E0266

Mercurial log glass Long Caption: 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 Constantine Jennings. After extensive trials by the Admiralty over many years, these glasses were found to be no better than the ordinary sand glasses. Credit line: © National Maritime Museum, Greenwich, London. Object: NAV0743 Artist: Gilbert & Sons Date: circa 1817 Medium: brass; glass; mercury Size: Overall: 175 mm Click here to buy a bespoke print of this image.