JOHN HARRISON is best known for his clocks used to determine longitude. In his later years he decided to build a new regulator which needed to be extremely accurate as part of the longitude project. His earlier regulators were becoming old, but still functioning after decades of use - remarkable for clocks that employed wood as the main construction material. Some of his wooden clocks are still working perfectly two and a half centuries later.

The new regulator was of more conventional brass construction. It was designed to be accurate to one second in 100 day’s running, something that was not to be achieved elsewhere until early in the 20th Century.

Owners of this clock today are the Royal Astronomical Society and it is generally known as the RAS Regulator. Earlier the clock had suffered from some neglect, and had two major restorations, the second by Rupert Gould who will be known to anyone who watched the TV series Longitude.

The RAS clock is the subject of a new book by Dr Stuart Harrison, John Harrison’s Contrivance, reviewed recently on these pages. The clock under construction here is by Ray Darnell (pictured) who has been working closely with  Stuart Harrison. As can be seen Ray’s clock at present has

Perspex plates a wise move in a developing design before committing to some large pieces of expensive brass sheet. That is fortunate for anyone who has the chance to see the clock (like at the London Model Engineering Exhibition here) as the most elegant mechanism is fully on view.

The design is of the clock as originally made by John Harrison, following research by his namesake. The absence of friction has to be seen to be believed. the movement can be set in motion with just the weight of a small coin - 20p piece - placed on a tooth of the large wheel. Everything is designed to reduce friction.

Stuart’s book contains CAD drawings, but it is not a ‘how to’ guide. Far from it. But we would expect many experienced clockmakers might be tempted to build the RAS Regulator, as designed by the Harrisons of the 18th and 21st Centuries. As such it is packed with features not found in clocks produced in home workshops today such as friction wheels, remontoire, Grasshopper Escapement,  and roller bearings.

It will be fascinating to see whether Ray’s clock will prove to be as accurate as old Harrison predicted. So far it looks pretty sensational.