Marine sandglass, is the ancient marine instrument employed to measure (or rather estimate) the time at sea. It used to be a 30 minutes hourglass. It was used as well, together with the chip log, to measure the boat speed through the water in knots (in this case with a 3 minute glass or less).
Originally, it consisted of two glass bottles one above the other and connected by both ends with a tube. The progress of glassblowing allowed, from one moment, to do it in one single piece. The Marine glass filled with sand or a suitable material (finely grinded lead or tin chips, used to avoid humidity) placed in the upper half. The sand was flowing slowly and steadily towards the lower half, by the action of gravity, taking a certain time to empty (wich could be calibrated.) Once the upper "glass" was empty the glass could be "turned" to measure another time period.
Although it was vital to cross the oceans, the Marine glass was not such an accurate element to measure the passage of time. Many factors could affect the duration of sand's flow: the humidity inside the "glass", the uniformity in fineness of the sand, the inner diameter of the connecting tube with a possible wear caused by the flow of sand, the more or less horizontal position, the effect of acceleration or deceleration of the ship's movements, all could influence the flow of sand and hence the time measured.
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Although no one knows exactly when was first used in the sea, for navigational purposes, the use of the Marine sandglass is recorded since the 14th century
The earliest recorded reference, which can be said with certainty to refer to a marine-glass, is dated 1345/46. It's contained in the Receipt of Thomas de Stetesham, Clerk of the King's Ship called "La George" within the reign of Edward III, which translated from Latin says:
"The same Thomas accounts has paid at Lescluse (Sluys) in Flanders for xii horologes of glass( pro xii orlogiis vitreis) a price of 4 1/2 'gross' each, in sterling 9s. Item, for iv horologes of the same type (de eadem secta), a price of five 'gross' each, making in sterling 3s. 4d."
Next reference is found in an inventory of effects of Charles V of France, which were in his possession at his death on September, 16, 1380. It's an inventory of about four thousand items and one of them describes an hour-glass in the king's study at his castle of St. Germain en Laye in the following terms.
"Item ung grant orloge de mer, de deux grans fiolles plains de sablon, en ung grant estuy de boys garny d'archal."
("Item a large sea clock, with two large phials filled with sand, in a large wooden brass-bound case.)
The most interesting thing about the second reference is that a common sand-glass is defined as a "sea clock", which suggests that, at this period, the sand-glass was more commonly used at sea and may have its origins in a maritime need.[1].
In the long-distance navigation through the open ocean, the "glass" to measure the time was a tool as important as the compass for the course. Filled with the amount of sand suitable for measuring a lapse of half an hour, each time the sand emptied was called a "glass" and eight "glasses" (four hours), defined a "watch". The compass and the "sandglass", along with the record in the logbook, of the speed measured with the "chip log", permitted the navigator to plot his position on a map. Multiplying the velocity by the time (measured with the glass) the course had been kept, gave the traveled distance, with the compass showing the sailing direction. This is the simple method called dead reckoning (an abbreviation for estimated calculation).
Until the beginning of the XIX century, when it was possible to navigate by the lunar distances, the dead reckoning navigation, periodically contrasted with the measurement of latitude taken with the quadrant,(backstaff, astrolabe, octant), was the only system available to mariners to navigate the globe, that's the reason why the "Marine sandglass" was so important for sailors. At the same time "on the mainland" mechanical clocks were used (apart from the sundials and hourglasses) to know the time, and this parallel situation has lasted during more than four hundred years (from 1350 to 1805)
Was used on ships to measure half-hour periods. The helmsman was responsible for turning it upside down. It began with the midday's sun, knocking the bell once, after half an hour twice, and so on .. But if a sailor wanted to shorten his guard, he only needed to turn the glass before it had fully emptied.
Columbus, on his ships, logged the pass of time by a half-hour "ampolleta" (glass) that was turned every time it emptied, to keep track of the "canonical" hours.
During the voyage of Ferdinand Magellan around the world, he equiped every ship with 18 hourglasses . It was the job of a ship's page to turn the hourglass, thus supplying the time to be registered on the ship's log. Noon was the reference time for navigation, not dependent on the glass as the sun would be in its zenith.[2]. A variable number of hourglasses were sometimes fixed in a common frame, each with a different operating time, for example, 1 hour, 45 minutes, 30 minutes and 15 minutes.
From the sixteenth century a smaller "glass" was used along with the chip log, to measure the speed (in knots) of the vessel over the water. The procedure was as follows:
A sailor ran the chip log and another sailor the sandglass. The slide of the pulled over the stern and let run the first length of line till the quadrant was stabilized in the water. The sailor was leaving to run the lineto pass freely leaving the slide by hand and touching the first knot sang "mark!" At the moment of the inverted glass and time began to run while the line was counting the knots as they passed until the sandglass sang "mark!" a second blow when they had dropped all the sand, then he caught the firmly the line, measuring the fraction of knot elapsed to the last mark! and cried "Five knots and fourth!
Marine sandglass. Nav. Hourglass. There were marine sandglasses of half an hour, of half a minute and of a quarter of a minute. The first used to be called "minuter", and the other two "seconders. Ant Range. That was reflected two Marine sandglasss, signifying the time to move sand two half-hour or one hour. | | Fras. Tues: GRIND THE -: move the sand from one to another glass. -STOP: stop change wings half past eleven of the morning to do at the instant of step the sun across the meridian, and thus have the time arranged. STEALING THE-: change before moving all the sand. Sometimes they did the coxswains or pages and the to shorten his guard. TALK BY AMPOLLETAS: talk to each shift and not touch her all to a time. GET GOOD -: Fig person is said very talkative.
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