Box, Box top open, Box open, Ivories, Box Front, Tub and gimbals, Tub, Dial, Movement, 2nd view of movement, Angle view, Potance closeup, Airy's bar,

This two day marine chronometer dates from ca 1860 and is fitted with Airy's bar adjustable temperature error compensator.

Airy was Astronomer Royal during the middle of the 19th Century. He had an active interest in timekeeping and chronometers in contrast to Maskelyne who was Astronomer Royal at the time of Harrison's triumph over the problem of longitude. The purpose of Airy's improvement was to allow the temperature error compensating weights to be moved without disturbing the poise of the balance. 

Most chronometer makers felt they were more trouble than they were worth and thus they only appear (in any number) on Admiralty purchased pieces. Purchases for the Admiralty had to have the approval of the Astronomer Royal so inclusion of Airy's Bar gave the maker a slight edge.

An alternative, less jaundiced view, is that the bar was only needed on chronometers expecting service in extremes of temperature such as both tropical and polar work. This would exclude most shipping vessels and leave only naval vessels and perhaps some fishing vessels. That would also explain the prevalence of Admiralty markings on chronometers with Airy's bar.