Marine sandglass is an ancient marine instrument employed to measure (or rather estimate) the time at sea when used as a 30 minute hourglass. It was also used 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 with both ends connected by a tube. Over time, the progress of glassblowing allowed it to be made in one single piece. The Marine glass was filled with sand or a suitable material such as finely ground lead or tin chips (used to avoid humidity). Placed in the upper half, the sand would flow slowly and steadily towards the lower half by the action of gravity, taking a certain time to empty (which could be calibrated). Once the upper "glass" was empty, the glass could be "turned" to measure another time period.
Although its use was vital to cross the oceans, the Marine glass was not such an accurate instrument 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.
Although no one knows exactly when it was first used on the seas for navigational purposes, the use of the Marine sandglass has been recorded since the 14th century.
The earliest recorded reference, which can be said with certainty to refer to a marine sand-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 4½ '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."
The next reference is found in an inventory of the property of Charles V of France in his possession when he died 16 September 1380, listing about four thousand items. One is described as an hour-glass in the king's study at his castle of St. Germain en Laye, as follows:
"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 originated in a maritime need.
In 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 19th century, when it was possible to navigate by the lunar distances, 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 lasted more than four hundred years from 1350 to 1805.
Watch sandglasses 
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 equipped 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. 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.
Ship-log sandglass 
From the 16th 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 line to 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!
- Diccionari Català Valencià Balear, Alcover-Moll: Rellotge de sorra. Item dos flascons d'hores, doc. mall., a. 1434 (Boll. Lul. Iii, 312)
- Over the Edge of the World: Magellan's terrifying circumnavigation of the globe. William Morrow. 2003. ISBN 0-06-621173-5.