Flagstaff Hill Maritime Museum and Village
Instrument - Hourglass
... finding latitude at sea... deduced reckoning finding latitude at sea sandglass ...
An hourglass or sandglass is an instrument for measuring a defined time and can be used perpetually by simply turning it over immediately the top bulb empties. The clear blown glass is shaped into two equal sized bulbs with a narrow passage in the centre and contains uniform sized sand or glass particles in the lower bulb. The width of the neck regulates the constant flow of the particles. The glass is held in a stand with top and bottom of equal shape and size.
Hourglasses can measure an infinite variety of time by gauging the size of the particles, the shape and size of the bulbs and the size of the passage between the bulbs, thus measuring hours or minutes or even seconds.
Generally an hourglass sits between discs of wood at the ends, which are joined by long wooden spindles between the ends and tightened by screw caps. The length of time can be adjusted by adding or removing sand particles.
The use of the marine sandglass (or hourglass) has been recorded in the 14th century in European shipping. A one minute sandglass was used in conjunction with the ship’s log for ‘dead reckoning’, (see below) that is, for measuring the ship’s speed through the water. They were also used to regulate ringing the ship’s timetable; for example a 4 hour sandglass was used for the length of the sailors’ watch, and a half hour timer for taking of readings for the ship’s log; the ship’s bell would be rung every half hour. It was usually the role of the cabin boy to watch and turn the sandglasses over at the exact time of them emptying their upper chambers and to ring the ship’s bell.
Hourglasses have been used historically for many hundreds of years. Some have been used for timing church sermons, in cooking, in industry and at sea. Even today they are used for measuring the cooking time of eggs and timing a player’s turn in games such as Boggle and Pictionary.
The sandglasses at sea were gradually replaced in the late 1700’s to early 1800’s by the more accurate chronometers (marine clocks) when they became reliable instruments.
DEAD RECKONING (or Deduced Reckoning)
Dead reckoning is the term used to describe the method of calculating the ship’s position from its speed and direction, used in early maritime travel, mostly in European waters. Both the (1) speed and the (2) direction of travel were recorded on a Traverse Board at half-hourly intervals during a helmsman’s watch of 4 hours. The navigator would record the readings in his ship’s log, plot them on his navigational chart and give his updated course directions to the next helmsman on watch, along with the cleared Traverse Board. This was a very approximate, but none-the-less helpful, method of navigation.
The wooden Traverse Board was a simple pegboard with a diagram of a compass with eight peg holes along the radius to each of the compass points, plus a grid with ascending half hours in the left column and increasing ship’s speed in knots in a row across the column headings, with a peg hole in each of the intersecting cells. A number of wooden pegs were attached to strings on the board. By placing one peg consecutively in the direction’s radius hole, starting from the centre, and the speed holes when the half hourly reading was taken, a picture of speed and direction for the whole 4 hour watch was created.
(1) To measure the ship’s speed a one minute hourglass timer was usually used to measure the ship’s speed through the water and help to calculate its longitude. A rope, with knots at regular standard intervals and a weight such as a log at the end, would be thrown overboard at the stern of the ship. At the same time the hourglass would be turned over and a seaman would start counting the number of knots on the rope that passed freely through his hands as the ship travelled. When the timer ran out the counting would be stopped. A timer of one minute (one-sixtieth of an hour), knots spaced one-sixtieth of a nautical mile apart, and simple arithmetic easily gave the speed of the ship in nautical miles per hour ("knots"). This would be recorded every half hour. The speed could however be inaccurate to the travel being affected by ocean currents and wind.
(2) To calculate the ship’s direction a compass sighting would be recorded each half hour.Marine hourglasses or sandglasses were used from around the 14th to 19th century during the time of sailing ships. This hourglass is representative of that era, which is during the time of the colonisation of Australia. Hourglass or sandglass; an instrument used to measure time. Two equal sized clear glass bulbs joined with a narrow passage between them, containing equal sized particles of sand grains in lower bulb. Glass sits in a brass collar at each end, in a frame comprising 3 decorative brass columns or posts, each attached top and bottom, using round screw-on feet, to round brass discs. Disc have Roman numerals for the numbers 1 - 12 pressed into their inner surfaces and hieroglyphics on the outer surfaces. Roman numerals on inner surface of discs " I II III IV V VI VII VIII IX X XI XII "
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Flagstaff Hill Maritime Museum and Village
Flagstaff Hill Maritime Museum and Village