worse !

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MichaelG.

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Best References found so far: __

https://mathshistory.st-andrews.ac.uk/HistTopics/U_of_St_Andrews_History/

https://royalsocietypublishing.org/doi/10.1098/rsnr.2014.0026#RSNR20140026F3

[Michael GilliganParticipant @michaelgilligan61133]

The plot thickens … This description does not sit well with the split-seconds-timer idea:

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The hood of the clock-case is square, surmounted by a triangular pediment, and measures over all 15 inches in height. Access to the movement is obtained by sliding up the hood. The dial is of brass, the time being registered on two silverised circles. The smaller circle, enclosed within the other, is engraved with the hours in Roman numerals. The larger circle is divided into sixty seconds, each of which is sub-divided into three, as the pendulum beats thirds of seconds. There is no minute hand. The movement of the clock is of simple construction and contains only three wheels with an ordinary tick-tack escapement driven by a weight, suspended on a slender chain, which requires to be pulled up every twenty-four hours. The pendulum is supported at the back on a knife-edge.

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Ref. __ https://mathshistory.st-andrews.ac.uk/Extras/Gregory_clock/

 

MichaelG.

[Peter Cook 6Participant @petercook6]

From (amongst others) – this seems to be the standard “history” of Knibb.

Carter Marsh & Co. Ltd (Antique Clocks) – Joseph Knibb Londini Fecit, Circa 1675

“By 1673 Professor James Gregory from St. Andrew’s university had commissioned two longcase clocks and a split second timer, all three clocks remain at St Andrews today. In a letter to John Flamsteed, dated 19th July 1673, Gregory commented:

I have 2 Pendulum Clocks making, with longe Swinges, Vibratinge Seconds; and Pointinge Houres, Minits and seconds, without Strikinge; And also one little Pendulum Clock, with a short Pendulum, vibratinge 4 times in a Second, alsoe without Strikinge; for discerninge small Intervalls; when there may be a pointe of a Second in Question.”

So it was a very short pendulum!

[Michael GilliganParticipant @michaelgilligan61133]

Thanks, Peter 

Yes, a very short pendulum … but we have conflicting accounts !

Yours quotes 4 times a second … mine quotes 3

[ please see my next post ]

The whole thing seems to be a curatorial muddle, and yet the clock still hangs there.

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Whilst I am willing to accept that there was some usefulness in having a sweep-seconds hand that simply ticked in sub-second increments … I do struggle to see how it would be used in astronomical observations if there was no ‘stop’ or ‘split’ facility.

I am astonished to find that such an important instrument has [it would appear] been so poorly documented.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Note: __ The Royal Society document that I linked yesterday explicitly contradicts the other quotation:

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Note: This is from the web-version, but the document is also freely downloadable as PDF

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Confused … Moi ? [as Miss Piggy might have put it]

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

In case anyone is wondering … here’s a good animation of the Tic-Tac escapement [which is, allegedly, what’s used in the little Knibb clock at the Wardlaw Museum]:

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https://www.youtube.com/watch?v=G7prVQiqE2A

https://youtu.be/G7prVQiqE2A?feature=shared

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… but note that he illustrates it with a long pendulum, not a short one.

MichaelG.

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Edit: __ I have no idea why neither of those links produces an embedded video; but life’s too short to worry about it.

[John HaineParticipant @johnhaine32865]

Neat escapement, impulses only on the rightward swing.  If the beat is 3/s, frequency is 1.5Hz, length is approx 1000/9=111.1mm, so presumably hidden in the case.

[Martin KyteParticipant @martinkyte99762]

My understanding is this. Say you are logging stellar transits to create a star map using the transit telescope at Greenwich. You need to know the exact time the star is on the cross hair of the instrument. You have an assistant who takes notes and calls out time marks when required maybe on the half minute. You the observer can then count the ticks of the clock until you judge the star to be on the meridian. You tell the assistant the number of ticks and they record the time.
regards Martin

[Martin Kyte]

On Martin Kyte Said:

My understanding is this. Say you are logging stellar transits to create a star map using the transit telescope at Greenwich. You need to know the exact time the star is on the cross hair of the instrument. You have an assistant who takes notes and calls out time marks when required maybe on the half minute. You the observer can then count the ticks of the clock until you judge the star to be on the meridian. You tell the assistant the number of ticks and they record the time.
regards Martin

Yes, Martin … that’s my understanding too

But the bit I find near-incomprehensible is the practicality of doing that to a resolution of a third of a second.

Tell me how to achieve that, and I will be much happier.

MichaelG.

[Peter Cook 6Participant @petercook6]

It would be interesting to see more detail of the dial markings, and (as you say) more detail of the mechanism.

I would have thought you did the opposite to Martin’s suggestion and have the assistant watch the dial and the astronomer call out “mark” when he judged the transit (possibly twice, once for the leading edge and once for the trailing edge crossing the hairline and take the average?). If the movement of the hand was sufficiently large for each discrete third of a second and the pair were practiced, I would have thought it would be possible. Certainly on a one seconds beating clock or watch, I think I can judge to within half a second (plus or minus a quarter of a second?).

If the hand stepped every third of a second, and the assistant well practiced, I would have thought it was well within the realms of possibility.

[Martin KyteParticipant @martinkyte99762]

Depends how fast you can count. You listen to the ticks which is easier than watching a dial. I have to admit to extrapolating from the descriptions I have read about using seconds pendulums in this manner. I do know that good observers were in the habit of estimating at least half second beats by counting. The ear is pretty good at rhythmic beats.

regards Martin

[Michael GilliganParticipant @michaelgilligan61133]

I sent an eMail to the museum this morning, enquiring whether there are any detail photographs or drawings available, to show the wheel-train and its arrangement

Every avenue that I have so far explored, however, ultimately leads me to the PhD thesis by Dr Helen C. Rawson [which I have already downloaded].

MichaelG.

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Ref.

https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/990/HelenRawsonPhDThesis.pdf;jsessionid=C10D32BF7A1C89A54F5D6F56B1CD5503?sequence=3

 

 

[Michael GilliganParticipant @michaelgilligan61133]

This is the most tantalising image:

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… but the jpeg artefacts mask the detail 🙁

I am guessing it comes from

H W Turnbull, James Gregory Tercentenary Memorial Volume (London, 1939).

as referenced in:

https://mathshistory.st-andrews.ac.uk/HistTopics/U_of_St_Andrews_History/

MichaelG.

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Edit: __ and here’s an animation by Redfern … the closing images of which look very familiar:

https://redfernanimation.com/the-tic-tac-escapement/

 

[Peter Cook 6Participant @petercook6]

Thank you Michael for finding that site. Some beautiful animations to sit are watch – an excellent way to kill an afternoon!

I think that dial and motion is definitely taken from the St Andrews clock – or an identical one. Watching the animation full screen – and it does seem to be real time – I think I could, with a bit of practice, time an event to within one third of a second.

I am still unsure about the  method of communication however. I still think the observer would identify the “mark” while the assistant got the time. Most people speak at up to 200 words/minute. With one word per tick of the clock you could achieve one third of a second. However once past twenty you start to need two words per tick and that would slow things down!

[Peter Cook 6]

On Peter Cook 6 Said:

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I think that dial and motion is definitely taken from the St Andrews clock – or an identical one. …

Without a doubt, Peter

I have just found two references in Antiquarian Horology, to John Redfern having worked on the St Andrews clocks and successfully reconstructed the escapement on the little one.

MichaelG.

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Edit: __ To avoid anyone suggesting that I contact Mr Redfern directly:

https://redfernanimation.com/further-information/

I propose to wait & see what response I get from the Museum.

[Peter Cook 6Participant @petercook6]

Michael, if you ever find out from the University how it works, please let us know.  You set me a puzzle.

Scaling from the pictures and animation, it is possible to deduce certain factors. The drop of the weights looks like about 60″ and the two ropes are about 1″ apart, so a great wheel drum (turning anticlockwise) one every 90 minutes would give about 30 hours running – which gels with the once per 24 hour wind.

If the great wheel rotates once per 90 minutes, then the minute wheel need gearing up 1:90, and the hour wheel gearing down 8:1. The only way I can see to do that in a three wheel clock would be to have a very high tooth count great wheel driving the minute arbour, and have a second pinion on the great wheel arbour driving the hour hand. All three wheel arbours are in a vertical line.

One possibility is

 

The Great wheel is 540 teeth (about Mod 0.2) driving a six tooth pinion on the escape arbour.

An 8 tooth pinion also on great wheel arbour drives a 64 tooth wheel (Mod 0.8) on the hour wheel (in red).

The escape wheel has 90 teeth.

The dark green circles are the positions of the main dial chapter ring and subsidiary dial, for positioning (taken from the animation), and the bright green disc is the great wheel drum.

It would be a great project – although that great wheel could be a nightmare to cut.

I hope this was interesting. Do let us know how Knibb did it if you ever find out.

PS apologies that the gears are involute – hat is the only kind my CAD program can draw

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for your Post, Peter … but unfortunately your included image is not [yet?] showing … I agree, it would make a fascinating project.

A prompt [and as-helpful-as-she-could-manage] reply came from Natasha at the Museum … but frankly we are not much further forward.

[ I will send you a Personal Message shortly ]

What she did kindly include was a considerably better photograph of the dial, a small crop from which I am including here:

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I was quite astonished by the elegant simplicity of the engraving on that chapter-ring … it looks like the best of modern work !

MichaelG.

[Peter Cook 6Participant @petercook6]

Sorry Michal that you can’t see the image. I can see it  on the machine I took it from. However I did upload it using the click and open file approach. This time I will try to drag and drop it.

Try again – if this works we may have found a bug!!

 

[Michael GilliganParticipant @michaelgilligan61133]

Excellent, thank you Peter

This new copy is visible
… the first one is still not.

MichaelG.

[Peter Cook 6Participant @petercook6]

I have reported the issue as a bug. If I select “click” and browse to the file, it fails. If I drag and drop the same file it does work. See if I get a response, but from here on in it’s drag & drop!!

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