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An hourglass (or sandglass, sand timer, or sand clock) is a device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated flow of a substance (historically sand) from the upper bulb to the lower one due to gravity. Typically, the upper and lower bulbs are symmetric as they ...
[2] [3] Filled with the amount of sand suitable for measuring a lapse of half an hour, each time the sand emptied was also called a "glass"; eight glasses (four hours) defined a "watch". [ 2 ] [ 3 ] The times determined by the sandglass, along with the record in the logbook of the speed measured with the " chip log ", permitted the ship's ...
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John Harrison, Thomas Tompion, and Mudge [7] built a number of clocks with 24-hour analog dials, particularly when building astronomical and nautical instruments. 24-hour dials were also used on sidereal clocks. The famous Big Ben clock in London has a 24-hour dial as part of the mechanism, although it is not visible from the outside. [8]
Hour: The next two rows of 8 light switches are the hour mark, using a similar BCD system, where the upper row is a multiplier of 5, and the lower row is a multiplier of 1. Therefore the maximum hour number is 4*5+4=24 and the minimal hour number is 0 where everything is OFF. Since a 24-hour clock uses a number from 0-23.
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The hours between 16:00 and 20:00 are so arranged because that watch (the "dog watch") was divided in two. The odd number of watches aimed to give each person a different watch each day; it also allowed the entire crew of a vessel to eat an evening meal, the normal time being at 17:00 with first dog watchmen eating at 18:00.