The log glass was used in conjunction with a log and line to time the period during which a log line was allowed to unroll from the log reel and so estimate the ship's speed from the amount of line paid out. Usually the glasses were filled with sand and timed a period of either 14 or 28 seconds. This example was an experimental model that was filled with mercury rather than sand and which could be used to time both 14 and 28 seconds - hence the two numbers on the bulbs. The maker's name is inscribed on the base as W & S Gilbert London Mathematical Instrument MAKERS To the Honble East India Company', as is the inscription 'Jennings PATENT', referring to Henry Conmstantine Jennings.

Not much is known about Jennings, who seems to have been a miscellaneous gentleman scientist. Among other things, he was described as the first man to liquefy prussic acid, the inventor of interesting new ways of producing sugar and destroying moths, and the instigator of a campaign against the waste of stationery in the House of Commons. He also submitted several navigational inventions to the Royal Navy and to the Board of Longitude, which was still assessing navigational ideas in the early nineteenth century. These proposals included an insulating compass (ACO1517), a log-reel and the mercury log-glass, for which he was awarded a medal from the Society for the Encouragement of Arts and Commerce. Extensive trials by the Admiralty, however, suggested that these glasses were no better than ordinary sand glasses.

This log glass is a pair with NAV0744