Martin Sheridan’s


The previous article deals with the complex Musical Clock. The Harrison RAS Regulator is in another dimension when it comes to complexity. The clock under construction shown here was also at the 2014 Midlands Exhibition, displayed by Martin Sheridan.

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 days running, something that was not to be achieved elsewhere until early in the 20th Century. It has much in common with Harrison’s H3 sea clock, but used a grid iron compensated pendulum as the oscillator instead of the large balance wheels on the sea clock.

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 book by Dr Stuart Harrison, John Harrison’s Contrivance, reviewed on these pages:

Articles on other RAS Regulators under construction can be found here:


At present the design is being refined by Stuart Harrison and we hope to bring details to these pages in due course.