Instead of a pendulum, it employs a pair of rocking bars with balls on the end and constrained with helical springs. The equal and opposite movement of these bars was less susceptible to being affected by a ships movement than a pendulum would be. Harrison's grasshopper escapement connects the bars with the rest of the mechanism. Some of the cog wheels are of wood which has self-lubricating properties. Gridirons provide temperature compensation by modifying the effective length of the helical springs.

For this invention Harrison received £250 (compared with the £20,000 offered for a full solution) from the Board of Longitude.

Harrison called it a "timekeeper".

The bar-balances like elongated dumbells do not run in conventional bearings. Instead they roll on pairs of plates set at 45° to the vertical and at 90° to each other. These plates, which only move through very short distances are on the ends of long arms pivoted near the bottom of the instrument. The counterweights to these arms are the brass knobs looking like control knobs at the very bottom.

This and other devices mean that the clock requires no lubrication.

• Harrison Jonathan Betts National Maritime Museum, Greenwich, 2007
• Ref: The Illustrated Longitude, Dava Sobel and William J. H. Andrews, Fourth Estate, London, 1998.

Better images: pixgood and my-time-machines.