Harrison’s early regulators were largely constructed in wood, yet contained several significant innovations, such as the grasshopper escapement which eliminated sliding friction, and the principle of an oil-free clock.  There is a growing appreciation that Harrison’s inventions form an integrated system, with each component influencing and affecting the other. His early background as a skilled woodworker also provided him with means to construct his segmented wheel construction. These were some of the ideas I wished to explore further. So my clock was to incorporate the wheel construction and escapement from the Brocklesby Park clock, with the remontoire from his later clocks. I gave myself a timeframe of six months to design and complete the project. This would necessitate making concessions by simplifying the design while trying to maintain timekeeping accuracy.

Numerous fine copies of Harrison’s clocks have been made, but I wanted to adopt some of his principles and incorporate them into my own design. I started researching this project over the 2009 Christmas break, and was fortunate to be loaned a copy of ‘The Science of John “Longitude” Harrison’ by William Laycock. It is a brilliant read, and he delves into the underlying scientific and mathematical principles of Harrison’s inventions. The other publications I used are listed in the bibliography. Finally, I was fortunate to be assisted by Peter Hastings, who calculated to geometry of the Grasshopper escapement, as well as putting up with my steady stream of questions.

Harrison’s Broc...

Harrison’s Broc...

Above: Harrison’s Brocklesby Park tower clock, 1722. The wheels are made from oak, the pinions from brass and the grasshopper escapement pallets from lignum vitae. (Photo from ‘John Harrison Clockmaker’ by Andrew King; page 497 and 505, Antiquarian Horology, Volume 29, June 2006).

• Harrison claims to have achieved an accuracy of one second a month with his regulators, something I will try to match with this wooden regulator.
• Use no lubrication in the movement.
• Segmented wooden wheel construction, including the escapement. Use Acacia Rhodoxylon, a very dense and stable hardwood from outback Queensland. The pallets of the grasshopper escapement to be of Lignum Vitae.
• Use the cycloidal tooth form on the wheels instead of Harrison’s chordal pitch gearing. Similarly, I used cycloidal brass pinions, instead of the lignum vitae roller pinions. Ideally the escape wheel pinion should be a roller pinion to absorb the recoil.   
• Use hybrid ceramic and stainless steel ball races are used throughout, instead of anti-friction wheels. The pendulum suspension also uses ball race suspension.
• The Grasshopper Escapement, Peter Hastings produced two designs for me, the constant force escapement, and the 2:3 ratio variable force escapement. I selected the constant force type for this clock. Pallet tooth form was modified to create safety action and prevent run-away.
• The remontoire is used to eliminate any variation in torque force delivered to the escape wheel through the drive train. It also frees the escapement from the effects of engaging friction when the pendulum is driving the escape wheel backwards during recoil. The remontoire layout for this clock is derived largely from H4, and as with the RAS regulator, is powered by a small spiral spring. I will leave the constant force remontoire used on H2 and H3 for another time. Ideally the remontoire should power the escape wheel as in the RAS, but this would necessitate a 120 tooth escape wheel, which would be difficult to construct reliably in wood.
• Temperature compensation: The temporary pendulum is made of a length of well seasoned rosewood. Likewise the bob is temporary. When time permits I would like to construct a grid-iron pendulum, as well as experiment with fused silica rod.
• Circular error compensation: Harrison used Huygens’ solution of cycloidal cheeks, but there is evidence that he modified it on the RAS regulator to a circular form. Either way, very fine tolerances are required to get the system to work as intended. Instead, I have deviated significantly from Harrison’s solution, and used a system developed by Dallas Cain and Pierre Boucheron using twin pendulums (Horological Science Newsletter 1993-1, 1994-1). I am not aware of any other clocks using this system.
• Barometric compensation: I doubt if this clock in its current form will be accurate enough to warrant any attempt at it its inclusion.
• The clock is currently running in a temporary stand. To get the rate to within I second a month, it would have to be housed in a cabinet, and securely mounted onto a masonry wall.

Wheel made from...

Wheel made from...

Wheels made from Outback acacia rhodoxylon with spoke inserts. As with the Harrison wheel, radial grain segments form the teeth. Each wheel comprises of total of 31 individual components.

X-ray image of ...

X-ray image of Harrison wooden clock wheel using radial grain oak segments for the teeth inserted into a solid oak disc (Photo from page 85, The Illustrated Longitude, by Dava Sobel and William J.H. Andrews).

Harrison developed his first grasshopper escapement in the early 1720’s as a redesign of the original Brocklesby Park clock anchor escapement. Problems with lubrication was causing the clock to stop, so Harrison’s solution was ingenious, by pivoting the pallets to the original pallet frame, he overcame sliding friction, eliminated drop and the associated loss of impulse, and produced an escapement that required no lubrication. Brilliant! It is still a recoil escapement, where the recoil is used release the pallet after impulse. The geometry of the Grasshopper also lends itself to a variety of configurations which can provide either a constant impulse to the pendulum, or an impulse of variable force. It also proved to be a versatile escapement in that it was used both on his land regulators and sea going clocks, H1, H2 and H3.

The Grasshopper...

The constant force escapement mock up is on the left, and a co-axial variable force escapement on the right.

The Grasshopper...

The final configuration of the constant force grasshopper. The pallets are made from a naturally oily wood, lignum vitae, also used by Harrison.

After completing working models of the grasshopper escapement and deciding to adopt the constant force design, I set about designing the wheel train and remontoire. The wheels and pinions were left overs or rejects from other projects, with the emphasis on producing a compact wheel train.

The Prototype

The Prototype

The materials used were high and medium density fibreboard, wood brass and steel. Judicious amounts of Blu-Tack, sticky tape and superglue were used to get something working as quickly as possible, and then modified as necessary. Despite the rickety nature of the construction, ball races were used in the prototype to give a more accurate indication of the forces required to drive the mechanism. Designing a clock from scratch involves a fair amount of trial and error as one navigates uncharted territory.

The Prototype

The Prototype

Circular Error ...

The completed clock movement with a temporary wood pendulum; running on a temporary stand. The escapement crutch is extended and a weight added to form the secondary pendulum which serves to counter circular error. The horizontal link connects it to the main pendulum. It is the intention is to make a grid iron pendulum, and mount the clock in a wall mounted clock case.

Circular Error ...

Circular Error ...

Circular Error ...

Graph 1. Free seconds pendulum coast-down from 7.5 degree semi-arc, without circular error correction. Pale blue graph show decreasing amplitude over time, red graph shows corresponding increase in rate, (i.e. pendulum swings faster over smaller arcs).
Graph 2. Free seconds pendulum coast-down from 7.5 degrees semi-arc with secondary pendulum attached by horizontal link. Pale blue graph again shows decreasing amplitude over time, and blue graph shows an initial increase in rate, then levels off between a pendulum amplitude of 6 to 4 degrees semi-arc, and then declines in rate after 4 degrees, (i.e. the pendulum rate remains constant between 6 and 4 degrees even though the amplitude is decreasing). The level portion of the rate graph can be varied by changing the relative masses of the two bobs, their relative lengths, and the position of the link.

Circular Error ...

Graph 3. Part of a Microset timer reading for the prototype clock movement. The graph shows the characteristic saw-tooth profile with the spikes in the rate coinciding with rewinding of the remontoire every minute. This variation in rate can be further reduced by adding Harrison’s solution of remontoire cams to provide a constant force on rewind.  The trial period was 7.4 days. The resolution on the vertical axis is 0.000049 seconds, the average error from target rate over that period was 3.4 seconds and the instability is 4 seconds per day. The clock itself was constructed from left over bits and pieces and scrap.
Will Matthysen
October 2010.

The Illustrated Longitude, by Dava Sobel and William J.H. Andrews.
The Science of Clocks and Watches, by A.L. Rawlings.
The Quest for Longitude.
The Lost Science of John ‘Longitude’ Harrison, by William  Laycock.
The Technology of John Harrison’s Portable Timekeepers, by A.G. Randall.
Antiquarian Horology, Number 4, Volume 29, June 2006.
Antiquarian Horology, Number 4, Volume 31, June 2009.
Horological Science Newsletter