The Experiment
The use of pendulums to measure the difference in gravity around the Earth was nothing new when George Biddell Airy and William Whewell set off to Cornwall in 1826. It had been known that the Earth’s gravity was variable since 1671 when the French scientist Jean Richer made measurements with a pendulum clock and discovered that gravity was not uniform and that it was running over 2 seconds slower in French Guiana than in Paris. In 1737 the French mathematician Pierre Bouguer swung pendulums at different elevations and from the rate of swing was able to make the first estimate of the density of the Earth. There was a lot of concern over the accuracy of these early experiments due to the difficulty in measuring the period of the pendulum. These concerns continued unabated until the invention in 1817 of a reversible pendulum by British Captain Henry Kater. The invention would offer the opportunity to measure the local acceleration of gravity with much more accuracy than ever before.
The experiment required the free-swinging pendulum to be hung in front of a tall grandfather-style precision clock and the timing of the swing would be measured against the clock-driven pendulum behind. To get a measurement for the gravity of the Earth you would need to run the experiment in two locations with different altitudes. Using mountains would be one option, but the mines of Cornwall offered another.
Dolcoath 1826
Dolcoath Mine in the early 1800s was the deepest mine in England. Known locally as The Queen of Cornish Mines it was located in the far west of England near Redruth. It was a very profitable working mine mainly extracting copper, but also tin, silver, arsenic and other minerals. By 1826 it was over 2000ft in depth, with its deepest recesses accessed by long series of ladders. Any equipment, mine spoil and occasionally men would travel up and down the large shafts in buckets called kibbles. The laborious activities would not be limited to the underground, on the surface were noisy pressing stamps and arsenic works which ran beside the engine house. These were working environments before the times of health and safety where accidents were frequent and life was hard for the men, women and children who made their living through mining.
The main way to get goods and occasionally people in and out of the mine – Riding the Kibble
Credit: Mines and Miners. Louis Simonin, 1868
It was in this environment that the two scientists Airy and Whewell arrived first in the summer of 1826 and brought with them precious and expensive precision instrumentation to conduct a scientific experiment. What could possibly go wrong? Well, as it turned out, quite a lot! On arrival, two stations were set up, one underground and one on the surface (perpendicular above to the underground station). Two pendulums (named Foster and Hall after their previous owners) in the design of an invariable Kater one-second pendulum were suspended on knife edges and hung in position in front of a clock pendulum. Alongside the pendulums, they arrived with seven precious and valuable chronometers, tripods, telescope sights and tents.
The surface station was set up in a tent on top of a hill which rises to the south of Dolcoath mine while the subsurface location was 1200ft underground. The cavern in which it was located was split into two with a wooden screen. The experiment was placed behind one side of the screen and separated from the person who would watch the motion from the other side through a sighting telescope looking through a small hole in the screen.
Each pendulum swung for 6-8 hours a day while being watched by either Airy or Whewell, and then the timings were compared between the two stations. The chronometers were transported between both sites at the start and end of the experiment to compare with the clocks. There were some initial concerns. The stands were not up to the job as they were not stable enough. The fragility of the chronometers and lack of agreement between the timings of these was a major concern (two soon had broken glass, damaged from being carried up and down the ladders). It was decided to make the observation runs shorter – just 5 hours of observation a day – so they could compare the chronometers to the ‘clock’ more regularly with an ambition to lead to better accuracy.
After the first cycle of measurements was made there was an attempt to raise the Foster pendulum to the surface. At this point, there was an accident and the straw packing within the kibble caught fire and the bottom of the bucket burnt through. The pendulum plunged downwards and was lost to the abyss. Airy believed it was sabotage and certainly the miners could have been to blame. They were a suspicious lot, living in a remote part of the UK and working in a job where death was commonplace. The arrival of two scientists from London with their strange requests couldn’t have gone down well. At best they were seen as an inconvenience at worst they were regarded as the source of bad luck. Sedgewick encapsulates this in his accounts of the experiment “One morning I listened to two men who had watched our descent the day before: “I think they’re no good. There must be something wicked about them – the little one (that was Airy) especially. I saw him stand with his back to the Church, and make strange faces.” (Sedgewick, 1890)
Airy immortalised the moment of disaster in a poem
The ladders of mighty Dolcoath I descended
Through caverns that yawned like an entrance to hell:
All was silent, save when through the levels came blended
The roar of the blast and the kibbul’s deep knell.
To the right, a vile path round the South Shaft was bending:
Behind, a chain-ladder from hooks was depending:
Our station’s white door in the front was ascending:
When I marked the sad spot where the pendulum fell.
Dark and drear was the spot in Dolcoath’s deepest level
Where the pendulum’s fragments were scattered around,
As when, at the close of some drunken men’s revel,
Broken bottles and plates encumber the ground
Yet though scatter’d they lay, not entirely neglected:
For the men who had packed them, with spirits dejected,
And Mid Cattell and deads the small pieces selected,
And sent up to grass all the bits that they found.
Taken from George Biddell Airy, ‘Dolcoath’, in P. D. Hingley and T. C. Daniel (eds.), A far off vision: a Cornishman at Greenwich Observatory. ‘Auto-Biographical Notes’ by Edwin Dunkin, F.R.S., F.R.A.S., (1821-1898), with notes on the lives & work of his father, brother and son, (Royal Institution of Cornwall: Truro, 1999), p. 182.
Dolcoath 1828
After the ill-fated first attempt, Airy and Whewell would return in the summer of 1828. This time they arrived with reinforcements; the intention was to keep the experiment running twenty-four/seven. Accompanying them were the eminent astronomer Richard Sheepshanks and geologist Adam Sedgewick. They also had a number of additional helpers including Airy’s younger brother.
In another change to the original experiment instead of bunking down in the local count house (now Miss Mollies Tea Rooms) they were to stay within the houses of local mine owners and aristocrats, the experiment had become in all ways something much grander. They arrived on site on the 8 July with two pendulums named Sabine and Brisbane after their previous owners and used the same setup as before except they had calculated that a one-second difference between chronometers was less significant over a longer period. So instead of 5-6 hour shifts, they would watch the pendulums continually with no breaks. There would be 3 people at the surface and 3 people below, they would work in shifts with shift work starting at 6am, 2pm and 10pm. Sheepshanks was in charge of the upper station while Airy was in charge of the lower station and he made sure he climbed the ladders to watch over every changeover: a gruelling schedule for himself while the experiment was in progress.
But by the 10 August the observations started to show an issue, something which Sheepshanks would work upon, eventually showing that it was an issue with the knife edge and agate plate. The steel knife edges of Sabine were not accurate enough. When the two pendulums were hung back to back the error was obvious. Airy fixed and made adjustments accordingly. Once this was resolved the experiment started again in earnest and the main experiment ran between the 16–19 August, when it was abandoned due to rising water in a lower part of the mine which infringed on their area. Airy left the experiment running for as long as he could until even he had to admit defeat once more and return everything to the surface.
In total 127 hours of observations were made. From this, they were able to draw the conclusion that the lower station accelerated by 2 seconds a day. These early results must have been exciting to the team, unfortunately, the experiment was cut short again this time the mine had slippage and they had to come out of the lower levels, and the experiment closed.
The Rosevale Reconstruction
The first hurdle we had was finding a location in Cornwall where we could conduct the experiment. Dolcoath has been decommissioned and flooded with water, as have many of the deep mines within Cornwall. We were in luck when we identified Rosevale mine in Zennor; this was a working mine run by enthusiasts and ex miners and offered us the opportunity of access, albeit not to the depths that Airy would encounter at Dolcoath. A predominantly Victorian mine, access to levels is by ladders and there are wonderful features such as the original tools and candle wax running down the walls. Like all mines, it is the environment, which is prone to change, and 2020 saw our first hurdle as the mine had been shut due to covid restrictions. The pumps had stopped working and the lower levels flooded, making access an impossibility, so we turned our attention instead to the manufacture of the pendulum.
One of the mine’s core team is a clock repairer, maker, and member of the Royal Horological Society. This was key as Kater pendulums were once quite common but are now incredibly rare instruments. We could not take an original into the damp, dirty and dangerous environment of a mine and quickly identified that we would need to build one of our own, which could at worst take damage from transport in and out to its subsurface location.
The making of the pendulum would require detailed information about materials, fixings, sizes and processes. We soon discovered that pendulums had been made from a number of different materials, brass, copper, wood and in some cases steel. Our investigations into exact measurements of the pendulums were equally elusive. This led us to the conclusion that we needed to see an original for ourselves. We found that one of these pendulums was housed at the Science Museum in London and after many delays due to covid restrictions one of our team Dr Daniel Belteki had the opportunity to photograph one in the late summer of 2021. This information has allowed Wayne Ridgeway the clockmaker to make a replica pendulum. A last-minute change at the end of December due to one of our team contracting covid saw us delaying the experiment until Spring 2022.
The Experiment
On 9 April 2022 we finally got the opportunity to run the experiment. Wayne Ridgeway had completed his manufacture of a Katar Pendulum replica and he had acquired a regulator to be positioned behind. We had chosen a location in the mine which was not too wet or had too much of a draft. The temperature was cool around 11 degrees and we could see your breath in the air. Dr Edward Gillin, Dr Daniel Belteki and I waited patiently as Wayne positioned and started the regulator, before leaving it to settle. He then hung the free-swinging Katar pendulum a short distance in front.
The regulator pendulum had a white dot upon it and the Katar pendulum had a black piece of wood which extended below the bob. It would be these two key elements that we would be watching during the course of the experiment. Positioning ourselves a short distance away we placed a small antique brass sighting scope on a table. Looking through this scope we would be watching for the moment that the white dot was ‘eclipsed’ by the black rod. In effect, it would disappear. This would be the timing that the two pendulums would be swinging together and the original experiments were called coincidences. We could anticipate when these would be forthcoming, as the pendulums visually started to look as though they were swinging in harmony, rather than in opposite directions. A coincidence deep in a mine would occur faster than one at the surface and it is this difference which would allow Airy and Whewell to undertake their calculations.
The first coincidence was witnessed by me. I found myself at the sighting scope just minutes before it was due to occur. It had been a challenge to focus on the pendulum through the small sighting scope, and I had felt a moment of panic when I thought that would be visually usable to undertake the measurement. A realisation that I had to use my peripheral vision, much in the way an astronomer would when teasing out detail on a planetary disc. I allowed my eyesight to settle and soon saw the inverted small image of the white dot and black rod through the sighting scope. As the first person to witness the event, it was very hard to know what to expect what I can claim to have seen is the eclipsing of the white dot, not once but twice, the first time for less than a second and then the second time for a longer period of 16 seconds. My experience was similar but unique to those that followed. Daniel Belteki made the observation of the second coincidence and he didn’t see a complete covering of the white dot, but he did see a maximum covering not once but twice, shortly spaced apart in time, Edward Gillin saw something similar when he had a go at the third coincidence.
Each coincidence was timed between 44 and 46 minutes apart and we felt this had more to do with the expertise of the instrument maker than the observers. With more time and coincidences I am sure we would have made improvements in the accuracy of the observations. It had taken a long time that morning to set up the equipment and get the regulator running smoothly. After nearly 5 hours of observations, we had to allow the mine to pack up and dismantle the experiment. Our limitations were very apparent, we had a lack of time to repeat the experiments to a similar length as Airy and Whewell (they conducted 127 hours in total), and we also lacked depth in which to conduct the experiment. Even so, this has whetted our appetite to try again and improve on our first attempt. We felt we were very successful in exploring the challenges involved in undertaking such an experiment in a less than ideal location and we are all looking forward to a time when we can reconvene and try it all over again.
See also:
George Biddell Airy, ‘Account of experiments made at Dolcoath Mine, in Cornwall, in 1826, & 1828 for the purpose of determining the density of the earth’, in P. D. Hingley and T. C. Daniel (eds.), A far-off vision: a Cornishman at Greenwich Observatory. ‘Auto-Biographical Notes’ by Edwin Dunkin, F.R.S., F.R.A.S., (1821-1898), with notes on the lives & work of his father, brother and son, (Royal Institution of Cornwall: Truro, 1999).
Sedgwick (1890) quoted in, John Willis Clark and Thomas McKenny Hughes, The Life and Letters of the Reverend Adam Sedgwick, LL.D., D.C.L., F.R.S., Fellow of Trinity College, Cambridge, Prebendary of Norwich, Woodwardian Professor of Geology, 1818-1873, Vol. I of II, (Cambridge University Press: Cambridge, 1890), p. 332.
Archaeoastronomy Cornwall 