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. After extensive trials by the Admiralty, however, these glasses were found to be no better than the ordinary sand glass. This log glass is a pair with NAV0743.